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<div class="section" id="persistence">
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<h1>Persistence<a class="headerlink" href="#persistence" title="Permalink to this headline">¶</a></h1>
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<p>Corda offers developers the option to expose all or some part of a contract state to an <em>Object Relational Mapping</em> (ORM) tool
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to be persisted in a RDBMS. The purpose of this is to assist <em>vault</em> development by effectively indexing
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persisted contract states held in the vault for the purpose of running queries over them and to allow relational joins
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between Corda data and private data local to the organisation owning a node.</p>
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<p>The ORM mapping is specified using the <a class="reference external" href="https://en.wikipedia.org/wiki/Java_Persistence_API">Java Persistence API</a> (JPA)
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as annotations and is converted to database table rows by the node automatically every time a state is recorded in the
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node’s local vault as part of a transaction.</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">Presently the node includes an instance of the H2 database but any database that supports JDBC is a candidate and
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the node will in the future support a range of database implementations via their JDBC drivers. Much of the node
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internal state is also persisted there. You can access the internal H2 database via JDBC, please see the info
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in “<a class="reference internal" href="node-administration.html"><span class="doc">Node administration</span></a>” for details.</p>
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</div>
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<div class="section" id="schemas">
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<h2>Schemas<a class="headerlink" href="#schemas" title="Permalink to this headline">¶</a></h2>
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<p>Every <code class="docutils literal"><span class="pre">ContractState</span></code> can implement the <code class="docutils literal"><span class="pre">QueryableState</span></code> interface if it wishes to be inserted into the node’s local
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database and accessible using SQL.</p>
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<p>The <code class="docutils literal"><span class="pre">QueryableState</span></code> interface requires the state to enumerate the different relational schemas it supports, for instance in
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cases where the schema has evolved, with each one being represented by a <code class="docutils literal"><span class="pre">MappedSchema</span></code> object return by the
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<code class="docutils literal"><span class="pre">supportedSchemas()</span></code> method. Once a schema is selected it must generate that representation when requested via the
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<code class="docutils literal"><span class="pre">generateMappedObject()</span></code> method which is then passed to the ORM.</p>
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<p>Nodes have an internal <code class="docutils literal"><span class="pre">SchemaService</span></code> which decides what to persist and what not by selecting the <code class="docutils literal"><span class="pre">MappedSchema</span></code>
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to use.</p>
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<p>The <code class="docutils literal"><span class="pre">SchemaService</span></code> can be configured by a node administrator to select the schemas used by each app. In this way the
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relational view of ledger states can evolve in a controlled fashion in lock-step with internal systems or other
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integration points and not necessarily with every upgrade to the contract code.
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It can select from the <code class="docutils literal"><span class="pre">MappedSchema</span></code> offered by a <code class="docutils literal"><span class="pre">QueryableState</span></code>, automatically upgrade to a
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later version of a schema or even provide a <code class="docutils literal"><span class="pre">MappedSchema</span></code> not originally offered by the <code class="docutils literal"><span class="pre">QueryableState</span></code>.</p>
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<p>It is expected that multiple different contract state implementations might provide mappings to some common schema.
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For example an Interest Rate Swap contract and an Equity OTC Option contract might both provide a mapping to a common
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Derivative schema. The schemas should typically not be part of the contract itself and should exist independently of it
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to encourage re-use of a common set within a particular business area or Cordapp.</p>
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<p><code class="docutils literal"><span class="pre">MappedSchema</span></code> offer a family name that is disambiguated using Java package style name-spacing derived from the class name
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of a <em>schema family</em> class that is constant across versions, allowing the <code class="docutils literal"><span class="pre">SchemaService</span></code> to select a preferred version
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of a schema.</p>
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<p>The <code class="docutils literal"><span class="pre">SchemaService</span></code> is also responsible for the <code class="docutils literal"><span class="pre">SchemaOptions</span></code> that can be configured for a particular <code class="docutils literal"><span class="pre">MappedSchema</span></code>
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which allow the configuration of a database schema or table name prefixes to avoid any clash with other <code class="docutils literal"><span class="pre">MappedSchema</span></code>.</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">It is intended that there should be plugin support for the <code class="docutils literal"><span class="pre">SchemaService</span></code> to offer the version upgrading and
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additional schemas as part of Cordapps, and that the active schemas be confgurable. However the present implementation
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offers none of this and simply results in all versions of all schemas supported by a <code class="docutils literal"><span class="pre">QueryableState</span></code> being persisted.
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This will change in due course. Similarly, it does not currently support configuring <code class="docutils literal"><span class="pre">SchemaOptions</span></code> but will do so in
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the future.</p>
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</div>
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</div>
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<div class="section" id="object-relational-mapping">
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<h2>Object Relational Mapping<a class="headerlink" href="#object-relational-mapping" title="Permalink to this headline">¶</a></h2>
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<p>The persisted representation of a <code class="docutils literal"><span class="pre">QueryableState</span></code> should be an instance of a <code class="docutils literal"><span class="pre">PersistentState</span></code> subclass, constructed
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either by the state itself or a plugin to the <code class="docutils literal"><span class="pre">SchemaService</span></code>. This allows the ORM layer to always associate a
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<code class="docutils literal"><span class="pre">StateRef</span></code> with a persisted representation of a <code class="docutils literal"><span class="pre">ContractState</span></code> and allows joining with the set of unconsumed states
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in the vault.</p>
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<p>The <code class="docutils literal"><span class="pre">PersistentState</span></code> subclass should be marked up as a JPA 2.1 <em>Entity</em> with a defined table name and having
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properties (in Kotlin, getters/setters in Java) annotated to map to the appropriate columns and SQL types. Additional
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entities can be included to model these properties where they are more complex, for example collections, so the mapping
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does not have to be <em>flat</em>. The <code class="docutils literal"><span class="pre">MappedSchema</span></code> must provide a list of all of the JPA entity classes for that schema in order
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to initialise the ORM layer.</p>
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<p>Several examples of entities and mappings are provided in the codebase, including <code class="docutils literal"><span class="pre">Cash.State</span></code> and
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<code class="docutils literal"><span class="pre">CommercialPaper.State</span></code>. For example, here’s the first version of the cash schema.</p>
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