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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="#suspendable-methods">Suspendable methods</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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<script type="text/javascript" src="_static/codesets.js"></script><div class="section" id="protocol-state-machines">
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<h1>Protocol state machines<a class="headerlink" href="#protocol-state-machines" title="Permalink to this headline">¶</a></h1>
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<p>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.</p>
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<div class="section" id="introduction">
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<h2>Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
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<p>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.</p>
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<p>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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<em>delivery versus payment</em> 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.</p>
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<p>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:</p>
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<ul class="simple">
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<li>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.</li>
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<li>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.</li>
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<li>Error handling.</li>
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<li>Message routing.</li>
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<li>Serialisation.</li>
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<li>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.</li>
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<li>Unit testing of the finished protocol.</li>
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</ul>
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<p>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.</p>
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<p>To put these problems in perspective, the <em>payment channel protocol</em> 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.</p>
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<p>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.</p>
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</div>
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<div class="section" id="theory">
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<h2>Theory<a class="headerlink" href="#theory" title="Permalink to this headline">¶</a></h2>
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<p>A <em>continuation</em> 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 concept is sometimes referred to as “fibers”. This may
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sound abstract but don’t worry, the examples below will make it clearer. The JVM does not natively support
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continuations, so we implement them using a library called Quasar which works through behind-the-scenes
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bytecode rewriting. You don’t have to know how this works to benefit from it, however.</p>
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<p>We use continuations for the following reasons:</p>
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<ul class="simple">
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<li>It allows us to write code that is free of callbacks, that looks like ordinary sequential code.</li>
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<li>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 thread stack can easily be 1mb in size.</li>
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<li>It frees the developer from thinking (much) about persistence and serialisation.</li>
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</ul>
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<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
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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.</p>
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</div>
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<div class="section" id="a-two-party-trading-protocol">
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<h2>A two party trading protocol<a class="headerlink" href="#a-two-party-trading-protocol" title="Permalink to this headline">¶</a></h2>
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<p>We would like to implement the “hello world” of shared transaction building protocols: a seller wishes to sell some
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<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
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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.</p>
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<p>Our protocol has two parties (B and S for buyer and seller) and will proceed as follows:</p>
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<ol class="arabic simple">
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<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
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B to pay.</li>
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<li>B sends to S a <code class="docutils literal"><span class="pre">SignedTransaction</span></code> 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.</li>
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<li>S signs it and hands the now finalised <code class="docutils literal"><span class="pre">SignedTransaction</span></code> back to B.</li>
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</ol>
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<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>
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<p>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.</p>
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<p>Note that it’s the <em>seller</em> who initiates contact with the buyer, not vice-versa as you might imagine.</p>
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<p>We start by defining a wrapper that namespaces the protocol code, two functions to start either the buy or sell side
