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Mike Hearn
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docs/build/html/_sources/index.txt
vendored
2
docs/build/html/_sources/index.txt
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@ -26,6 +26,8 @@ Read on to learn:
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overview
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getting-set-up
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tutorial
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messaging
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protocol-state-machines
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visualiser
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roadmap
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97
docs/build/html/_sources/messaging.txt
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97
docs/build/html/_sources/messaging.txt
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Networking and messaging
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========================
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Although the platform does not currently provide a network backend, some preliminary interfaces are defined along with
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an in-memory implementation provided for use by unit tests and other exploratory code. An implementation based on Apache
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Kafka is also being developed, which should be sufficient for real use cases to be implemented in the short run, even
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though in the long run a fully peer to peer protocol will be required.
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This article quickly explains the basic networking interfaces in the code.
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Messaging vs networking
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-----------------------
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It is important to understand that the code expects any networking module to provide the following services:
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- Persistent, reliable and secure delivery of complete messages. The module is expected to retry delivery if initial
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attempts fail.
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- Ability to send messages both 1:1 and 1:many, where 'many' may mean the entire group of network users.
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The details of how this is achieved are not exposed to the rest of the code.
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Interfaces
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----------
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The most important interface is called ``MessagingService`` and is defined in the ``core/messaging/Messaging.kt`` file.
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It declares an interface with the following operations:
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- ``addMessageHandler(topic: String, executor: Executor, callback: (Message, MessageHandlerRegistration) -> Unit)``
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- ``createMessage(topic: String, data: ByteArray): Message``
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- ``send(message: Message, targetRecipients: MessageRecipients)``
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- ``stop()``
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along with a few misc others that are not important enough to discuss here.
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A *topic* is simply a string that identifies the kind of message that is being sent. When a message is received, the
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topic is compared exactly to the list of registered message handlers and if it matches, the callback is invoked.
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Adding a handler returns a ``MessageHandlerRegistration`` object that can be used to remove the handler, and that
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registration object is also passed to each invocation to simplify the case where a handler wishes to remove itself.
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Some helper functions are also provided that simplify the process of sending a message by using Kryo serialisation, and
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registering one-shot handlers that remove themselves once they finished running, but those don't need to be implemented
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by network module authors themselves.
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Destinations are represented using opaque classes (i.e. their contents are defined by the implementation). The
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``MessageRecipients`` interface represents any possible set of recipients: it's used when a piece of code doesn't
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care who is going to get a message, just that someone does. The ``SingleMessageRecipient`` interface inherits from
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``MessageRecipients`` and represents a handle to some specific individual receiver on the network. Whether they are
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identified by IP address, public key, message router ID or some other kind of address is not exposed at this level.
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``MessageRecipientGroup`` is not used anywhere at the moment but represents multiple simultaneous recipients. And
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finally ``AllPossibleRecipients`` is used for network wide broadcast. It's also unused right now, outside of unit tests.
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In memory implementation
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------------------------
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To ease unit testing of business logic, a simple in-memory messaging service is provided. To access this you can inherit
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your test case class from the ``TestWithInMemoryNetwork`` class. This provides a few utility methods to help test
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code that involves message passing.
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You can run a mock network session in one of two modes:
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- Manually "pumped"
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- Automatically pumped with background threads
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"Pumping" is the act of telling a mock network node to pop a message off its queue and process it. Typically you want
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unit tests to be fast, repeatable and you want to be able to insert your own changes into the middle of any given
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message sequence. This is what the manual mode is for. In this mode, all logic runs on the same thread (the thread
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running the unit tests). You can create and use a node like this:
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.. container:: codeset
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.. sourcecode:: kotlin
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val (aliceAddr, aliceNode) = makeNode(inBackground = false)
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val (bobAddr, bobNode) = makeNode(false)
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aliceNode.send("test.topic", aliceAddr, "foo")
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bobNode.pump(blocking = false)
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.. note:: Currently only Kotlin examples are available for networking and protocol state machines. Java examples may
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follow later. Naming arguments in Kotlin like above is optional but sometimes useful to make code examples clearer.
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The above code won't actually do anything because no message handler is registered for "test.topic" so the message will
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go into a holding area. If/when we add a handler that can accept test.topic, the message will be delivered then.
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Sometimes you don't want to have to call the pump method over and over again. You can use the ``runNetwork { .. }``
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construct to fix this: any code inside the block will be run, and then all nodes you created will be pumped over and
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over until all of them have reported that they have no work left to do. This means any ping-pongs of messages will
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be run until everything settles.
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You can see more examples of how to use this in the file ``InMemoryMessagingTests.kt``.
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If you specify ``inBackground = true`` to ``makeNode`` then each node will create its own background thread which will
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sit around waiting for messages to be delivered. Handlers will then be invoked on that background thread. This is a
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more difficult style of programming that can be used to increase the realism of the unit tests by ensuring multiple
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nodes run in parallel, just as they would on a real network spread over multiple machines.
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417
docs/build/html/_sources/protocol-state-machines.txt
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417
docs/build/html/_sources/protocol-state-machines.txt
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@ -0,0 +1,417 @@
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.. highlight:: kotlin
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.. raw:: html
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<script type="text/javascript" src="_static/jquery.js"></script>
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<script type="text/javascript" src="_static/codesets.js"></script>
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Protocol state machines
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=======================
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This article explains our experimental approach to modelling financial protocols in code. It explains how the
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platform's state machine framework is used, and takes you through the code for a simple 2-party asset trading protocol
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which is included in the source.
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Introduction
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------------
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Shared distributed ledgers are interesting because they allow many different, mutually distrusting parties to
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share a single source of truth about the ownership of assets. Digitally signed transactions are used to update that
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shared ledger, and transactions may alter many states simultaneously and atomically.
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Blockchain systems such as Bitcoin support the idea of building up a finished, signed transaction by passing around
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partially signed invalid transactions outside of the main network, and by doing this you can implement
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*delivery versus payment* such that there is no chance of settlement failure, because the movement of cash and the
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traded asset are performed atomically by the same transaction. To perform such a trade involves a multi-step protocol
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in which messages are passed back and forth privately between parties, checked, signed and so on.
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Despite how useful these protocols are, platforms such as Bitcoin and Ethereum do not assist the developer with the rather
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tricky task of actually building them. That is unfortunate. There are many awkward problems in their implementation
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that a good platform would take care of for you, problems like:
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* Avoiding "callback hell" in which code that should ideally be sequential is turned into an unreadable mess due to the
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desire to avoid using up a thread for every protocol instantiation.
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* Surviving node shutdowns/restarts that may occur in the middle of the protocol without complicating things. This
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implies that the state of the protocol must be persisted to disk.
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* Error handling.
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* Message routing.
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* Serialisation.
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* Catching type errors, in which the developer gets temporarily confused and expects to receive/send one type of message
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when actually they need to receive/send another.
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* Unit testing of the finished protocol.
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Actor frameworks can solve some of the above but they are often tightly bound to a particular messaging layer, and
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we would like to keep a clean separation. Additionally, they are typically not type safe, and don't make persistence or
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writing sequential code much easier.
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To put these problems in perspective the *payment channel protocol* in the bitcoinj library, which allows bitcoins to
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be temporarily moved off-chain and traded at high speed between two parties in private, consists of about 7000 lines of
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Java and took over a month of full time work to develop. Most of that code is concerned with the details of persistence,
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message passing, lifecycle management, error handling and callback management. Because the business logic is quite
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spread out the code can be difficult to read and debug.
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As small contract-specific trading protocols are a common occurence in finance, we provide a framework for the
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construction of them that automatically handles many of the concerns outlined above.
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Theory
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------
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A *continuation* is a suspended stack frame stored in a regular object that can be passed around, serialised,
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unserialised and resumed from where it was suspended. This may sound abstract but don't worry, the examples below
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will make it clearer. The JVM does not natively support continuations, so we implement them using a a library called
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JavaFlow which works through behind-the-scenes bytecode rewriting. You don't have to know how this works to benefit
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from it, however.
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We use continuations for the following reasons:
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* It allows us to write code that is free of callbacks, that looks like ordinary sequential code.
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* A suspended continuation takes far less memory than a suspended thread. It can be as low as a few hundred bytes.
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In contrast a suspended Java stack can easily be 1mb in size.
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* It frees the developer from thinking (much) about persistence and serialisation.
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A *state machine* is a piece of code that moves through various *states*. These are not the same as states in the data
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model (that represent facts about the world on the ledger), but rather indicate different stages in the progression
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of a multi-stage protocol. Typically writing a state machine would require the use of a big switch statement and some
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explicit variables to keep track of where you're up to. The use of continuations avoids this hassle.
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A two party trading protocol
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----------------------------
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We would like to implement the "hello world" of shared transaction building protocols: a seller wishes to sell some
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*asset* (e.g. some commercial paper) in return for *cash*. The buyer wishes to purchase the asset using his cash. They
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want the trade to be atomic so neither side is exposed to the risk of settlement failure. We assume that the buyer
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and seller have found each other and arranged the details on some exchange, or over the counter. The details of how
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the trade is arranged isn't covered in this article.
