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<div class="section" id="flow-framework">
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<h1>Flow framework<a class="headerlink" href="#flow-framework" title="Permalink to this headline">¶</a></h1>
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<p>In Corda all communication takes the form of structured sequences of messages passed between parties which we call flows.</p>
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<p>Flows enable complex multi-step, multi-party business interactions to be modelled as blocking code without a central controller.
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The code is transformed into an asynchronous state machine, with checkpoints written to the node’s backing database when messages are sent and received.
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A node may potentially have millions of flows active at once and they may last days, across node restarts and even upgrades.</p>
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<p>A flow library is provided to enable developers to re-use common flow types such as notarisation, membership broadcast,
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transaction resolution and recording, and so on.</p>
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<p>APIs are provided to send and receive object graphs to and from other identities on the network, embed sub-flows,
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report progress information to observers and even interact with people (for manual resolution of exceptional scenarios)</p>
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<p>Flows are embedded within CorDapps and deployed to a participant’s node for execution.</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">We will be implementing the concept of a flow hospital to provide a means for a node administrator to decide
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whether a paused flow should be killed or repaired. Flows enter this state if they throw exceptions or explicitly request human assistance.</p>
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</div>
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<p>Section 4 of the <a class="reference external" href="_static/corda-technical-whitepaper.pdf">Technical white paper</a> provides further detail of the above features.</p>
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<p>The following diagram illustrates a sample multi-party business flow:</p>
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<img alt="_images/flowFramework.png" src="_images/flowFramework.png" />
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<p>Note the following:</p>
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<ul class="simple">
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<li>there are 3 participants in this workflow including the notary</li>
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<li>the Buyer and Seller flows (depicted in green) are custom written by developers and deployed within a CorDapp</li>
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<li>the custom written flows invoke both financial library flows such as <code class="docutils literal"><span class="pre">TwoPartyTradeFlow</span></code> (depicted in orange) and core
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library flows such as <code class="docutils literal"><span class="pre">ResolveTransactionsFlow</span></code> and <code class="docutils literal"><span class="pre">NotaryFlow</span></code> (depicted in yellow)</li>
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<li>each side of the flow illustrates the stage of execution with a progress tracker notification</li>
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<li>activities within a flow directly or indirectly interact with its node’s ledger (eg. to record a signed, notarised transaction) and vault (eg. to perform a spend of some fungible asset)</li>
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<li>flows interact across parties using send, receive and sendReceive messaging semantics (by implementing the <code class="docutils literal"><span class="pre">FlowLogic</span></code> interface)</li>
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</ul>
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