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of the protocol, and two classes that will contain the protocol definition. We also pick what data will be used by
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each side.</p>
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<div class="codeset container">
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<div class="highlight-kotlin"><div class="highlight"><pre><span class="k">object</span> <span class="nc">TwoPartyTradeProtocol</span> <span class="p">{</span>
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<span class="k">val</span> <span class="py">TRADE_TOPIC</span> <span class="p">=</span> <span class="s">"platform.trade"</span>
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<span class="k">fun</span> <span class="nf">runSeller</span><span class="p">(</span><span class="n">smm</span><span class="p">:</span> <span class="n">StateMachineManager</span><span class="p">,</span> <span class="n">timestampingAuthority</span><span class="p">:</span> <span class="n">LegallyIdentifiableNode</span><span class="p">,</span>
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<span class="n">otherSide</span><span class="p">:</span> <span class="n">SingleMessageRecipient</span><span class="p">,</span> <span class="n">assetToSell</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="n">price</span><span class="p">:</span> <span class="n">Amount</span><span class="p">,</span>
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<span class="n">myKeyPair</span><span class="p">:</span> <span class="n">KeyPair</span><span class="p">,</span> <span class="n">buyerSessionID</span><span class="p">:</span> <span class="n">Long</span><span class="p">):</span> <span class="n">ListenableFuture</span><span class="p"><</span><span class="n">SignedTransaction</span><span class="p">></span> <span class="p">{</span>
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<span class="k">val</span> <span class="py">seller</span> <span class="p">=</span> <span class="n">Seller</span><span class="p">(</span><span class="n">otherSide</span><span class="p">,</span> <span class="n">timestampingAuthority</span><span class="p">,</span> <span class="n">assetToSell</span><span class="p">,</span> <span class="n">price</span><span class="p">,</span> <span class="n">myKeyPair</span><span class="p">,</span> <span class="n">buyerSessionID</span><span class="p">)</span>
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<span class="n">smm</span><span class="p">.</span><span class="n">add</span><span class="p">(</span><span class="s">"$TRADE_TOPIC.seller"</span><span class="p">,</span> <span class="n">seller</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">seller</span><span class="p">.</span><span class="n">resultFuture</span>
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<span class="p">}</span>
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<span class="k">fun</span> <span class="nf">runBuyer</span><span class="p">(</span><span class="n">smm</span><span class="p">:</span> <span class="n">StateMachineManager</span><span class="p">,</span> <span class="n">timestampingAuthority</span><span class="p">:</span> <span class="n">LegallyIdentifiableNode</span><span class="p">,</span>
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<span class="n">otherSide</span><span class="p">:</span> <span class="n">SingleMessageRecipient</span><span class="p">,</span> <span class="n">acceptablePrice</span><span class="p">:</span> <span class="n">Amount</span><span class="p">,</span> <span class="n">typeToBuy</span><span class="p">:</span> <span class="n">Class</span><span class="p"><</span><span class="k">out</span> <span class="n">OwnableState</span><span class="p">>,</span>
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<span class="n">sessionID</span><span class="p">:</span> <span class="n">Long</span><span class="p">):</span> <span class="n">ListenableFuture</span><span class="p"><</span><span class="n">SignedTransaction</span><span class="p">></span> <span class="p">{</span>
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<span class="k">val</span> <span class="py">buyer</span> <span class="p">=</span> <span class="n">Buyer</span><span class="p">(</span><span class="n">otherSide</span><span class="p">,</span> <span class="n">timestampingAuthority</span><span class="p">.</span><span class="n">identity</span><span class="p">,</span> <span class="n">acceptablePrice</span><span class="p">,</span> <span class="n">typeToBuy</span><span class="p">,</span> <span class="n">sessionID</span><span class="p">)</span>
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<span class="n">smm</span><span class="p">.</span><span class="n">add</span><span class="p">(</span><span class="s">"$TRADE_TOPIC.buyer"</span><span class="p">,</span> <span class="n">buyer</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">buyer</span><span class="p">.</span><span class="n">resultFuture</span>
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<span class="p">}</span>
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<span class="c1">// This object is serialised to the network and is the first protocol message the seller sends to the buyer.</span>
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<span class="k">class</span> <span class="nc">SellerTradeInfo</span><span class="p">(</span>
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<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>
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<span class="k">val</span> <span class="py">price</span><span class="p">:</span> <span class="n">Amount</span><span class="p">,</span>
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<span class="k">val</span> <span class="py">sellerOwnerKey</span><span class="p">:</span> <span class="n">PublicKey</span><span class="p">,</span>
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<span class="k">val</span> <span class="py">sessionID</span><span class="p">:</span> <span class="n">Long</span>
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<span class="p">)</span>
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<span class="k">class</span> <span class="nc">SignaturesFromSeller</span><span class="p">(</span><span class="k">val</span> <span class="py">timestampAuthoritySig</span><span class="p">:</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">WithKey</span><span class="p">,</span> <span class="k">val</span> <span class="py">sellerSig</span><span class="p">:</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">WithKey</span><span class="p">)</span>