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Our protocol has two parties (B and S for buyer and seller) and will proceed as follows:
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1. S sends a ``StateAndRef`` pointing to the state they want to sell to B, along with info about the price they require
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B to pay.
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2. B sends to S a ``SignedWireTransaction`` that includes the state as input, B's cash as input, the state with the new
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owner key as output, and any change cash as output. It contains a single signature from B but isn't valid because
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it lacks a signature from S authorising movement of the asset.
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3. S signs it and hands the now finalised ``SignedWireTransaction`` back to B.
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You can find the implementation of this protocol in the file ``contracts/protocols/TwoPartyTradeProtocol.kt``.
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Assuming no malicious termination, they both end the protocol being in posession of a valid, signed transaction that
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represents an atomic asset swap.
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Note that it's the *seller* who initiates contact with the buyer, not vice-versa as you might imagine.
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We start by defining an abstract base class to encapsulate the protocol. This is what code that invokes the protocol
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will see:
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.. container:: codeset
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.. sourcecode:: kotlin
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abstract class TwoPartyTradeProtocol {
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class SellerInitialArgs(
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val assetToSell: StateAndRef<OwnableState>,
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val price: Amount,
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val myKeyPair: KeyPair,
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val buyerSessionID: Long
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)
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abstract fun runSeller(otherSide: SingleMessageRecipient, args: SellerInitialArgs): Seller
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class BuyerInitialArgs(
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val acceptablePrice: Amount,
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val typeToBuy: Class<out OwnableState>,
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val sessionID: Long
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)
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abstract fun runBuyer(otherSide: SingleMessageRecipient, args: BuyerInitialArgs): Buyer
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abstract class Buyer : ProtocolStateMachine<BuyerInitialArgs, Pair<TimestampedWireTransaction, LedgerTransaction>>()
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abstract class Seller : ProtocolStateMachine<SellerInitialArgs, Pair<TimestampedWireTransaction, LedgerTransaction>>()
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companion object {
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@JvmStatic fun create(smm: StateMachineManager): TwoPartyTradeProtocol {
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return TwoPartyTradeProtocolImpl(smm)
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}
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}
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}
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Let's unpack what this code does:
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- It defines a several classes nested inside the main ``TwoPartyTradeProtocol`` class, and a couple of methods, one to
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run the buyer side of the protocol and one to run the seller side.
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- Two of the classes are simply wrappers for parameters to the trade; things like what is being sold, what the price
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of the asset is, how much the buyer is willing to pay and so on. The ``myKeyPair`` field is simply the public key
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that the seller wishes the buyer to send the cash to. The session ID field is sent from buyer to seller when the
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trade is being set up and is just a big random number. It's used to keep messages separated on the network, and stop
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malicious entities trying to interfere with the message stream.
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- The other two classes define empty abstract classes called ``Buyer`` and ``Seller``. These inherit from a class
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called ``ProtocolStateMachine`` and provide two type parameters: the arguments class we just defined for each side
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and the type of the object that the protocol finally produces (this doesn't have to be identical for each side, even
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though in this case it is).
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- Finally it simply defines a static method that creates an instance of an object that inherits from this base class
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and returns it, with a ``StateMachineManager`` as an instance. The Impl class will be defined below.
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Alright, so using this protocol shouldn't be too hard: in the simplest case we can just pass in the details of the trade
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to either runBuyer or runSeller, depending on who we are, and then call ``.get()`` on the resulting future to block the
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calling thread until the protocol has finished. Or we could register a callback on the returned future that will be
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invoked when it's done, where we could e.g. update a user interface.
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The only tricky part is how to get one of these things. We need a ``StateMachineManager``. Where does that come from
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and why do we need one?
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The state machine manager
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-------------------------
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The SMM is a class responsible for taking care of all running protocols in a node. It knows how to register handlers
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with a ``MessagingService`` and iterate the right state machine when the time comes. It provides the
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send/receive/sendAndReceive calls that let the code request network interaction and it will store a serialised copy of
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each state machine before it's suspended to wait for the network.
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To get a ``StateMachineManager``, you currently have to build one by passing in a ``ServiceHub`` and a thread or thread
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pool which it can use. This will change in future so don't worry about the details of this too much: just check the
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unit tests to see how it's done.
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Implementing the seller
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-----------------------
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.. container:: codeset
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.. sourcecode:: kotlin
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private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) : TwoPartyTradeProtocol() {
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companion object {
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val TRADE_TOPIC = "com.r3cev.protocols.trade"
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fun makeSessionID() = Math.abs(SecureRandom.getInstanceStrong().nextLong())
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}
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class SellerImpl : Seller() {
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override fun call(args: SellerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
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TODO()
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}
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}
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class BuyerImpl : Buyer() {
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override fun call(args: BuyerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
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TODO()
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}
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}
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override fun runSeller(otherSide: SingleMessageRecipient, args: SellerInitialArgs): Seller {
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return smm.add(otherSide, args, "$TRADE_TOPIC.seller", SellerImpl::class.java)
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}
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override fun runBuyer(otherSide: SingleMessageRecipient, args: BuyerInitialArgs): Buyer {
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return smm.add(otherSide, args, "$TRADE_TOPIC.buyer", BuyerImpl::class.java)
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}
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}
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We start with a skeleton on which we will build the protocol. Putting things in a *companion object* in Kotlin is like
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declaring them as static members in Java. Here, we define a "topic" that will identify trade related messages that
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arrive at a node (see :doc:`messaging` for details), and a convenience function to pick a large random session ID.
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.. note:: Session IDs keep different traffic streams separated, so for security they must be large and random enough
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to be unguessable. 63 bits is good enough.
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The runSeller and runBuyer methods simply start the state machines, passing in a reference to the classes and the topics
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each side will use.
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Now let's try implementing the seller side. Firstly, we're going to need a message to send to the buyer describing what
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we want to trade. Remember: this data comes from whatever system was used to find the trading partner to begin with.
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It could be as simple as a chat room or as complex as a 24/7 exchange.
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.. container:: codeset
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.. sourcecode:: kotlin
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// This object is serialised to the network and is the first protocol message the seller sends to the buyer.
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class SellerTradeInfo(
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val assetForSale: StateAndRef<OwnableState>,
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val price: Amount,
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val sellerOwnerKey: PublicKey,
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val buyerSessionID: Long
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)
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That's simple enough: our opening protocol message will be serialised before being sent over the wire, and it contains
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the details that were agreed so we can double check them. It also contains a session ID so we can identify this
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trade's messages, and a pointer to where the asset that is being sold can be found on the ledger.
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Next we add some code to the ``SellerImpl.call`` method:
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.. container:: codeset
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.. sourcecode:: kotlin
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val sessionID = makeSessionID()
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// Make the first message we'll send to kick off the protocol.
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val hello = SellerTradeInfo(args.assetToSell, args.price, args.myKeyPair.public, sessionID)
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// Zero is a special session ID that is being listened to by the buyer (i.e. before a session is started).
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val partialTX = sendAndReceive<SignedWireTransaction>(TRADE_TOPIC, args.buyerSessionID, sessionID, hello)
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logger().trace { "Received partially signed transaction" }
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That's pretty straight forward. We generate a session ID to identify what's happening on the seller side, fill out
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the initial protocol message, and then call ``sendAndReceive``. This function takes a few arguments:
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- A type argument, which is the object we're expecting to receive from the other side.
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- The topic string that ensures the message is routed to the right bit of code in the other side's node.
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- The session IDs that ensure the messages don't get mixed up with other simultaneous trades.
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- And finally, the thing to send. It'll be serialised and sent automatically.
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Once sendAndReceive is called, the call method will be suspended into a continuation. When it gets back we'll do a log
|
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message. The buyer is supposed to send us a transaction with all the right inputs/outputs/commands in return, with their
|
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cash put into the transaction and their signature on it authorising the movement of the cash.
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.. note:: There are a few rules you need to bear in mind when writing a class that will be used as a continuation.
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The first is that anything on the stack when the function is suspended will be stored into the heap and kept alive by
|
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the garbage collector. So try to avoid keeping enormous data structures alive unless you really have to.
|
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|
||||
The second is that as well as being kept on the heap, objects reachable from the stack will be serialised. The state
|
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of the function call may be resurrected much later! Kryo doesn't require objects be marked as serialisable, but even so,
|
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doing things like creating threads from inside these calls would be a bad idea. They should only contain business
|
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logic.
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|
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The third rule to bear in mind is that you can't declare variables or methods in these classes and access
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them from outside of the class, due to the bytecode rewriting and classloader tricks that are used to make this all
|
||||
work. If you want access to something inside the BuyerImpl or SellerImpl classes, you must define a super-interface
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or super-class (like ``Buyer``/``Seller``) and put what you want to access there.
