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<div class="section" id="client-rpc">
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<h1>Client RPC<a class="headerlink" href="#client-rpc" title="Permalink to this headline">¶</a></h1>
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<p>There are multiple ways to interact with a node from a <em>client program</em>, but if your client is written in a JVM
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compatible language the easiest way to do so is using the client library. The library connects to your running
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node using a message queue protocol and then provides a simple RPC interface to interact with it. You make calls
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on a Java object as normal, and the marshalling back and forth is handled for you.</p>
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<p>The starting point for the client library is the <a class="reference external" href="api/com.r3corda.client/-corda-r-p-c-client/index.html">CordaRPCClient</a> class. This provides a <code class="docutils literal"><span class="pre">proxy</span></code> method that
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returns an implementation of the <a class="reference external" href="api/com.r3corda.node.services.messaging/-corda-r-p-c-ops/index.html">CordaRPCOps</a> interface. A timeout parameter can be specified, and observables that
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are returned by RPCs can be subscribed to in order to receive an ongoing stream of updates from the node. More
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detail on how to use this is provided in the docs for the proxy method.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">The returned object is somewhat expensive to create and consumes a small amount of server side
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resources. When you’re done with it, cast it to <code class="docutils literal"><span class="pre">Closeable</span></code> or <code class="docutils literal"><span class="pre">AutoCloseable</span></code> and close it. Don’t create
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one for every call you make - create a proxy and reuse it.</p>
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</div>
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<p>For a brief tutorial on how one can use the RPC API see <a class="reference internal" href="tutorial-clientrpc-api.html"><span class="doc">Client RPC API</span></a>.</p>
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<div class="section" id="observables">
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<h2>Observables<a class="headerlink" href="#observables" title="Permalink to this headline">¶</a></h2>
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<p>The RPC system handles observables in a special way. When a method returns an observable, whether directly or
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as a sub-object of the response object graph, an observable is created on the client to match the one on the
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server. Objects emitted by the server-side observable are pushed onto a queue which is then drained by the client.
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The returned observable may even emit object graphs with even more observables in them, and it all works as you
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would expect.</p>
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<p>This feature comes with a cost: the server must queue up objects emitted by the server-side observable until you
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download them. Therefore RPCs that use this feature are marked with the <code class="docutils literal"><span class="pre">@RPCReturnsObservables</span></code> annotation, and
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you are expected to subscribe to all the observables returned. If you don’t want an observable then subscribe
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then unsubscribe immediately to clear the buffers and indicate that you aren’t interested. If your app quits then
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server side resources will be freed automatically.</p>
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<p>When all the observables returned by an RPC are unsubscribed on the client side, that unsubscription propagates
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through to the server where the corresponding server-side observables are also unsubscribed.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">If you leak an observable or proxy on the client side and it gets garbage collected, you will get
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a warning printed to the logs and the proxy will be closed for you. But don’t rely on this, as garbage
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collection is non-deterministic.</p>
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</div>
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</div>
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<div class="section" id="versioning">
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<h2>Versioning<a class="headerlink" href="#versioning" title="Permalink to this headline">¶</a></h2>
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<p>The client RPC protocol is versioned with a simple incrementing integer. When a proxy is created the server is
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queried for its protocol version, and you can specify your minimum requirement. Methods added in later versions
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are tagged with the <code class="docutils literal"><span class="pre">@RPCSinceVersion</span></code> annotation. If you try to use a method that the server isn’t advertising
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support of, an <code class="docutils literal"><span class="pre">UnsupportedOperationException</span></code> is thrown. If you want to know the version of the server, just
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use the <code class="docutils literal"><span class="pre">protocolVersion</span></code> property (i.e. <code class="docutils literal"><span class="pre">getProtocolVersion</span></code> in Java).</p>
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</div>
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<div class="section" id="thread-safety">
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<h2>Thread safety<a class="headerlink" href="#thread-safety" title="Permalink to this headline">¶</a></h2>
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<p>A proxy is thread safe, blocking, and will only allow a single RPC to be in flight at once. Any observables that
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are returned and you subscribe to will have objects emitted on a background thread. Observables returned as part
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of one RPC and observables returned from another may have their callbacks invoked in parallel, but observables
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returned as part of the same specific RPC invocation are processed serially and will not be invoked in parallel.</p>
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<p>If you want to make multiple calls to the server in parallel you can do that by creating multiple proxies, but
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be aware that the server itself may <em>not</em> process your work in parallel even if you make your requests that way.</p>
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</div>
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<div class="section" id="error-handling">
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<h2>Error handling<a class="headerlink" href="#error-handling" title="Permalink to this headline">¶</a></h2>
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<p>If something goes wrong with the RPC infrastructure itself, an <code class="docutils literal"><span class="pre">RPCException</span></code> is thrown. If you call a method that
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requires a higher version of the protocol than the server supports, <code class="docutils literal"><span class="pre">UnsupportedOperationException</span></code> is thrown.
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Otherwise, if the server implementation throws an exception, that exception is serialised and rethrown on the client
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side as if it was thrown from inside the called RPC method. These exceptions can be caught as normal.</p>
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</div>
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<div class="section" id="wire-protocol">
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<h2>Wire protocol<a class="headerlink" href="#wire-protocol" title="Permalink to this headline">¶</a></h2>
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<p>The client RPC wire protocol is not currently documented. To use it you must use the client library provided.
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This is likely to change in a future release.</p>
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