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<span class="k">class</span> <span class="nc">Seller</span><span class="p">(</span><span class="k">val</span> <span class="py">otherSide</span><span class="p">:</span> <span class="n">SingleMessageRecipient</span><span class="p">,</span>
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<span class="k">val</span> <span class="py">timestampingAuthority</span><span class="p">:</span> <span class="n">LegallyIdentifiableNode</span><span class="p">,</span>
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<span class="k">val</span> <span class="py">assetToSell</span><span class="p">:</span> <span class="n">StateAndRef</span><span class="p"><</span><span class="n">OwnableState</span><span class="p">>,</span>
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<span class="k">val</span> <span class="py">price</span><span class="p">:</span> <span class="n">Amount</span><span class="p">,</span>
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<span class="k">val</span> <span class="py">myKeyPair</span><span class="p">:</span> <span class="n">KeyPair</span><span class="p">,</span>
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<span class="k">val</span> <span class="py">buyerSessionID</span><span class="p">:</span> <span class="n">Long</span><span class="p">)</span> <span class="p">:</span> <span class="n">ProtocolLogic</span><span class="p"><</span><span class="n">SignedTransaction</span><span class="p">>()</span> <span class="p">{</span>
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<span class="n">@Suspendable</span>
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<span class="k">override</span> <span class="k">fun</span> <span class="nf">call</span><span class="p">():</span> <span class="n">SignedTransaction</span> <span class="p">{</span>
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<span class="n">TODO</span><span class="p">()</span>
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<span class="p">}</span>
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<span class="p">}</span>
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<span class="k">class</span> <span class="nc">UnacceptablePriceException</span><span class="p">(</span><span class="k">val</span> <span class="py">givenPrice</span><span class="p">:</span> <span class="n">Amount</span><span class="p">)</span> <span class="p">:</span> <span class="n">Exception</span><span class="p">()</span>
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<span class="k">class</span> <span class="nc">AssetMismatchException</span><span class="p">(</span><span class="k">val</span> <span class="py">expectedTypeName</span><span class="p">:</span> <span class="n">String</span><span class="p">,</span> <span class="k">val</span> <span class="py">typeName</span><span class="p">:</span> <span class="n">String</span><span class="p">)</span> <span class="p">:</span> <span class="n">Exception</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">override</span> <span class="k">fun</span> <span class="nf">toString</span><span class="p">()</span> <span class="p">=</span> <span class="s">"The submitted asset didn't match the expected type: $expectedTypeName vs $typeName"</span>
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<span class="p">}</span>
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<span class="k">class</span> <span class="nc">Buyer</span><span class="p">(</span><span class="k">val</span> <span class="py">otherSide</span><span class="p">:</span> <span class="n">SingleMessageRecipient</span><span class="p">,</span>
|
|
<span class="k">val</span> <span class="py">timestampingAuthority</span><span class="p">:</span> <span class="n">Party</span><span class="p">,</span>
|
|
<span class="k">val</span> <span class="py">acceptablePrice</span><span class="p">:</span> <span class="n">Amount</span><span class="p">,</span>
|
|
<span class="k">val</span> <span class="py">typeToBuy</span><span class="p">:</span> <span class="n">Class</span><span class="p"><</span><span class="k">out</span> <span class="n">OwnableState</span><span class="p">>,</span>
|
|
<span class="k">val</span> <span class="py">sessionID</span><span class="p">:</span> <span class="n">Long</span><span class="p">)</span> <span class="p">:</span> <span class="n">ProtocolLogic</span><span class="p"><</span><span class="n">SignedTransaction</span><span class="p">>()</span> <span class="p">{</span>
|
|
<span class="n">@Suspendable</span>
|
|
<span class="k">override</span> <span class="k">fun</span> <span class="nf">call</span><span class="p">():</span> <span class="n">SignedTransaction</span> <span class="p">{</span>
|
|
<span class="n">TODO</span><span class="p">()</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</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> singleton, and a couple of methods, one
|
|
to run the buyer side of the protocol and one to run the seller side. Some of the classes are simply protocol messages.</li>
|
|
<li>It defines the “trade topic”, which is just a string that namespaces this protocol. The prefix “platform.” is reserved
|
|
by the DLG, but you can define your own protocols using standard Java-style reverse DNS notation.</li>
|
|
<li>The <code class="docutils literal"><span class="pre">runBuyer</span></code> and <code class="docutils literal"><span class="pre">runSeller</span></code> methods take a number of parameters that specialise the protocol for this run,
|
|
use them to construct a <code class="docutils literal"><span class="pre">Buyer</span></code> or <code class="docutils literal"><span class="pre">Seller</span></code> object respectively, and then add the new instances to the
|
|
<code class="docutils literal"><span class="pre">StateMachineManager</span></code>. The purpose of this class is described below. The <code class="docutils literal"><span class="pre">smm.add</span></code> method takes a logger name as
|
|
the first parameter, this is just a standard JDK logging identifier string, and the instance to add.</li>
|
|
</ul>
|
|
<p>Going through the data needed to become a seller, we have:</p>
|
|
<ul class="simple">
|
|
<li><code class="docutils literal"><span class="pre">timestampingAuthority:</span> <span class="pre">LegallyIdentifiableNode</span></code> - a reference to a node on the P2P network that acts as a trusted
|
|