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OK, let's keep going:
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.. container:: codeset
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||||
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.. sourcecode:: kotlin
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partialTX.verifySignatures()
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val wtx = partialTX.txBits.deserialize<WireTransaction>()
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requireThat {
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"transaction sends us the right amount of cash" by (wtx.outputStates.sumCashBy(args.myKeyPair.public) == args.price)
|
||||
// There are all sorts of funny games a malicious secondary might play here, we should fix them:
|
||||
//
|
||||
// - This tx may attempt to send some assets we aren't intending to sell to the secondary, if
|
||||
// we're reusing keys! So don't reuse keys!
|
||||
// - This tx may not be valid according to the contracts of the input states, so we must resolve
|
||||
// and fully audit the transaction chains to convince ourselves that it is actually valid.
|
||||
// - This tx may include output states that impose odd conditions on the movement of the cash,
|
||||
// once we implement state pairing.
|
||||
//
|
||||
// but the goal of this code is not to be fully secure, but rather, just to find good ways to
|
||||
// express protocol state machines on top of the messaging layer.
|
||||
}
|
||||
|
||||
val ourSignature = args.myKeyPair.signWithECDSA(partialTX.txBits.bits)
|
||||
val fullySigned: SignedWireTransaction = partialTX.copy(sigs = partialTX.sigs + ourSignature)
|
||||
// We should run it through our full TransactionGroup of all transactions here.
|
||||
fullySigned.verify()
|
||||
val timestamped: TimestampedWireTransaction = fullySigned.toTimestampedTransaction(serviceHub.timestampingService)
|
||||
logger().trace { "Built finished transaction, sending back to secondary!" }
|
||||
|
||||
send(TRADE_TOPIC, sessionID, timestamped)
|
||||
|
||||
return Pair(timestamped, timestamped.verifyToLedgerTransaction(serviceHub.timestampingService, serviceHub.identityService))
|
||||
|
||||
Here, we see some assertions and signature checking to satisfy ourselves that we're not about to sign something
|
||||
incorrect. Once we're happy, we calculate a signature over the transaction to authorise the movement of the asset
|
||||
we are selling, and then we verify things to make sure it's all OK. Finally, we request timestamping of the
|
||||
transaction, and send the now finalised and validated transaction back to the buyer.
|
||||
|
||||
.. warning:: This code is **not secure**. Other than not checking for all possible invalid constructions, if the
|
||||
seller stops before sending the finalised transaction to the buyer, the seller is left with a valid transaction
|
||||
but the buyer isn't, so they can't spend the asset they just purchased! This sort of thing will be fixed in a
|
||||
future version of the code.
|
||||
|
||||
Finally, the call function returns with the result of the protocol: in our case, the final transaction in two different
|
||||
forms.
|
||||
|
||||
Implementing the buyer
|
||||
----------------------
|
||||
|
||||
OK, let's do the same for the buyer side:
|
||||
|
||||
.. container:: codeset
|
||||
|
||||
.. sourcecode:: kotlin
|
||||
|
||||
class BuyerImpl : Buyer() {
|
||||
override fun call(args: BuyerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
|
||||
// Wait for a trade request to come in on our pre-provided session ID.
|
||||
val tradeRequest = receive<SellerTradeInfo>(TRADE_TOPIC, args.sessionID)
|
||||
|
||||
// What is the seller trying to sell us?
|
||||
val assetTypeName = tradeRequest.assetForSale.state.javaClass.name
|
||||
logger().trace { "Got trade request for a $assetTypeName" }
|
||||
|
||||
// Check the start message for acceptability.
|
||||
check(tradeRequest.sessionID > 0)
|
||||
if (tradeRequest.price > args.acceptablePrice)
|
||||
throw UnacceptablePriceException(tradeRequest.price)
|
||||
if (!args.typeToBuy.isInstance(tradeRequest.assetForSale.state))
|
||||
throw AssetMismatchException(args.typeToBuy.name, assetTypeName)
|
||||
|
||||
// TODO: Either look up the stateref here in our local db, or accept a long chain of states and
|
||||
// validate them to audit the other side and ensure it actually owns the state we are being offered!
|
||||
// For now, just assume validity!
|
||||
|
||||
// Generate the shared transaction that both sides will sign, using the data we have.
|
||||
val ptx = PartialTransaction()
|
||||
// Add input and output states for the movement of cash, by using the Cash contract to generate the states.
|
||||
val wallet = serviceHub.walletService.currentWallet
|
||||
val cashStates = wallet.statesOfType<Cash.State>()
|
||||
val cashSigningPubKeys = Cash().craftSpend(ptx, tradeRequest.price, tradeRequest.sellerOwnerKey, cashStates)
|
||||
// Add inputs/outputs/a command for the movement of the asset.
|
||||
ptx.addInputState(tradeRequest.assetForSale.ref)
|
||||
// Just pick some new public key for now.
|
||||
val freshKey = serviceHub.keyManagementService.freshKey()
|
||||
val (command, state) = tradeRequest.assetForSale.state.withNewOwner(freshKey.public)
|
||||
ptx.addOutputState(state)
|
||||
ptx.addArg(WireCommand(command, tradeRequest.assetForSale.state.owner))
|
||||
|
||||
// Now sign the transaction with whatever keys we need to move the cash.
|
||||
for (k in cashSigningPubKeys) {
|
||||
val priv = serviceHub.keyManagementService.toPrivate(k)
|
||||
ptx.signWith(KeyPair(k, priv))
|
||||
}
|
||||
|
||||
val stx = ptx.toSignedTransaction(checkSufficientSignatures = false)
|
||||
stx.verifySignatures() // Verifies that we generated a signed transaction correctly.
|
||||
|
||||
// TODO: Could run verify() here to make sure the only signature missing is the sellers.
|
||||
|
||||
logger().trace { "Sending partially signed transaction to seller" }
|
||||
|
||||
// TODO: Protect against the buyer terminating here and leaving us in the lurch without the final tx.
|
||||
// TODO: Protect against a malicious buyer sending us back a different transaction to the one we built.
|
||||
val fullySigned = sendAndReceive<TimestampedWireTransaction>(TRADE_TOPIC,
|
||||
tradeRequest.sessionID, args.sessionID, stx)
|
||||
|
||||
logger().trace { "Got fully signed transaction, verifying ... "}
|
||||
|
||||
val ltx = fullySigned.verifyToLedgerTransaction(serviceHub.timestampingService, serviceHub.identityService)
|
||||
|
||||
logger().trace { "Fully signed transaction was valid. Trade complete! :-)" }
|
||||
|
||||
return Pair(fullySigned, ltx)
|
||||
}
|
||||
}
|
||||
|
||||
This code is fairly straightforward. Here are some things to pay attention to:
|
||||
|
||||
1. We do some sanity checking on the received message to ensure we're being offered what we expected to be offered.
|
||||
2. We create a cash spend in the normal way, by using ``Cash().craftSpend``.
|
||||
3. We access the *service hub* when we need it to access things that are transient and may change or be recreated
|
||||
whilst a protocol is suspended, things like the wallet or the timestamping service. Remember that a protocol may
|
||||
be suspended when it waits to receive a message across node or computer restarts, so objects representing a service
|
||||
or data which may frequently change should be accessed 'just in time'.
|
||||
4. Finally, we send the unfinsished, invalid transaction to the seller so they can sign it. They are expected to send
|
||||
back to us a ``TimestampedWireTransaction``, which once we verify it, should be the final outcome of the trade.
|
||||
|
||||
As you can see, the protocol logic is straightforward and does not contain any callbacks or network glue code, despite
|
||||
the fact that it takes minimal resources and can survive node restarts.
|
||||
|
||||
.. warning:: When accessing things via the ``serviceHub`` field, avoid the temptation to stuff a reference into a local variable.
|
||||
If you do this then next time your protocol waits to receive an object, the system will try and serialise all your
|
||||
local variables and end up trying to serialise, e.g. the timestamping service, which doesn't make any conceptual
|
||||
sense. The ``serviceHub`` field is defined by the ``ProtocolStateMachine`` superclass and is marked transient so
|
||||
this problem doesn't occur. It's also restored for you after a protocol state machine is restored after a node
|
||||
restart.
