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<div class="section" id="networking-and-messaging">
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<h1>Networking and messaging<a class="headerlink" href="#networking-and-messaging" title="Permalink to this headline">¶</a></h1>
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<p>Corda uses AMQP/1.0 over TLS between nodes which is currently implemented using Apache Artemis, an embeddable message
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queue broker. Building on established MQ protocols gives us features like persistence to disk, automatic delivery
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retries with backoff and dead-letter routing, security, large message streaming and so on.</p>
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<p>Artemis is hidden behind a thin interface that also has an in-memory only implementation suitable for use in
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unit tests and visualisation tools.</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">A future version of Corda will allow the MQ broker to be split out of the main node and run as a
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separate server. We may also support non-Artemis implementations via JMS, allowing the broker to be swapped
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out for alternative implementations.</p>
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</div>
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<p>There are multiple ways of interacting with the network. When writing an application you typically won’t use the
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messaging subsystem directly. Instead you will build on top of the <a class="reference internal" href="flow-state-machines.html"><span class="doc">flow framework</span></a>,
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which adds a layer on top of raw messaging to manage multi-step flows and let you think in terms of identities
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rather than specific network endpoints.</p>
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<div class="section" id="messaging-types">
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<h2>Messaging types<a class="headerlink" href="#messaging-types" title="Permalink to this headline">¶</a></h2>
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<p>Every <code class="docutils literal"><span class="pre">Message</span></code> object has an associated <em>topic</em> and may have a <em>session ID</em>. These are wrapped in a <code class="docutils literal"><span class="pre">TopicSession</span></code>.
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An implementation of <code class="docutils literal"><span class="pre">MessagingService</span></code> can be used to create messages and send them. You can get access to the
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messaging service via the <code class="docutils literal"><span class="pre">ServiceHub</span></code> object that is provided to your app. Endpoints on the network are
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identified at the lowest level using <code class="docutils literal"><span class="pre">SingleMessageRecipient</span></code> which may be e.g. an IP address, or in future
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versions perhaps a routing path through the network.</p>
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</div>
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<div class="section" id="network-map-service">
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<h2>Network Map Service<a class="headerlink" href="#network-map-service" title="Permalink to this headline">¶</a></h2>
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<p>Supporting the messaging layer is a network map service, which is responsible for tracking public nodes on the network.</p>
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<p>Nodes have an internal component, the network map cache, which contains a copy of the network map (which is just a
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document). When a node starts up its cache fetches a copy of the full network map, and requests to be notified of
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changes. The node then registers itself with the network map service, and the service notifies subscribers that a new
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node has joined the network. Nodes do not automatically deregister themselves, so (for example) nodes going offline
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briefly for maintenance are retained in the network map, and messages for them will be queued, minimising disruption.</p>
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<p>Nodes submit signed changes to the map service, which then forwards update notifications on to nodes which have
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requested to be notified of changes.</p>
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<p>The network map currently supports:</p>
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<ul class="simple">
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<li>Looking up nodes by service</li>
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<li>Looking up node for a party</li>
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<li>Suggesting a node providing a specific service, based on suitability for a contract and parties, for example suggesting
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an appropriate interest rates oracle for a interest rate swap contract. Currently no recommendation logic is in place.
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The code simply picks the first registered node that supports the required service.</li>
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