timestamper. The use of timestamping is described in <a class="reference internal" href="data-model.html"><em>Data model</em></a>.</li>
|
|
<li><code class="docutils literal"><span class="pre">otherSide:</span> <span class="pre">SingleMessageRecipient</span></code> - the network address of the node with which you are trading.</li>
|
|
<li><code class="docutils literal"><span class="pre">assetToSell:</span> <span class="pre">StateAndRef<OwnableState></span></code> - a pointer to the ledger entry that represents the thing being sold.</li>
|
|
<li><code class="docutils literal"><span class="pre">price:</span> <span class="pre">Amount</span></code> - the agreed on price that the asset is being sold for.</li>
|
|
<li><code class="docutils literal"><span class="pre">myKeyPair:</span> <span class="pre">KeyPair</span></code> - the key pair that controls the asset being sold. It will be used to sign the transaction.</li>
|
|
<li><code class="docutils literal"><span class="pre">buyerSessionID:</span> <span class="pre">Long</span></code> - a unique number that identifies this trade to the buyer. It is expected that the buyer
|
|
knows that the trade is going to take place and has sent you such a number already. (This field may go away in a future
|
|
iteration of the framework)</li>
|
|
</ul>
|
|
<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>And for the buyer:</p>
|
|
<ul class="simple">
|
|
<li><code class="docutils literal"><span class="pre">acceptablePrice:</span> <span class="pre">Amount</span></code> - the price that was agreed upon out of band. If the seller specifies a price less than
|
|
or equal to this, then the trade will go ahead.</li>
|
|
<li><code class="docutils literal"><span class="pre">typeToBuy:</span> <span class="pre">Class<out</span> <span class="pre">OwnableState></span></code> - the type of state that is being purchased. This is used to check that the
|
|
sell side of the protocol isn’t trying to sell us the wrong thing, whether by accident or on purpose.</li>
|
|
<li><code class="docutils literal"><span class="pre">sessionID:</span> <span class="pre">Long</span></code> - the session ID that was handed to the seller in order to start the protocol.</li>
|
|
</ul>
|
|
<p>The run methods return a <code class="docutils literal"><span class="pre">ListenableFuture</span></code> that will complete when the protocol has finished.</p>
|
|
<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 resulting object 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>Finally, we define a couple of exceptions, and two classes that will be used as a protocol message called
|
|
<code class="docutils literal"><span class="pre">SellerTradeInfo</span></code> and <code class="docutils literal"><span class="pre">SignaturesFromSeller</span></code>.</p>
|
|
</div>
|
|
<div class="section" id="suspendable-methods">
|
|
<h2>Suspendable methods<a class="headerlink" href="#suspendable-methods" title="Permalink to this headline">¶</a></h2>
|
|
<p>The <code class="docutils literal"><span class="pre">call</span></code> method of the buyer/seller classes is marked with the <code class="docutils literal"><span class="pre">@Suspendable</span></code> annotation. What does this mean?</p>
|
|
<p>As mentioned above, our protocol framework will at points suspend the code and serialise it to disk. For this to work,
|
|
any methods on the call stack must have been pre-marked as <code class="docutils literal"><span class="pre">@Suspendable</span></code> so the bytecode rewriter knows to modify
|
|
the underlying code to support this new feature. A protocol is suspended when calling either <code class="docutils literal"><span class="pre">receive</span></code>, <code class="docutils literal"><span class="pre">send</span></code> or
|
|
<code class="docutils literal"><span class="pre">sendAndReceive</span></code> which we will learn more about below. For now, just be aware that when one of these methods is
|
|
invoked, all methods on the stack must have been marked. If you forget, then in the unit test environment you will
|
|
get a useful error message telling you which methods you didn’t mark. The fix is simple enough: just add the annotation
|
|
and try again.</p>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last">A future version of Java is likely to remove this pre-marking requirement completely.</p>
|
|
</div>
|
|
</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 messages arrive. 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>
|
|
<p>Let’s implement the <code class="docutils literal"><span class="pre">Seller.call</span></code> method. This will be invoked by the platform when the protocol is started by the
|
|
<code class="docutils literal"><span class="pre">StateMachineManager</span></code>.</p>
|
|
<div class="codeset container">
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span class="k">val</span> <span class="py">partialTX</span><span class="p">:</span> <span class="n">SignedTransaction</span> <span class="p">=</span> <span class="n">receiveAndCheckProposedTransaction</span><span class="p">()</span>
|
|
|
|
<span class="c1">// These two steps could be done in parallel, in theory. Our framework doesn't support that yet though.</span>
|
|
<span class="k">val</span> <span class="py">ourSignature</span> <span class="p">=</span> <span class="n">signWithOurKey</span><span class="p">(</span><span class="n">partialTX</span><span class="p">)</span>
|
|
<span class="k">val</span> <span class="py">tsaSig</span> <span class="p">=</span> <span class="n">subProtocol</span><span class="p">(</span><span class="n">TimestampingProtocol</span><span class="p">(</span><span class="n">timestampingAuthority</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="k">val</span> <span class="py">stx</span><span class="p">:</span> <span class="n">SignedTransaction</span> <span class="p">=</span> <span class="n">sendSignatures</span><span class="p">(</span><span class="n">partialTX</span><span class="p">,</span> <span class="n">ourSignature</span><span class="p">,</span> <span class="n">tsaSig</span><span class="p">)</span>
|
|
|
|
<span class="k">return</span> <span class="n">stx</span>
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
<p>Here we see the outline of the procedure. We receive a proposed trade transaction from the buyer and check that it’s
|
|
valid. Then we sign with our own key, request a timestamping authority to assert with another signature that the
|
|
timestamp in the transaction (if any) is valid, and finally we send back both our signature and the TSA’s signature.