|
||||
|
||||
2
docs/build/html/genindex.html
vendored
2
docs/build/html/genindex.html
vendored
@ -85,6 +85,8 @@
|
||||
<li class="toctree-l1"><a class="reference internal" href="overview.html">Overview</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="getting-set-up.html">Getting set up</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="tutorial.html">Tutorial</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="messaging.html">Networking and messaging</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="protocol-state-machines.html">Protocol state machines</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="visualiser.html">Using the visualiser</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="roadmap.html">Roadmap</a></li>
|
||||
</ul>
|
||||
|
||||
2
docs/build/html/getting-set-up.html
vendored
2
docs/build/html/getting-set-up.html
vendored
@ -90,6 +90,8 @@
|
||||
</ul>
|
||||
</li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="tutorial.html">Tutorial</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="messaging.html">Networking and messaging</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="protocol-state-machines.html">Protocol state machines</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="visualiser.html">Using the visualiser</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="roadmap.html">Roadmap</a></li>
|
||||
</ul>
|
||||
|
||||
17
docs/build/html/index.html
vendored
17
docs/build/html/index.html
vendored
@ -85,6 +85,8 @@
|
||||
<li class="toctree-l1"><a class="reference internal" href="overview.html">Overview</a></li>
|
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<li class="toctree-l1"><a class="reference internal" href="getting-set-up.html">Getting set up</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="tutorial.html">Tutorial</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="messaging.html">Networking and messaging</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="protocol-state-machines.html">Protocol state machines</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="visualiser.html">Using the visualiser</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="roadmap.html">Roadmap</a></li>
|
||||
</ul>
|
||||
@ -172,6 +174,21 @@ prove or disprove the following hypothesis:</p>
|
||||
<li class="toctree-l2"><a class="reference internal" href="tutorial.html#non-asset-oriented-based-smart-contracts">Non-asset-oriented based smart contracts</a></li>
|
||||
</ul>
|
||||
</li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="messaging.html">Networking and messaging</a><ul>
|
||||
<li class="toctree-l2"><a class="reference internal" href="messaging.html#messaging-vs-networking">Messaging vs networking</a></li>
|
||||
<li class="toctree-l2"><a class="reference internal" href="messaging.html#interfaces">Interfaces</a></li>
|
||||
<li class="toctree-l2"><a class="reference internal" href="messaging.html#in-memory-implementation">In memory implementation</a></li>
|
||||
</ul>
|
||||
</li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="protocol-state-machines.html">Protocol state machines</a><ul>
|
||||
<li class="toctree-l2"><a class="reference internal" href="protocol-state-machines.html#introduction">Introduction</a></li>
|
||||
<li class="toctree-l2"><a class="reference internal" href="protocol-state-machines.html#theory">Theory</a></li>
|
||||
<li class="toctree-l2"><a class="reference internal" href="protocol-state-machines.html#a-two-party-trading-protocol">A two party trading protocol</a></li>
|
||||
<li class="toctree-l2"><a class="reference internal" href="protocol-state-machines.html#the-state-machine-manager">The state machine manager</a></li>
|
||||
<li class="toctree-l2"><a class="reference internal" href="protocol-state-machines.html#implementing-the-seller">Implementing the seller</a></li>
|
||||
<li class="toctree-l2"><a class="reference internal" href="protocol-state-machines.html#implementing-the-buyer">Implementing the buyer</a></li>
|
||||
</ul>
|
||||
</li>
|
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<li class="toctree-l1"><a class="reference internal" href="visualiser.html">Using the visualiser</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="roadmap.html">Roadmap</a></li>
|
||||
</ul>
|
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|
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2
docs/build/html/inthebox.html
vendored
2
docs/build/html/inthebox.html
vendored
@ -86,6 +86,8 @@
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<li class="toctree-l1"><a class="reference internal" href="overview.html">Overview</a></li>
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<li class="toctree-l1"><a class="reference internal" href="getting-set-up.html">Getting set up</a></li>
|
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<li class="toctree-l1"><a class="reference internal" href="tutorial.html">Tutorial</a></li>
|
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<li class="toctree-l1"><a class="reference internal" href="messaging.html">Networking and messaging</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="protocol-state-machines.html">Protocol state machines</a></li>
|
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<li class="toctree-l1"><a class="reference internal" href="visualiser.html">Using the visualiser</a></li>
|
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<li class="toctree-l1"><a class="reference internal" href="roadmap.html">Roadmap</a></li>
|
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</ul>
|
||||
|
||||
295
docs/build/html/messaging.html
vendored
Normal file
295
docs/build/html/messaging.html
vendored
Normal file
@ -0,0 +1,295 @@
|
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|
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|
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<!DOCTYPE html>
|
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<li class="toctree-l1"><a class="reference internal" href="tutorial.html">Tutorial</a></li>
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<li class="toctree-l1 current"><a class="current reference internal" href="">Networking and messaging</a><ul>
|
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<li class="toctree-l2"><a class="reference internal" href="#messaging-vs-networking">Messaging vs networking</a></li>
|
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<li class="toctree-l2"><a class="reference internal" href="#interfaces">Interfaces</a></li>
|
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<li class="toctree-l2"><a class="reference internal" href="#in-memory-implementation">In memory implementation</a></li>
|
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<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
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|
||||
<div class="section" id="networking-and-messaging">
|
||||
<h1>Networking and messaging<a class="headerlink" href="#networking-and-messaging" title="Permalink to this headline">¶</a></h1>
|
||||
<p>Although the platform does not currently provide a network backend, some preliminary interfaces are defined along with
|
||||
an in-memory implementation provided for use by unit tests and other exploratory code. An implementation based on Apache
|
||||
Kafka is also being developed, which should be sufficient for real use cases to be implemented in the short run, even
|
||||
though in the long run a fully peer to peer protocol will be required.</p>
|
||||
<p>This article quickly explains the basic networking interfaces in the code.</p>
|
||||
<div class="section" id="messaging-vs-networking">
|
||||
<h2>Messaging vs networking<a class="headerlink" href="#messaging-vs-networking" title="Permalink to this headline">¶</a></h2>
|
||||
<p>It is important to understand that the code expects any networking module to provide the following services:</p>
|
||||
<ul class="simple">
|
||||
<li>Persistent, reliable and secure delivery of complete messages. The module is expected to retry delivery if initial
|
||||
attempts fail.</li>
|
||||
<li>Ability to send messages both 1:1 and 1:many, where ‘many’ may mean the entire group of network users.</li>
|
||||
</ul>
|
||||
<p>The details of how this is achieved are not exposed to the rest of the code.</p>
|
||||
</div>
|
||||
<div class="section" id="interfaces">
|
||||
<h2>Interfaces<a class="headerlink" href="#interfaces" title="Permalink to this headline">¶</a></h2>
|
||||
<p>The most important interface is called <code class="docutils literal"><span class="pre">MessagingService</span></code> and is defined in the <code class="docutils literal"><span class="pre">core/messaging/Messaging.kt</span></code> file.
|
||||
It declares an interface with the following operations:</p>
|
||||
<ul class="simple">
|
||||
<li><code class="docutils literal"><span class="pre">addMessageHandler(topic:</span> <span class="pre">String,</span> <span class="pre">executor:</span> <span class="pre">Executor,</span> <span class="pre">callback:</span> <span class="pre">(Message,</span> <span class="pre">MessageHandlerRegistration)</span> <span class="pre">-></span> <span class="pre">Unit)</span></code></li>
|
||||
<li><code class="docutils literal"><span class="pre">createMessage(topic:</span> <span class="pre">String,</span> <span class="pre">data:</span> <span class="pre">ByteArray):</span> <span class="pre">Message</span></code></li>
|
||||
<li><code class="docutils literal"><span class="pre">send(message:</span> <span class="pre">Message,</span> <span class="pre">targetRecipients:</span> <span class="pre">MessageRecipients)</span></code></li>
|
||||
<li><code class="docutils literal"><span class="pre">stop()</span></code></li>
|
||||
</ul>
|
||||
<p>along with a few misc others that are not important enough to discuss here.</p>
|
||||
<p>A <em>topic</em> is simply a string that identifies the kind of message that is being sent. When a message is received, the
|
||||
topic is compared exactly to the list of registered message handlers and if it matches, the callback is invoked.
|
||||
Adding a handler returns a <code class="docutils literal"><span class="pre">MessageHandlerRegistration</span></code> object that can be used to remove the handler, and that
|
||||
registration object is also passed to each invocation to simplify the case where a handler wishes to remove itself.</p>
|
||||
<p>Some helper functions are also provided that simplify the process of sending a message by using Kryo serialisation, and
|
||||
registering one-shot handlers that remove themselves once they finished running, but those don’t need to be implemented
|
||||
by network module authors themselves.</p>
|
||||
<p>Destinations are represented using opaque classes (i.e. their contents are defined by the implementation). The
|
||||
<code class="docutils literal"><span class="pre">MessageRecipients</span></code> interface represents any possible set of recipients: it’s used when a piece of code doesn’t
|
||||
care who is going to get a message, just that someone does. The <code class="docutils literal"><span class="pre">SingleMessageRecipient</span></code> interface inherits from
|
||||
<code class="docutils literal"><span class="pre">MessageRecipients</span></code> and represents a handle to some specific individual receiver on the network. Whether they are
|
||||
identified by IP address, public key, message router ID or some other kind of address is not exposed at this level.
