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<li class="toctree-l1 current"><a class="current reference internal" href="#">Data model</a><ul>
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<li class="toctree-l1"><a class="reference internal" href="tutorial-attachments.html">Using attachments</a></li>
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<p class="caption"><span class="caption-text">Other</span></p>
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<li class="toctree-l1"><a class="reference internal" href="merkle-trees.html">Transaction tear-offs</a></li>
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<p class="caption"><span class="caption-text">Appendix</span></p>
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<li class="toctree-l1"><a class="reference internal" href="setting-up-a-corda-network.html">What is a corda network?</a></li>
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<li class="toctree-l1"><a class="reference internal" href="secure-coding-guidelines.html">Secure coding guidelines</a></li>
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<li class="toctree-l1"><a class="reference internal" href="azure-vm.html">Working with the Corda Demo on Azure Marketplace</a></li>
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<div class="section" id="data-model">
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<h1>Data model<a class="headerlink" href="#data-model" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="overview">
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<h2>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</a></h2>
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<p>Corda uses the so-called “UTXO set” model (unspent transaction output). In this model, the database
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does not track accounts or balances. An entry is either spent or not spent but it cannot be changed. In this model the
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database is a set of immutable rows keyed by (hash:output index). Transactions define outputs that append new rows and
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inputs which consume existing rows.</p>
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<p>The Corda ledger is defined as a set of immutable <strong>states</strong>, which are created and destroyed by digitally signed <strong>transactions</strong>.
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Each transaction points to a set of states that it will consume/destroy, these are called <strong>inputs</strong>, and contains a set
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of new states that it will create, these are called <strong>outputs</strong>.
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Although the ledger is shared, it is not always the case that transactions and ledger entries are globally visible.
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In cases where a set of transactions stays within a small subgroup of users it is possible to keep the relevant
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data purely within that group. To ensure consistency, we rely heavily on secure hashes like SHA-256 to identify things.</p>
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<p>The Corda model provides the following additional features:</p>
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<ul class="simple">
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<li>There is no global broadcast at any point.</li>
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<li>States can include arbitrary typed data.</li>
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<li>Transactions invoke not only input contracts but also the contracts of the outputs.</li>
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<li>Contracts refer to a bundle of business logic that may handle various different tasks, beyond transaction verification.</li>
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<li>Contracts are Turing-complete and can be written in any ordinary programming language that targets the JVM.</li>
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<li>Arbitrarily-precise time-bounds may be specified in transactions (which must be attested to by a notary)</li>
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<li>Primary consensus implementations use block-free conflict resolution algorithms.</li>
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<li>Transactions are not ordered using a block chain and by implication Corda does not use miners or proof-of-work.
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Instead each state points to a notary, which is a service that guarantees it will sign a transaction only if all the
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input states are un-consumed.</li>
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</ul>
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<p>Corda provides three main tools to achieve global distributed consensus:</p>
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<ul class="simple">
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<li>Smart contract logic to ensure state transitions are valid according to the pre-agreed rules.</li>
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<li>Uniqueness and timestamping services to order transactions temporally and eliminate conflicts.</li>
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<li>An <a class="reference internal" href="key-concepts-flow-framework.html"><span class="doc">orchestration framework</span></a> which simplifies the process of writing complex multi-step protocols between multiple different parties.</li>
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</ul>
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<p>Comparisons of the Corda data model with Bitcoin and Ethereum can be found in the white papers.</p>
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</div>
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<div class="section" id="states">
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<h2>States<a class="headerlink" href="#states" title="Permalink to this headline">¶</a></h2>
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<p>A state object represents an agreement between two or more parties, the evolution of which governed by machine-readable contract code.
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This code references, and is intended to implement, portions of human-readable legal prose.
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It is intended to be shared only with those who have a legitimate reason to see it.</p>
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<p>The following diagram illustrates a state object:</p>
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<img alt="_images/contract.png" src="_images/contract.png" />
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<p>In the diagram above, we see a state object representing a cash claim of £100 against a commercial bank, owned by a fictional shipping company.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">Legal prose (depicted above in grey-shade) is currently implemented as an unparsed reference to the natural language
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contract that the code is supposed to express (usually a hash of the contract’s contents).</p>
|
|||
|
|
</div>
|
|||
|
|
<p>States contain arbitrary data, but they always contain at minimum a hash of the bytecode of a
|
|||
|
|
<strong>contract code</strong> file, which is a program expressed in JVM byte code that runs sandboxed inside a Java virtual machine.