|
|
Finally, we hand back to the code that invoked the protocol the finished transaction in a couple of different forms.</p>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last"><code class="docutils literal"><span class="pre">ProtocolLogic</span></code> classes can be composed together. Here, we see the use of the <code class="docutils literal"><span class="pre">subProtocol</span></code> method, which
|
|
is given an instance of <code class="docutils literal"><span class="pre">TimestampingProtocol</span></code>. This protocol will run to completion and yield a result, almost
|
|
as if it’s a regular method call. In fact, under the hood, all the <code class="docutils literal"><span class="pre">subProtocol</span></code> method does is pass the current
|
|
fiber object into the newly created object and then run <code class="docutils literal"><span class="pre">call()</span></code> on it ... so it basically _is_ just a method call.
|
|
This is where we can see the benefits of using continuations/fibers as a programming model.</p>
|
|
</div>
|
|
<p>Let’s fill out the <code class="docutils literal"><span class="pre">receiveAndCheckProposedTransaction()</span></code> method.</p>
|
|
<div class="codeset container">
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span class="n">@Suspendable</span>
|
|
<span class="k">open</span> <span class="k">fun</span> <span class="nf">receiveAndCheckProposedTransaction</span><span class="p">():</span> <span class="n">SignedTransaction</span> <span class="p">{</span>
|
|
<span class="k">val</span> <span class="py">sessionID</span> <span class="p">=</span> <span class="n">random63BitValue</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">assetToSell</span><span class="p">,</span> <span class="n">price</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="k">val</span> <span class="py">maybeSTX</span> <span class="p">=</span> <span class="n">sendAndReceive</span><span class="p"><</span><span class="n">SignedTransaction</span><span class="p">>(</span><span class="n">TRADE_TOPIC</span><span class="p">,</span> <span class="n">otherSide</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">maybeSTX</span><span class="p">.</span><span class="n">validate</span> <span class="p">{</span>
|
|
<span class="c1">// Check that the tx proposed by the buyer is valid.</span>
|
|
<span class="k">val</span> <span class="py">missingSigs</span> <span class="p">=</span> <span class="n">it</span><span class="p">.</span><span class="n">verify</span><span class="p">(</span><span class="n">throwIfSignaturesAreMissing</span> <span class="p">=</span> <span class="k">false</span><span class="p">)</span>
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">missingSigs</span> <span class="p">!=</span> <span class="n">setOf</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">timestampingAuthority</span><span class="p">.</span><span class="n">identity</span><span class="p">.</span><span class="n">owningKey</span><span class="p">))</span>
|
|
<span class="k">throw</span> <span class="n">SignatureException</span><span class="p">(</span><span class="s">"The set of missing signatures is not as expected: $missingSigs"</span><span class="p">)</span>
|
|
|
|
<span class="k">val</span> <span class="py">wtx</span><span class="p">:</span> <span class="n">WireTransaction</span> <span class="p">=</span> <span class="n">it</span><span class="p">.</span><span class="n">tx</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: ${it.id}"</span> <span class="p">}</span>
|
|
|
|
<span class="n">checkDependencies</span><span class="p">(</span><span class="n">it</span><span class="p">)</span>
|
|
|
|
<span class="c1">// This verifies that the transaction is contract-valid, even though it is missing signatures.</span>
|
|
<span class="n">serviceHub</span><span class="p">.</span><span class="n">verifyTransaction</span><span class="p">(</span><span class="n">wtx</span><span class="p">.</span><span class="n">toLedgerTransaction</span><span class="p">(</span><span class="n">serviceHub</span><span class="p">.</span><span class="n">identityService</span><span class="p">))</span>
|
|
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">wtx</span><span class="p">.</span><span class="n">outputs</span><span class="p">.</span><span class="n">sumCashBy</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">price</span><span class="p">)</span>
|
|
<span class="k">throw</span> <span class="n">IllegalArgumentException</span><span class="p">(</span><span class="s">"Transaction is not sending us the right amounnt of cash"</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 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 (yet), 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="k">return</span> <span class="n">it</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
<p>That’s pretty straightforward. 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>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>The thing to send. It’ll be serialised and sent automatically.</li>
|
|
<li>Finally a type argument, which is the kind of object we’re expecting to receive from the other side.</li>
|
|
</ul>
|
|
<p>Once <code class="docutils literal"><span class="pre">sendAndReceive</span></code> 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 couple of 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 class="last">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>
|
|
</div>
|
|
