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< h1 > Flow framework< a class = "headerlink" href = "#flow-framework" title = "Permalink to this headline" > ¶< / a > < / h1 >
< p > In Corda all communication takes the form of structured sequences of messages passed between parties which we call flows.< / p >
< p > Flows enable complex multi-step, multi-party business interactions to be modelled as blocking code without a central controller.
The code is transformed into an asynchronous state machine, with checkpoints written to the node’
A node may potentially have millions of flows active at once and they may last days, across node restarts and even upgrades.< / p >
< p > A flow library is provided to enable developers to re-use common flow types such as notarisation, membership broadcast,
transaction resolution and recording, and so on.< / p >
< p > APIs are provided to send and receive object graphs to and from other identities on the network, embed sub-flows,
report progress information to observers and even interact with people (for manual resolution of exceptional scenarios)< / p >
< p > Flows are embedded within CorDapps and deployed to a participant’ s node for execution.< / p >
< div class = "admonition note" >
< p class = "first admonition-title" > Note< / p >
< p class = "last" > We will be implementing the concept of a flow hospital to provide a means for a node administrator to decide
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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< 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 >
< p > The following diagram illustrates a sample multi-party business flow:< / p >
< img alt = "_images/flowFramework.png" src = "_images/flowFramework.png" / >
< p > Note the following:< / p >
< ul class = "simple" >
< li > there are 3 participants in this workflow including the notary< / li >
< li > the Buyer and Seller flows (depicted in green) are custom written by developers and deployed within a CorDapp< / li >
< 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
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 >
< li > each side of the flow illustrates the stage of execution with a progress tracker notification< / li >
< 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 >
< 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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