|
||||
<code class="docutils literal"><span class="pre">MessageRecipientGroup</span></code> is not used anywhere at the moment but represents multiple simultaneous recipients. And
|
||||
finally <code class="docutils literal"><span class="pre">AllPossibleRecipients</span></code> is used for network wide broadcast. It’s also unused right now, outside of unit tests.</p>
|
||||
</div>
|
||||
<div class="section" id="in-memory-implementation">
|
||||
<h2>In memory implementation<a class="headerlink" href="#in-memory-implementation" title="Permalink to this headline">¶</a></h2>
|
||||
<p>To ease unit testing of business logic, a simple in-memory messaging service is provided. To access this you can inherit
|
||||
your test case class from the <code class="docutils literal"><span class="pre">TestWithInMemoryNetwork</span></code> class. This provides a few utility methods to help test
|
||||
code that involves message passing.</p>
|
||||
<p>You can run a mock network session in one of two modes:</p>
|
||||
<ul class="simple">
|
||||
<li>Manually “pumped”</li>
|
||||
<li>Automatically pumped with background threads</li>
|
||||
</ul>
|
||||
<p>“Pumping” is the act of telling a mock network node to pop a message off its queue and process it. Typically you want
|
||||
unit tests to be fast, repeatable and you want to be able to insert your own changes into the middle of any given
|
||||
message sequence. This is what the manual mode is for. In this mode, all logic runs on the same thread (the thread
|
||||
running the unit tests). You can create and use a node like this:</p>
|
||||
<div class="codeset container">
|
||||
<div class="highlight-kotlin"><div class="highlight"><pre>val (aliceAddr, aliceNode) = makeNode(inBackground = false)
|
||||
val (bobAddr, bobNode) = makeNode(false)
|
||||
|
||||
aliceNode.send("test.topic", aliceAddr, "foo")
|
||||
bobNode.pump(blocking = false)
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<div class="admonition note">
|
||||
<p class="first admonition-title">Note</p>
|
||||
<p class="last">Currently only Kotlin examples are available for networking and protocol state machines. Java examples may
|
||||
follow later. Naming arguments in Kotlin like above is optional but sometimes useful to make code examples clearer.</p>
|
||||
</div>
|
||||
<p>The above code won’t actually do anything because no message handler is registered for “test.topic” so the message will
|
||||
go into a holding area. If/when we add a handler that can accept test.topic, the message will be delivered then.</p>
|
||||
<p>Sometimes you don’t want to have to call the pump method over and over again. You can use the <code class="docutils literal"><span class="pre">runNetwork</span> <span class="pre">{</span> <span class="pre">..</span> <span class="pre">}</span></code>
|
||||
construct to fix this: any code inside the block will be run, and then all nodes you created will be pumped over and
|
||||
over until all of them have reported that they have no work left to do. This means any ping-pongs of messages will
|
||||
be run until everything settles.</p>
|
||||
<p>You can see more examples of how to use this in the file <code class="docutils literal"><span class="pre">InMemoryMessagingTests.kt</span></code>.</p>
|
||||
<p>If you specify <code class="docutils literal"><span class="pre">inBackground</span> <span class="pre">=</span> <span class="pre">true</span></code> to <code class="docutils literal"><span class="pre">makeNode</span></code> then each node will create its own background thread which will
|
||||
sit around waiting for messages to be delivered. Handlers will then be invoked on that background thread. This is a
|
||||
more difficult style of programming that can be used to increase the realism of the unit tests by ensuring multiple
|
||||
nodes run in parallel, just as they would on a real network spread over multiple machines.</p>
|
||||
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|
||||
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<li class="toctree-l1 current"><a class="current reference internal" href="">Protocol state machines</a><ul>
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<li class="toctree-l2"><a class="reference internal" href="#introduction">Introduction</a></li>
|
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<li class="toctree-l2"><a class="reference internal" href="#theory">Theory</a></li>
|
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<li class="toctree-l2"><a class="reference internal" href="#a-two-party-trading-protocol">A two party trading protocol</a></li>
|
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<li class="toctree-l2"><a class="reference internal" href="#the-state-machine-manager">The state machine manager</a></li>
|
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<li class="toctree-l2"><a class="reference internal" href="#implementing-the-seller">Implementing the seller</a></li>
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<li class="toctree-l2"><a class="reference internal" href="#implementing-the-buyer">Implementing the buyer</a></li>
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<script type="text/javascript" src="_static/jquery.js"></script>
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<script type="text/javascript" src="_static/codesets.js"></script><div class="section" id="protocol-state-machines">
|
||||
<h1>Protocol state machines<a class="headerlink" href="#protocol-state-machines" title="Permalink to this headline">¶</a></h1>
|
||||
<p>This article explains our experimental approach to modelling financial protocols in code. It explains how the
|
||||
platform’s state machine framework is used, and takes you through the code for a simple 2-party asset trading protocol
|
||||
which is included in the source.</p>
|
||||
<div class="section" id="introduction">
|
||||
<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
|
||||
<p>Shared distributed ledgers are interesting because they allow many different, mutually distrusting parties to
|
||||
share a single source of truth about the ownership of assets. Digitally signed transactions are used to update that
|
||||
shared ledger, and transactions may alter many states simultaneously and atomically.</p>
|
||||
<p>Blockchain systems such as Bitcoin support the idea of building up a finished, signed transaction by passing around
|
||||
partially signed invalid transactions outside of the main network, and by doing this you can implement
|
||||
<em>delivery versus payment</em> such that there is no chance of settlement failure, because the movement of cash and the
|
||||
traded asset are performed atomically by the same transaction. To perform such a trade involves a multi-step protocol
|
||||
in which messages are passed back and forth privately between parties, checked, signed and so on.</p>
|
||||
<p>Despite how useful these protocols are, platforms such as Bitcoin and Ethereum do not assist the developer with the rather
|
||||
tricky task of actually building them. That is unfortunate. There are many awkward problems in their implementation
|
||||
that a good platform would take care of for you, problems like:</p>
|
||||
<ul class="simple">
|
||||
<li>Avoiding “callback hell” in which code that should ideally be sequential is turned into an unreadable mess due to the
|
||||
desire to avoid using up a thread for every protocol instantiation.</li>
|
||||
<li>Surviving node shutdowns/restarts that may occur in the middle of the protocol without complicating things. This
|
||||
implies that the state of the protocol must be persisted to disk.</li>
|
||||
<li>Error handling.</li>
|
||||
<li>Message routing.</li>
|
||||
<li>Serialisation.</li>
|
||||
<li>Catching type errors, in which the developer gets temporarily confused and expects to receive/send one type of message
|
||||
when actually they need to receive/send another.</li>
|
||||
<li>Unit testing of the finished protocol.</li>
|
||||
</ul>
|
||||
<p>Actor frameworks can solve some of the above but they are often tightly bound to a particular messaging layer, and
|
||||
we would like to keep a clean separation. Additionally, they are typically not type safe, and don’t make persistence or
|
||||
writing sequential code much easier.</p>
|
||||
<p>To put these problems in perspective the <em>payment channel protocol</em> in the bitcoinj library, which allows bitcoins to
|
||||
be temporarily moved off-chain and traded at high speed between two parties in private, consists of about 7000 lines of
|
||||
Java and took over a month of full time work to develop. Most of that code is concerned with the details of persistence,
|
||||
message passing, lifecycle management, error handling and callback management. Because the business logic is quite
|
||||
spread out the code can be difficult to read and debug.</p>
|
||||
<p>As small contract-specific trading protocols are a common occurence in finance, we provide a framework for the
|
||||
construction of them that automatically handles many of the concerns outlined above.</p>
|
||||
</div>
|
||||
<div class="section" id="theory">
|
||||
<h2>Theory<a class="headerlink" href="#theory" title="Permalink to this headline">¶</a></h2>
|
||||
<p>A <em>continuation</em> is a suspended stack frame stored in a regular object that can be passed around, serialised,
|
||||
unserialised and resumed from where it was suspended. This may sound abstract but don’t worry, the examples below
|
||||
will make it clearer. The JVM does not natively support continuations, so we implement them using a a library called
|
||||
JavaFlow which works through behind-the-scenes bytecode rewriting. You don’t have to know how this works to benefit
|
||||
from it, however.</p>
|
||||
<p>We use continuations for the following reasons:</p>
|
||||
<ul class="simple">
|
||||
<li>It allows us to write code that is free of callbacks, that looks like ordinary sequential code.</li>
|
||||
<li>A suspended continuation takes far less memory than a suspended thread. It can be as low as a few hundred bytes.