<p>You get back a simple wrapper class, <code class="docutils literal"><span class="pre">UntrustworthyData<SignedTransaction></span></code>, which is just a marker class that reminds
|
|
us that the data came from a potentially malicious external source and may have been tampered with or be unexpected in
|
|
other ways. It doesn’t add any functionality, but acts as a reminder to “scrub” the data before use. Here, our scrubbing
|
|
simply involves checking the signatures on it. Then we go ahead and check all the dependencies of this partial
|
|
transaction for validity. Here’s the code to do that:</p>
|
|
<div class="codeset container">
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span class="n">@Suspendable</span>
|
|
<span class="k">private</span> <span class="k">fun</span> <span class="nf">checkDependencies</span><span class="p">(</span><span class="n">stx</span><span class="p">:</span> <span class="n">SignedTransaction</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="c1">// Download and check all the transactions that this transaction depends on, but do not check this</span>
|
|
<span class="c1">// transaction itself.</span>
|
|
<span class="k">val</span> <span class="py">dependencyTxIDs</span> <span class="p">=</span> <span class="n">stx</span><span class="p">.</span><span class="n">tx</span><span class="p">.</span><span class="n">inputs</span><span class="p">.</span><span class="n">map</span> <span class="p">{</span> <span class="n">it</span><span class="p">.</span><span class="n">txhash</span> <span class="p">}.</span><span class="n">toSet</span><span class="p">()</span>
|
|
<span class="n">subProtocol</span><span class="p">(</span><span class="n">ResolveTransactionsProtocol</span><span class="p">(</span><span class="n">dependencyTxIDs</span><span class="p">,</span> <span class="n">otherSide</span><span class="p">))</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
<p>This is simple enough: we mark the method as <code class="docutils literal"><span class="pre">@Suspendable</span></code> because we’re going to invoke a sub-protocol, extract the
|
|
IDs of the transactions the proposed transaction depends on, and then uses a protocol provided by the system to download
|
|
and check them all. This protocol does a breadth-first search over the dependency graph, bottoming out at issuance
|
|
transactions that don’t have any inputs themselves. Once the node has audited the transaction history, all the dependencies
|
|
are committed to the node’s local database so they won’t be checked again next time.</p>
|
|
<div class="admonition note">
|
|
<p class="first admonition-title">Note</p>
|
|
<p class="last">Transaction dependency resolution assumes that the peer you got the transaction from has all of the
|
|
dependencies itself. It must do, otherwise it could not have convinced itself that the dependencies were themselves
|
|
valid. It’s important to realise that requesting only the transactions we require is a privacy leak, because if
|
|
we don’t download a transaction from the peer, they know we must have already seen it before. Fixing this privacy
|
|
leak will come later.</p>
|
|
</div>
|
|
<p>After the dependencies, we check the proposed trading transaction for validity by running the contracts for that as
|
|
well (but having handled the fact that some signatures are missing ourselves).</p>
|
|
<p>Here’s the rest of the code:</p>
|
|
<div class="codeset container">
|
|
<div class="highlight-kotlin"><div class="highlight"><pre><span class="k">open</span> <span class="k">fun</span> <span class="nf">signWithOurKey</span><span class="p">(</span><span class="n">partialTX</span><span class="p">:</span> <span class="n">SignedTransaction</span><span class="p">)</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">@Suspendable</span>
|
|
<span class="k">open</span> <span class="k">fun</span> <span class="nf">sendSignatures</span><span class="p">(</span><span class="n">partialTX</span><span class="p">:</span> <span class="n">SignedTransaction</span><span class="p">,</span> <span class="n">ourSignature</span><span class="p">:</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">WithKey</span><span class="p">,</span>
|
|
<span class="n">tsaSig</span><span class="p">:</span> <span class="n">DigitalSignature</span><span class="p">.</span><span class="n">LegallyIdentifiable</span><span class="p">):</span> <span class="n">SignedTransaction</span> <span class="p">{</span>
|
|
<span class="k">val</span> <span class="py">fullySigned</span> <span class="p">=</span> <span class="n">partialTX</span> <span class="p">+</span> <span class="n">tsaSig</span> <span class="p">+</span> <span class="n">ourSignature</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">otherSide</span><span class="p">,</span> <span class="n">buyerSessionID</span><span class="p">,</span> <span class="n">SignaturesFromSeller</span><span class="p">(</span><span class="n">tsaSig</span><span class="p">,</span> <span class="n">ourSignature</span><span class="p">))</span>
|
|
<span class="k">return</span> <span class="n">fullySigned</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
<p>It’s should be all pretty straightforward: here, <code class="docutils literal"><span class="pre">txBits</span></code> is the raw byte array representing the transaction.</p>
|
|
<p>In <code class="docutils literal"><span class="pre">sendSignatures</span></code>, we take the two signatures we calculated, then add them to the partial transaction we were sent.