|
|||
|
|
Contract code (or just “contracts” in the rest of this document) are globally shared pieces of business logic.</p>
|
|||
|
|
<div class="admonition note">
|
|||
|
|
<p class="first admonition-title">Note</p>
|
|||
|
|
<p class="last">In the current code dynamic loading of contracts is not implemented. This will change in the near future.</p>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
<div class="section" id="contracts">
|
|||
|
|
<h2>Contracts<a class="headerlink" href="#contracts" title="Permalink to this headline">¶</a></h2>
|
|||
|
|
<p>Contracts define part of the business logic of the ledger.</p>
|
|||
|
|
<p>Corda enforces business logic through smart contract code, which is constructed as a pure function (called “verify”) that either accepts
|
|||
|
|
or rejects a transaction, and which can be composed from simpler, reusable functions. The functions interpret transactions
|
|||
|
|
as taking states as inputs and producing output states through the application of (smart contract) commands, and accept
|
|||
|
|
the transaction if the proposed actions are valid. Given the same transaction, a contract’s “verify” function always yields
|
|||
|
|
exactly the same result. Contracts do not have storage or the ability to interact with anything.</p>
|
|||
|
|
<div class="admonition note">
|
|||
|
|
<p class="first admonition-title">Note</p>
|
|||
|
|
<p class="last">In the future, contracts will be mobile. Nodes will download and run contracts inside a sandbox without any review in some deployments,
|
|||
|
|
although we envisage the use of signed code for Corda deployments in the regulated sphere. Corda will use an augmented
|
|||
|
|
JVM custom sandbox that is radically more restrictive than the ordinary JVM sandbox, and it will enforce not only
|
|||
|
|
security requirements but also deterministic execution.</p>
|
|||
|
|
</div>
|
|||
|
|
<p>To further aid writing contracts we introduce the concept of <a class="reference internal" href="clauses.html"><span class="doc">Clauses</span></a> which provide a means of re-using common
|
|||
|
|
verification logic.</p>
|
|||
|
|
</div>
|
|||
|
|
<div class="section" id="transactions">
|
|||
|
|
<h2>Transactions<a class="headerlink" href="#transactions" title="Permalink to this headline">¶</a></h2>
|
|||
|
|
<p>Transaction are used to update the ledger by consuming existing state objects and producing new state objects.</p>
|
|||
|
|
<p>A transaction update is accepted according to the following two aspects of consensus:</p>
|
|||
|
|
<blockquote>
|
|||
|
|
<div><ol class="arabic simple">
|
|||
|
|
<li>Transaction validity: parties can ensure that the proposed transaction and all its ancestors are valid
|
|||
|
|
by checking that the associated contract code runs successfully and has all the required signatures</li>
|
|||
|
|
<li>Transaction uniqueness: parties can ensure there exists no other transaction, over which we have previously reached
|
|||
|
|
consensus (validity and uniqueness), that consumes any of the same states. This is the responsibility of a notary service.</li>
|
|||
|
|
</ol>
|
|||
|
|
</div></blockquote>
|
|||
|
|
<p>Beyond inputs and outputs, transactions may also contain <strong>commands</strong>, small data packets that
|
|||
|
|
the platform does not interpret itself but which parameterise execution of the contracts. They can be thought of as
|
|||
|
|
arguments to the verify function. Each command has a list of <strong>composite keys</strong> associated with it. The platform ensures
|
|||
|
|
that the transaction has signatures matching every key listed in the commands before the contracts start to execute. Thus, a verify
|
|||
|
|
function can trust that all listed keys have signed the transaction, but is responsible for verifying that any keys required
|
|||
|
|
for the transaction to be valid from the verify function’s perspective are included in the list. Public keys
|
|||
|
|
may be random/identityless for privacy, or linked to a well known legal identity, for example via a
|
|||
|
|
<em>public key infrastructure</em> (PKI).</p>
|
|||
|
|
<div class="admonition note">
|
|||
|
|
<p class="first admonition-title">Note</p>
|
|||
|
|
<p class="last">Linkage of keys with identities via a PKI is only partially implemented in the current code.</p>
|
|||
|
|
</div>
|
|||
|
|
<p>Commands are always embedded inside a transaction. Sometimes, there’s a larger piece of data that can be reused across
|
|||
|
|
many different transactions. For this use case, we have <strong>attachments</strong>. Every transaction can refer to zero or more
|
|||
|
|
attachments by hash. Attachments are always ZIP/JAR files, which may contain arbitrary content. These files are