|
||||
In contrast a suspended Java stack can easily be 1mb in size.</li>
|
||||
<li>It frees the developer from thinking (much) about persistence and serialisation.</li>
|
||||
</ul>
|
||||
<p>A <em>state machine</em> is a piece of code that moves through various <em>states</em>. These are not the same as states in the data
|
||||
model (that represent facts about the world on the ledger), but rather indicate different stages in the progression
|
||||
of a multi-stage protocol. Typically writing a state machine would require the use of a big switch statement and some
|
||||
explicit variables to keep track of where you’re up to. The use of continuations avoids this hassle.</p>
|
||||
</div>
|
||||
<div class="section" id="a-two-party-trading-protocol">
|
||||
<h2>A two party trading protocol<a class="headerlink" href="#a-two-party-trading-protocol" title="Permalink to this headline">¶</a></h2>
|
||||
<p>We would like to implement the “hello world” of shared transaction building protocols: a seller wishes to sell some
|
||||
<em>asset</em> (e.g. some commercial paper) in return for <em>cash</em>. The buyer wishes to purchase the asset using his cash. They
|
||||
want the trade to be atomic so neither side is exposed to the risk of settlement failure. We assume that the buyer
|
||||
and seller have found each other and arranged the details on some exchange, or over the counter. The details of how
|
||||
the trade is arranged isn’t covered in this article.</p>
|
||||
<p>Our protocol has two parties (B and S for buyer and seller) and will proceed as follows:</p>
|
||||
<ol class="arabic simple">
|
||||
<li>S sends a <code class="docutils literal"><span class="pre">StateAndRef</span></code> pointing to the state they want to sell to B, along with info about the price they require
|
||||
B to pay.</li>
|
||||
<li>B sends to S a <code class="docutils literal"><span class="pre">SignedWireTransaction</span></code> that includes the state as input, B’s cash as input, the state with the new
|
||||
owner key as output, and any change cash as output. It contains a single signature from B but isn’t valid because
|
||||
it lacks a signature from S authorising movement of the asset.</li>
|
||||
<li>S signs it and hands the now finalised <code class="docutils literal"><span class="pre">SignedWireTransaction</span></code> back to B.</li>
|
||||
</ol>
|
||||
<p>You can find the implementation of this protocol in the file <code class="docutils literal"><span class="pre">contracts/protocols/TwoPartyTradeProtocol.kt</span></code>.</p>
|
||||
<p>Assuming no malicious termination, they both end the protocol being in posession of a valid, signed transaction that
|
||||
represents an atomic asset swap.</p>
|
||||
<p>Note that it’s the <em>seller</em> who initiates contact with the buyer, not vice-versa as you might imagine.</p>
|
||||
<p>We start by defining an abstract base class to encapsulate the protocol. This is what code that invokes the protocol
|
||||
will see:</p>
|
||||
<div class="codeset container">
|
||||
<div class="highlight-kotlin"><div class="highlight"><pre>abstract class TwoPartyTradeProtocol {
|
||||
class SellerInitialArgs(
|
||||
val assetToSell: StateAndRef<OwnableState>,
|
||||
val price: Amount,
|
||||
val myKeyPair: KeyPair,
|
||||
val buyerSessionID: Long
|
||||
)
|
||||
|
||||
abstract fun runSeller(otherSide: SingleMessageRecipient, args: SellerInitialArgs): Seller
|
||||
|
||||
class BuyerInitialArgs(
|
||||
val acceptablePrice: Amount,
|
||||
val typeToBuy: Class<out OwnableState>,
|
||||
val sessionID: Long
|
||||
)
|
||||
|
||||
abstract fun runBuyer(otherSide: SingleMessageRecipient, args: BuyerInitialArgs): Buyer
|
||||
|
||||
abstract class Buyer : ProtocolStateMachine<BuyerInitialArgs, Pair<TimestampedWireTransaction, LedgerTransaction>>()
|
||||
abstract class Seller : ProtocolStateMachine<SellerInitialArgs, Pair<TimestampedWireTransaction, LedgerTransaction>>()
|
||||
|
||||
companion object {
|
||||
@JvmStatic fun create(smm: StateMachineManager): TwoPartyTradeProtocol {
|
||||
return TwoPartyTradeProtocolImpl(smm)
|
||||
}
|
||||
}
|
||||
}
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<p>Let’s unpack what this code does:</p>
|
||||
<ul class="simple">
|
||||
<li>It defines a several classes nested inside the main <code class="docutils literal"><span class="pre">TwoPartyTradeProtocol</span></code> class, and a couple of methods, one to
|
||||
run the buyer side of the protocol and one to run the seller side.</li>
|
||||
<li>Two of the classes are simply wrappers for parameters to the trade; things like what is being sold, what the price
|
||||
of the asset is, how much the buyer is willing to pay and so on. The <code class="docutils literal"><span class="pre">myKeyPair</span></code> field is simply the public key
|
||||
that the seller wishes the buyer to send the cash to. The session ID field is sent from buyer to seller when the
|
||||
trade is being set up and is just a big random number. It’s used to keep messages separated on the network, and stop
|
||||
malicious entities trying to interfere with the message stream.</li>
|
||||
<li>The other two classes define empty abstract classes called <code class="docutils literal"><span class="pre">Buyer</span></code> and <code class="docutils literal"><span class="pre">Seller</span></code>. These inherit from a class
|
||||
called <code class="docutils literal"><span class="pre">ProtocolStateMachine</span></code> and provide two type parameters: the arguments class we just defined for each side
|
||||
and the type of the object that the protocol finally produces (this doesn’t have to be identical for each side, even
|
||||
though in this case it is).</li>
|
||||
<li>Finally it simply defines a static method that creates an instance of an object that inherits from this base class
|
||||
and returns it, with a <code class="docutils literal"><span class="pre">StateMachineManager</span></code> as an instance. The Impl class will be defined below.</li>
|
||||
</ul>
|
||||
<p>Alright, so using this protocol shouldn’t be too hard: in the simplest case we can just pass in the details of the trade
|
||||
to either runBuyer or runSeller, depending on who we are, and then call <code class="docutils literal"><span class="pre">.get()</span></code> on the resulting future to block the
|
||||
calling thread until the protocol has finished. Or we could register a callback on the returned future that will be
|
||||
invoked when it’s done, where we could e.g. update a user interface.</p>
|
||||
<p>The only tricky part is how to get one of these things. We need a <code class="docutils literal"><span class="pre">StateMachineManager</span></code>. Where does that come from
|
||||
and why do we need one?</p>
|
||||
</div>
|
||||
<div class="section" id="the-state-machine-manager">
|
||||
<h2>The state machine manager<a class="headerlink" href="#the-state-machine-manager" title="Permalink to this headline">¶</a></h2>
|
||||
<p>The SMM is a class responsible for taking care of all running protocols in a node. It knows how to register handlers
|
||||
with a <code class="docutils literal"><span class="pre">MessagingService</span></code> and iterate the right state machine when the time comes. It provides the
|
||||
send/receive/sendAndReceive calls that let the code request network interaction and it will store a serialised copy of
|
||||
each state machine before it’s suspended to wait for the network.</p>
|
||||
<p>To get a <code class="docutils literal"><span class="pre">StateMachineManager</span></code>, you currently have to build one by passing in a <code class="docutils literal"><span class="pre">ServiceHub</span></code> and a thread or thread
|
||||
pool which it can use. This will change in future so don’t worry about the details of this too much: just check the
|
||||
unit tests to see how it’s done.</p>
|
||||
</div>
|
||||
<div class="section" id="implementing-the-seller">
|
||||
<h2>Implementing the seller<a class="headerlink" href="#implementing-the-seller" title="Permalink to this headline">¶</a></h2>
|
||||
<div class="codeset container">
|
||||
<div class="highlight-kotlin"><div class="highlight"><pre>private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) : TwoPartyTradeProtocol() {
|
||||
companion object {
|
||||
val TRADE_TOPIC = "com.r3cev.protocols.trade"
|
||||
fun makeSessionID() = Math.abs(SecureRandom.getInstanceStrong().nextLong())
|
||||
}
|
||||
|
||||
class SellerImpl : Seller() {
|
||||
override fun call(args: SellerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
|
||||
TODO()
|
||||
}
|
||||
}
|
||||
|
||||
|
||||
class BuyerImpl : Buyer() {
|
||||
override fun call(args: BuyerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
|
||||
TODO()
|
||||
}
|
||||
}
|
||||
|
||||
override fun runSeller(otherSide: SingleMessageRecipient, args: SellerInitialArgs): Seller {
|
||||
return smm.add(otherSide, args, "$TRADE_TOPIC.seller", SellerImpl::class.java)
|
||||
}
|
||||
|
||||
override fun runBuyer(otherSide: SingleMessageRecipient, args: BuyerInitialArgs): Buyer {
|
||||
return smm.add(otherSide, args, "$TRADE_TOPIC.buyer", BuyerImpl::class.java)
|
||||
}
|
||||
}
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<p>We start with a skeleton on which we will build the protocol. Putting things in a <em>companion object</em> in Kotlin is like
|
||||
declaring them as static members in Java. Here, we define a “topic” that will identify trade related messages that
|
||||
arrive at a node (see <a class="reference internal" href="messaging.html"><em>Networking and messaging</em></a> for details), and a convenience function to pick a large random session ID.</p>
|
||||
<div class="admonition note">
|
||||
<p class="first admonition-title">Note</p>
|
||||
<p class="last">Session IDs keep different traffic streams separated, so for security they must be large and random enough
|
||||
to be unguessable. 63 bits is good enough.</p>
|
||||
</div>
|
||||
<p>The runSeller and runBuyer methods simply start the state machines, passing in a reference to the classes and the topics
|
||||
each side will use.</p>
|
||||
<p>Now let’s try implementing the seller side. Firstly, we’re going to need a message to send to the buyer describing what
|
||||
we want to trade. Remember: this data comes from whatever system was used to find the trading partner to begin with.