|
|
We provide an overload for the + operator so signatures can be added to a SignedTransaction easily. Finally, we wrap the
|
|
two signatures in a simple wrapper message class and send it back. The send won’t block waiting for an acknowledgement,
|
|
but the underlying message queue software will retry delivery if the other side has gone away temporarily.</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>
|
|
</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>@Suspendable
|
|
override fun call(): SignedTransaction {
|
|
val tradeRequest = receiveAndValidateTradeRequest()
|
|
val (ptx, cashSigningPubKeys) = assembleSharedTX(tradeRequest)
|
|
val stx = signWithOurKeys(cashSigningPubKeys, ptx)
|
|
val signatures = swapSignaturesWithSeller(stx, tradeRequest.sessionID)
|
|
|
|
logger.trace { "Got signatures from seller, verifying ... "}
|
|
val fullySigned = stx + signatures.timestampAuthoritySig + signatures.sellerSig
|
|
fullySigned.verify()
|
|
|
|
logger.trace { "Fully signed transaction was valid. Trade complete! :-)" }
|
|
return fullySigned
|
|
}
|
|
|
|
@Suspendable
|
|
open fun receiveAndValidateTradeRequest(): SellerTradeInfo {
|
|
// Wait for a trade request to come in on our pre-provided session ID.
|
|
val maybeTradeRequest = receive<SellerTradeInfo>(TRADE_TOPIC, sessionID)
|
|
|
|
maybeTradeRequest.validate {
|
|
// What is the seller trying to sell us?
|
|
val asset = it.assetForSale.state
|
|
val assetTypeName = asset.javaClass.name
|
|
logger.trace { "Got trade request for a $assetTypeName: ${it.assetForSale}" }
|
|
|
|
// Check the start message for acceptability.
|
|
check(it.sessionID > 0)
|
|
if (it.price > acceptablePrice)
|
|
throw UnacceptablePriceException(it.price)
|
|
if (!typeToBuy.isInstance(asset))
|
|
throw AssetMismatchException(typeToBuy.name, assetTypeName)
|
|
|
|
// Check the transaction that contains the state which is being resolved.
|
|
// We only have a hash here, so if we don't know it already, we have to ask for it.
|
|
subProtocol(ResolveTransactionsProtocol(setOf(it.assetForSale.ref.txhash), otherSide))
|
|
|
|
return it
|
|
}
|
|
}
|
|
|
|
@Suspendable
|
|
open fun swapSignaturesWithSeller(stx: SignedTransaction, theirSessionID: Long): SignaturesFromSeller {
|
|
logger.trace { "Sending partially signed transaction to seller" }
|
|
|
|
// TODO: Protect against the seller terminating here and leaving us in the lurch without the final tx.
|
|
|
|
return sendAndReceive(TRADE_TOPIC, otherSide, theirSessionID, sessionID, stx, SignaturesFromSeller::class.java).validate { it }
|
|
}
|
|
|
|
open fun signWithOurKeys(cashSigningPubKeys: List<PublicKey>, ptx: TransactionBuilder): SignedTransaction {
|
|
// 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))
|
|
}
|
|
|
|
return ptx.toSignedTransaction(checkSufficientSignatures = false)
|
|
}
|
|
|
|
open fun assembleSharedTX(tradeRequest: SellerTradeInfo): Pair<TransactionBuilder, List<PublicKey>> {
|
|
val ptx = TransactionBuilder()
|
|
// 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().generateSpend(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. This won't be linked with our identity in any way, which is what
|
|
// we want for privacy reasons: the key is here ONLY to manage and control ownership, it is not intended to
|
|
// reveal who the owner actually is. The key management service is expected to derive a unique key from some
|
|
// initial seed in order to provide privacy protection.
|
|
val freshKey = serviceHub.keyManagementService.freshKey()
|
|
val (command, state) = tradeRequest.assetForSale.state.withNewOwner(freshKey.public)
|
|
ptx.addOutputState(state)
|
|
ptx.addCommand(command, tradeRequest.assetForSale.state.owner)
|
|
|
|
// And add a request for timestamping: it may be that none of the contracts need this! But it can't hurt
|
|
// to have one.