|
|||
|
|
then exposed on the classpath and so can be opened by contract code in the same manner as any JAR resources
|
|||
|
|
would be loaded.</p>
|
|||
|
|
<p>Note that there is nothing that explicitly binds together specific inputs, outputs, commands or attachments. Instead,
|
|||
|
|
it’s up to the contract code to interpret the pieces inside the transaction and ensure they fit together correctly. This
|
|||
|
|
is done to maximise flexibility for the contract developer.</p>
|
|||
|
|
<p>Transactions may sometimes need to provide a contract with data from the outside world. Examples may include stock
|
|||
|
|
prices, facts about events or the statuses of legal entities (e.g. bankruptcy), and so on. The providers of such
|
|||
|
|
facts are called <strong>oracles</strong> and they provide facts to the ledger by signing transactions that contain commands they
|
|||
|
|
recognise, or by creating signed attachments. The commands contain the fact and the signature shows agreement to that fact.</p>
|
|||
|
|
<p>Time is also modelled as a fact and represented as a <strong>timestamping command</strong> placed inside the transaction. This specifies a
|
|||
|
|
time window in which the transaction is considered valid for notarisation. The time window can be open ended (i.e. with a start but no end or vice versa).
|
|||
|
|
In this way transactions can be linked to the notary’s clock.</p>
|
|||
|
|
<p>It is possible for a single Corda network to have multiple competing notaries. A new (output) state is tied to a specific
|
|||
|
|
notary when it is created. Transactions can only consume (input) states that are all associated with the same notary.
|
|||
|
|
A special type of transaction is provided that can move a state (or set of states) from one notary to another.</p>
|
|||
|
|
<div class="admonition note">
|
|||
|
|
<p class="first admonition-title">Note</p>
|
|||
|
|
<p class="last">Currently the platform code will not automatically re-assign states to a single notary. This is a future planned feature.</p>
|
|||
|
|
</div>
|
|||
|
|
<div class="section" id="transaction-validation">
|
|||
|
|
<h3>Transaction Validation<a class="headerlink" href="#transaction-validation" title="Permalink to this headline">¶</a></h3>
|
|||
|
|
<p>When a transaction is presented to a node as part of a flow it may need to be checked. Checking original transaction validity is
|
|||
|
|
the responsibility of the <code class="docutils literal"><span class="pre">ResolveTransactions</span></code> flow. This flow performs a breadth-first search over the transaction graph,
|
|||
|
|
downloading any missing transactions into local storage and validating them. The search bottoms out at transactions without inputs
|
|||
|
|
(eg. these are mostly created from issuance transactions). A transaction is not considered valid if any of its transitive dependencies are invalid.</p>
|
|||
|
|
<div class="admonition note">
|
|||
|
|
<p class="first admonition-title">Note</p>
|
|||
|
|
<p class="last">Non-validating notaries assume transaction validity and do not request transaction data or their dependencies
|
|||
|
|
beyond the list of states consumed.</p>
|
|||
|
|
</div>
|
|||
|
|
<p>The tutorial ” <a class="reference internal" href="tutorial-contract.html"><span class="doc">Writing a contract</span></a> “provides a hand-ons walk-through using these concepts.</p>
|
|||
|
|
</div>
|
|||
|
|
<div class="section" id="transaction-representation">
|
|||
|
|
<h3>Transaction Representation<a class="headerlink" href="#transaction-representation" title="Permalink to this headline">¶</a></h3>
|
|||
|
|
<p>By default, all transaction data (input and output states, commands, attachments) is visible to all participants in
|
|||
|
|
a multi-party, multi-flow business workflow. <a class="reference internal" href="merkle-trees.html"><span class="doc">Transaction tear-offs</span></a> describes how Corda uses Merkle trees to
|
|||
|
|
ensure data integrity and hiding of sensitive data within a transaction that shouldn’t be visible in its entirety to all
|
|||
|
|
participants (eg. oracles nodes providing facts).</p>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
|
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|
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|
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<a href="key-concepts-core-types.html" class="btn btn-neutral float-right" title="Core types" accesskey="n">Next <span class="fa fa-arrow-circle-right"></span></a>
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<p>
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© Copyright 2016, R3 Limited.
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