|
||||
It could be as simple as a chat room or as complex as a 24/7 exchange.</p>
|
||||
<div class="codeset container">
|
||||
<div class="highlight-kotlin"><div class="highlight"><pre><span class="c1">// This object is serialised to the network and is the first protocol message the seller sends to the buyer.</span>
|
||||
<span class="k">class</span> <span class="nc">SellerTradeInfo</span><span class="p">(</span>
|
||||
<span class="k">val</span> <span class="py">assetForSale</span><span class="p">:</span> <span class="n">StateAndRef</span><span class="p"><</span><span class="n">OwnableState</span><span class="p">>,</span>
|
||||
<span class="k">val</span> <span class="py">price</span><span class="p">:</span> <span class="n">Amount</span><span class="p">,</span>
|
||||
<span class="k">val</span> <span class="py">sellerOwnerKey</span><span class="p">:</span> <span class="n">PublicKey</span><span class="p">,</span>
|
||||
<span class="k">val</span> <span class="py">buyerSessionID</span><span class="p">:</span> <span class="n">Long</span>
|
||||
<span class="p">)</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<p>That’s simple enough: our opening protocol message will be serialised before being sent over the wire, and it contains
|
||||
the details that were agreed so we can double check them. It also contains a session ID so we can identify this
|
||||
trade’s messages, and a pointer to where the asset that is being sold can be found on the ledger.</p>
|
||||
<p>Next we add some code to the <code class="docutils literal"><span class="pre">SellerImpl.call</span></code> method:</p>
|
||||
<div class="codeset container">
|
||||
<div class="highlight-kotlin"><div class="highlight"><pre><span class="k">val</span> <span class="py">sessionID</span> <span class="p">=</span> <span class="n">makeSessionID</span><span class="p">()</span>
|
||||
|
||||
<span class="c1">// Make the first message we'll send to kick off the protocol.</span>
|
||||
<span class="k">val</span> <span class="py">hello</span> <span class="p">=</span> <span class="n">SellerTradeInfo</span><span class="p">(</span><span class="n">args</span><span class="p">.</span><span class="n">assetToSell</span><span class="p">,</span> <span class="n">args</span><span class="p">.</span><span class="n">price</span><span class="p">,</span> <span class="n">args</span><span class="p">.</span><span class="n">myKeyPair</span><span class="p">.</span><span class="k">public</span><span class="p">,</span> <span class="n">sessionID</span><span class="p">)</span>
|
||||
|
||||
<span class="c1">// Zero is a special session ID that is being listened to by the buyer (i.e. before a session is started).</span>
|
||||
<span class="k">val</span> <span class="py">partialTX</span> <span class="p">=</span> <span class="n">sendAndReceive</span><span class="p"><</span><span class="n">SignedWireTransaction</span><span class="p">>(</span><span class="n">TRADE_TOPIC</span><span class="p">,</span> <span class="n">args</span><span class="p">.</span><span class="n">buyerSessionID</span><span class="p">,</span> <span class="n">sessionID</span><span class="p">,</span> <span class="n">hello</span><span class="p">)</span>
|
||||
<span class="n">logger</span><span class="p">().</span><span class="n">trace</span> <span class="p">{</span> <span class="s">"Received partially signed transaction"</span> <span class="p">}</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<p>That’s pretty straight forward. We generate a session ID to identify what’s happening on the seller side, fill out
|
||||
the initial protocol message, and then call <code class="docutils literal"><span class="pre">sendAndReceive</span></code>. This function takes a few arguments:</p>
|
||||
<ul class="simple">
|
||||
<li>A type argument, which is the object we’re expecting to receive from the other side.</li>
|
||||
<li>The topic string that ensures the message is routed to the right bit of code in the other side’s node.</li>
|
||||
<li>The session IDs that ensure the messages don’t get mixed up with other simultaneous trades.</li>
|
||||
<li>And finally, the thing to send. It’ll be serialised and sent automatically.</li>
|
||||
</ul>
|
||||
<p>Once sendAndReceive is called, the call method will be suspended into a continuation. When it gets back we’ll do a log
|
||||
message. The buyer is supposed to send us a transaction with all the right inputs/outputs/commands in return, with their
|
||||
cash put into the transaction and their signature on it authorising the movement of the cash.</p>
|
||||
<div class="admonition note">
|
||||
<p class="first admonition-title">Note</p>
|
||||
<p>There are a few rules you need to bear in mind when writing a class that will be used as a continuation.
|
||||
The first is that anything on the stack when the function is suspended will be stored into the heap and kept alive by
|
||||
the garbage collector. So try to avoid keeping enormous data structures alive unless you really have to.</p>
|
||||
<p>The second is that as well as being kept on the heap, objects reachable from the stack will be serialised. The state
|
||||
of the function call may be resurrected much later! Kryo doesn’t require objects be marked as serialisable, but even so,
|
||||
doing things like creating threads from inside these calls would be a bad idea. They should only contain business
|
||||
logic.</p>
|
||||
<p class="last">The third rule to bear in mind is that you can’t declare variables or methods in these classes and access
|
||||
them from outside of the class, due to the bytecode rewriting and classloader tricks that are used to make this all
|
||||
work. If you want access to something inside the BuyerImpl or SellerImpl classes, you must define a super-interface
|
||||
or super-class (like <code class="docutils literal"><span class="pre">Buyer</span></code>/<code class="docutils literal"><span class="pre">Seller</span></code>) and put what you want to access there.</p>
|
||||
</div>
|
||||
<p>OK, let’s keep going:</p>
|
||||
<div class="codeset container">
|
||||
<div class="highlight-kotlin"><div class="highlight"><pre><span class="n">partialTX</span><span class="p">.</span><span class="n">verifySignatures</span><span class="p">()</span>
|
||||
<span class="k">val</span> <span class="py">wtx</span> <span class="p">=</span> <span class="n">partialTX</span><span class="p">.</span><span class="n">txBits</span><span class="p">.</span><span class="n">deserialize</span><span class="p"><</span><span class="n">WireTransaction</span><span class="p">>()</span>
|
||||
|
||||
<span class="n">requireThat</span> <span class="p">{</span>
|
||||
<span class="s">"transaction sends us the right amount of cash"</span> <span class="k">by</span> <span class="p">(</span><span class="n">wtx</span><span class="p">.</span><span class="n">outputStates</span><span class="p">.</span><span class="n">sumCashBy</span><span class="p">(</span><span class="n">args</span><span class="p">.</span><span class="n">myKeyPair</span><span class="p">.</span><span class="k">public</span><span class="p">)</span> <span class="p">==</span> <span class="n">args</span><span class="p">.</span><span class="n">price</span><span class="p">)</span>
|
||||
<span class="c1">// There are all sorts of funny games a malicious secondary might play here, we should fix them:</span>
|
||||
<span class="c1">//</span>
|
||||
<span class="c1">// - This tx may attempt to send some assets we aren't intending to sell to the secondary, if</span>
|
||||
<span class="c1">// we're reusing keys! So don't reuse keys!</span>
|
||||
<span class="c1">// - This tx may not be valid according to the contracts of the input states, so we must resolve</span>
|
||||
<span class="c1">// and fully audit the transaction chains to convince ourselves that it is actually valid.</span>
|
||||
<span class="c1">// - This tx may include output states that impose odd conditions on the movement of the cash,</span>
|
||||
<span class="c1">// once we implement state pairing.</span>
|
||||
<span class="c1">//</span>
|
||||
<span class="c1">// but the goal of this code is not to be fully secure, but rather, just to find good ways to</span>
|
||||
<span class="c1">// express protocol state machines on top of the messaging layer.</span>
|
||||
<span class="p">}</span>
|
||||
|
||||
<span class="k">val</span> <span class="py">ourSignature</span> <span class="p">=</span> <span class="n">args</span><span class="p">.</span><span class="n">myKeyPair</span><span class="p">.</span><span class="n">signWithECDSA</span><span class="p">(</span><span class="n">partialTX</span><span class="p">.</span><span class="n">txBits</span><span class="p">.</span><span class="n">bits</span><span class="p">)</span>
|
||||
<span class="k">val</span> <span class="py">fullySigned</span><span class="p">:</span> <span class="n">SignedWireTransaction</span> <span class="p">=</span> <span class="n">partialTX</span><span class="p">.</span><span class="n">copy</span><span class="p">(</span><span class="n">sigs</span> <span class="p">=</span> <span class="n">partialTX</span><span class="p">.</span><span class="n">sigs</span> <span class="p">+</span> <span class="n">ourSignature</span><span class="p">)</span>
|
||||
<span class="c1">// We should run it through our full TransactionGroup of all transactions here.</span>
|
||||
<span class="n">fullySigned</span><span class="p">.</span><span class="n">verify</span><span class="p">()</span>
|
||||
<span class="k">val</span> <span class="py">timestamped</span><span class="p">:</span> <span class="n">TimestampedWireTransaction</span> <span class="p">=</span> <span class="n">fullySigned</span><span class="p">.</span><span class="n">toTimestampedTransaction</span><span class="p">(</span><span class="n">serviceHub</span><span class="p">.</span><span class="n">timestampingService</span><span class="p">)</span>
|
||||
<span class="n">logger</span><span class="p">().</span><span class="n">trace</span> <span class="p">{</span> <span class="s">"Built finished transaction, sending back to secondary!"</span> <span class="p">}</span>
|
||||
|
||||
<span class="n">send</span><span class="p">(</span><span class="n">TRADE_TOPIC</span><span class="p">,</span> <span class="n">sessionID</span><span class="p">,</span> <span class="n">timestamped</span><span class="p">)</span>
|
||||
|
||||
<span class="k">return</span> <span class="n">Pair</span><span class="p">(</span><span class="n">timestamped</span><span class="p">,</span> <span class="n">timestamped</span><span class="p">.</span><span class="n">verifyToLedgerTransaction</span><span class="p">(</span><span class="n">serviceHub</span><span class="p">.</span><span class="n">timestampingService</span><span class="p">,</span> <span class="n">serviceHub</span><span class="p">.</span><span class="n">identityService</span><span class="p">))</span>
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<p>Here, we see some assertions and signature checking to satisfy ourselves that we’re not about to sign something
|
||||
incorrect. Once we’re happy, we calculate a signature over the transaction to authorise the movement of the asset
|
||||
we are selling, and then we verify things to make sure it’s all OK. Finally, we request timestamping of the
|
||||
transaction, and send the now finalised and validated transaction back to the buyer.</p>
|
||||
<div class="admonition warning">
|
||||
<p class="first admonition-title">Warning</p>
|
||||
<p class="last">This code is <strong>not secure</strong>. Other than not checking for all possible invalid constructions, if the
|
||||
seller stops before sending the finalised transaction to the buyer, the seller is left with a valid transaction
|
||||
but the buyer isn’t, so they can’t spend the asset they just purchased! This sort of thing will be fixed in a
|
||||
future version of the code.</p>
|
||||
</div>
|
||||
<p>Finally, the call function returns with the result of the protocol: in our case, the final transaction in two different
|
||||
forms.</p>
|
||||
</div>
|
||||
<div class="section" id="implementing-the-buyer">
|
||||
<h2>Implementing the buyer<a class="headerlink" href="#implementing-the-buyer" title="Permalink to this headline">¶</a></h2>
|
||||
<p>OK, let’s do the same for the buyer side:</p>
|
||||
<div class="codeset container">
|
||||
<div class="highlight-kotlin"><div class="highlight"><pre>class BuyerImpl : Buyer() {
|
||||
override fun call(args: BuyerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
|
||||
// Wait for a trade request to come in on our pre-provided session ID.