|
|
ptx.setTime(Instant.now(), timestampingAuthority, 30.seconds)
|
|
return Pair(ptx, cashSigningPubKeys)
|
|
}
|
|
</pre></div>
|
|
</div>
|
|
</div>
|
|
<p>This code is longer but still 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().generateSpend</span></code>. See the contracts tutorial if this isn’t
|
|
clear.</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 unfinished, 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">SignaturesFromSeller</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 when a protocol state machine is restored after a node
|
|
restart.</p>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="progress-tracking">
|
|
<h2>Progress tracking<a class="headerlink" href="#progress-tracking" title="Permalink to this headline">¶</a></h2>
|
|
<p>Not shown in the code snippets above is the usage of the <code class="docutils literal"><span class="pre">ProgressTracker</span></code> API. Progress tracking exports information
|
|
from a protocol about where it’s got up to in such a way that observers can render it in a useful manner to humans who
|
|
may need to be informed. It may be rendered via an API, in a GUI, onto a terminal window, etc.</p>
|
|
<p>A <code class="docutils literal"><span class="pre">ProgressTracker</span></code> is constructed with a series of <code class="docutils literal"><span class="pre">Step</span></code> objects, where each step is an object representing a
|
|
stage in a piece of work. It is therefore typical to use singletons that subclass <code class="docutils literal"><span class="pre">Step</span></code>, which may be defined easily
|
|
in one line when using Kotlin. Typical steps might be “Waiting for response from peer”, “Waiting for signature to be
|
|
approved”, “Downloading and verifying data” etc.</p>
|
|
<p>Each step exposes a label. By default labels are fixed, but by subclassing <code class="docutils literal"><span class="pre">RelabelableStep</span></code>
|
|
you can make a step that can update its label on the fly. That’s useful for steps that want to expose non-structured
|
|
progress information like the current file being downloaded. By defining your own step types, you can export progress
|
|
in a way that’s both human readable and machine readable.</p>
|
|
<p>Progress trackers are hierarchical. Each step can be the parent for another tracker. By altering the
|
|
<code class="docutils literal"><span class="pre">ProgressTracker.childrenFor[step]</span> <span class="pre">=</span> <span class="pre">tracker</span></code> map, a tree of steps can be created. It’s allowed to alter the hierarchy
|
|
at runtime, on the fly, and the progress renderers will adapt to that properly. This can be helpful when you don’t
|
|
fully know ahead of time what steps will be required. If you _do_ know what is required, configuring as much of the
|
|
hierarchy ahead of time is a good idea, as that will help the users see what is coming up.</p>
|
|
<p>Every tracker has not only the steps given to it at construction time, but also the singleton
|
|
<code class="docutils literal"><span class="pre">ProgressTracker.UNSTARTED</span></code> step and the <code class="docutils literal"><span class="pre">ProgressTracker.DONE</span></code> step. Once a tracker has become <code class="docutils literal"><span class="pre">DONE</span></code> its
|
|
position may not be modified again (because e.g. the UI may have been removed/cleaned up), but until that point, the
|
|
position can be set to any arbitrary set both forwards and backwards. Steps may be skipped, repeated, etc. Note that
|
|
rolling the current step backwards will delete any progress trackers that are children of the steps being reversed, on
|
|
the assumption that those subtasks will have to be repeated.</p>
|
|
<p>Trackers provide an <a class="reference external" href="http://reactivex.io/">Rx observable</a> which streams changes to the hierarchy. The top level
|
|
observable exposes all the events generated by its children as well. The changes are represented by objects indicating
|
|
whether the change is one of position (i.e. progress), structure (i.e. new subtasks being added/removed) or some other
|
|
aspect of rendering (i.e. a step has changed in some way and is requesting a re-render).</p>
|
|
<p>The protocol framework is somewhat integrated with this API. Each <code class="docutils literal"><span class="pre">ProtocolLogic</span></code> may optionally provide a tracker by
|
|
overriding the <code class="docutils literal"><span class="pre">protocolTracker</span></code> property (<code class="docutils literal"><span class="pre">getProtocolTracker</span></code> method in Java). If the
|
|
<code class="docutils literal"><span class="pre">ProtocolLogic.subProtocol</span></code> method is used, then the tracker of the sub-protocol will be made a child of the current
|
|
step in the parent protocol automatically, if the parent is using tracking in the first place. The framework will also
|
|
automatically set the current step to <code class="docutils literal"><span class="pre">DONE</span></code> for you, when the protocol is finished.</p>
|
|
<p>Because a protocol may sometimes wish to configure the children in its progress hierarchy _before_ the sub-protocol
|
|
is constructed, for sub-protocols that always follow the same outline regardless of their parameters it’s conventional
|
|
to define a companion object/static method (for Kotlin/Java respectively) that constructs a tracker, and then allow
|
|
the sub-protocol to have the tracker it will use be passed in as a parameter. This allows all trackers to be built
|
|
and linked ahead of time.</p>
|
|
<p>In future, the progress tracking framework will become a vital part of how exceptions, errors, and other faults are
|
|
surfaced to human operators for investigation and resolution.</p>
|
|
</div>
|
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