|
||||
val tradeRequest = receive<SellerTradeInfo>(TRADE_TOPIC, args.sessionID)
|
||||
|
||||
// What is the seller trying to sell us?
|
||||
val assetTypeName = tradeRequest.assetForSale.state.javaClass.name
|
||||
logger().trace { "Got trade request for a $assetTypeName" }
|
||||
|
||||
// Check the start message for acceptability.
|
||||
check(tradeRequest.sessionID > 0)
|
||||
if (tradeRequest.price > args.acceptablePrice)
|
||||
throw UnacceptablePriceException(tradeRequest.price)
|
||||
if (!args.typeToBuy.isInstance(tradeRequest.assetForSale.state))
|
||||
throw AssetMismatchException(args.typeToBuy.name, assetTypeName)
|
||||
|
||||
// TODO: Either look up the stateref here in our local db, or accept a long chain of states and
|
||||
// validate them to audit the other side and ensure it actually owns the state we are being offered!
|
||||
// For now, just assume validity!
|
||||
|
||||
// Generate the shared transaction that both sides will sign, using the data we have.
|
||||
val ptx = PartialTransaction()
|
||||
// Add input and output states for the movement of cash, by using the Cash contract to generate the states.
|
||||
val wallet = serviceHub.walletService.currentWallet
|
||||
val cashStates = wallet.statesOfType<Cash.State>()
|
||||
val cashSigningPubKeys = Cash().craftSpend(ptx, tradeRequest.price, tradeRequest.sellerOwnerKey, cashStates)
|
||||
// Add inputs/outputs/a command for the movement of the asset.
|
||||
ptx.addInputState(tradeRequest.assetForSale.ref)
|
||||
// Just pick some new public key for now.
|
||||
val freshKey = serviceHub.keyManagementService.freshKey()
|
||||
val (command, state) = tradeRequest.assetForSale.state.withNewOwner(freshKey.public)
|
||||
ptx.addOutputState(state)
|
||||
ptx.addArg(WireCommand(command, tradeRequest.assetForSale.state.owner))
|
||||
|
||||
// Now sign the transaction with whatever keys we need to move the cash.
|
||||
for (k in cashSigningPubKeys) {
|
||||
val priv = serviceHub.keyManagementService.toPrivate(k)
|
||||
ptx.signWith(KeyPair(k, priv))
|
||||
}
|
||||
|
||||
val stx = ptx.toSignedTransaction(checkSufficientSignatures = false)
|
||||
stx.verifySignatures() // Verifies that we generated a signed transaction correctly.
|
||||
|
||||
// TODO: Could run verify() here to make sure the only signature missing is the sellers.
|
||||
|
||||
logger().trace { "Sending partially signed transaction to seller" }
|
||||
|
||||
// TODO: Protect against the buyer terminating here and leaving us in the lurch without the final tx.
|
||||
// TODO: Protect against a malicious buyer sending us back a different transaction to the one we built.
|
||||
val fullySigned = sendAndReceive<TimestampedWireTransaction>(TRADE_TOPIC,
|
||||
tradeRequest.sessionID, args.sessionID, stx)
|
||||
|
||||
logger().trace { "Got fully signed transaction, verifying ... "}
|
||||
|
||||
val ltx = fullySigned.verifyToLedgerTransaction(serviceHub.timestampingService, serviceHub.identityService)
|
||||
|
||||
logger().trace { "Fully signed transaction was valid. Trade complete! :-)" }
|
||||
|
||||
return Pair(fullySigned, ltx)
|
||||
}
|
||||
}
|
||||
</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<p>This code is fairly straightforward. Here are some things to pay attention to:</p>
|
||||
<ol class="arabic simple">
|
||||
<li>We do some sanity checking on the received message to ensure we’re being offered what we expected to be offered.</li>
|
||||
<li>We create a cash spend in the normal way, by using <code class="docutils literal"><span class="pre">Cash().craftSpend</span></code>.</li>
|
||||
<li>We access the <em>service hub</em> when we need it to access things that are transient and may change or be recreated
|
||||
whilst a protocol is suspended, things like the wallet or the timestamping service. Remember that a protocol may
|
||||
be suspended when it waits to receive a message across node or computer restarts, so objects representing a service
|
||||
or data which may frequently change should be accessed ‘just in time’.</li>
|
||||
<li>Finally, we send the unfinsished, invalid transaction to the seller so they can sign it. They are expected to send
|
||||
back to us a <code class="docutils literal"><span class="pre">TimestampedWireTransaction</span></code>, which once we verify it, should be the final outcome of the trade.</li>
|
||||
</ol>
|
||||
<p>As you can see, the protocol logic is straightforward and does not contain any callbacks or network glue code, despite
|
||||
the fact that it takes minimal resources and can survive node restarts.</p>
|
||||
<div class="admonition warning">
|
||||
<p class="first admonition-title">Warning</p>
|
||||
<p class="last">When accessing things via the <code class="docutils literal"><span class="pre">serviceHub</span></code> field, avoid the temptation to stuff a reference into a local variable.
|
||||
If you do this then next time your protocol waits to receive an object, the system will try and serialise all your
|
||||
local variables and end up trying to serialise, e.g. the timestamping service, which doesn’t make any conceptual
|
||||
sense. The <code class="docutils literal"><span class="pre">serviceHub</span></code> field is defined by the <code class="docutils literal"><span class="pre">ProtocolStateMachine</span></code> superclass and is marked transient so
|
||||
this problem doesn’t occur. It’s also restored for you after a protocol state machine is restored after a node
|
||||
restart.</p>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
|
||||
|
||||
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|
||||
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|
||||
<footer>
|
||||
|
||||
<div class="rst-footer-buttons" role="navigation" aria-label="footer navigation">
|
||||
|
||||
<a href="visualiser.html" class="btn btn-neutral float-right" title="Using the visualiser" accesskey="n">Next <span class="fa fa-arrow-circle-right"></span></a>
|
||||
|
||||
|
||||
<a href="messaging.html" class="btn btn-neutral" title="Networking and messaging" accesskey="p"><span class="fa fa-arrow-circle-left"></span> Previous</a>
|
||||
|
||||
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|
||||
|
||||
|
||||
<hr/>
|
||||
|
||||
<div role="contentinfo">
|
||||
<p>
|
||||
© Copyright 2015, R3 CEV.
|
||||
|
||||
</p>
|
||||
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|
||||
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||||
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|
||||
|
||||
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|
||||
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|
||||
|
||||
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|
||||
|
||||
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|
||||
|
||||
|
||||
|
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|
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|
||||
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|
||||
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|
||||
2
docs/build/html/search.html
vendored
2
docs/build/html/search.html
vendored
@ -84,6 +84,8 @@
|
||||
<li class="toctree-l1"><a class="reference internal" href="overview.html">Overview</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="getting-set-up.html">Getting set up</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="tutorial.html">Tutorial</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="messaging.html">Networking and messaging</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="protocol-state-machines.html">Protocol state machines</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="visualiser.html">Using the visualiser</a></li>
|
||||
<li class="toctree-l1"><a class="reference internal" href="roadmap.html">Roadmap</a></li>
|
||||
</ul>
|
||||
|
||||
2
docs/build/html/searchindex.js
vendored
2
docs/build/html/searchindex.js
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