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79 lines
4.4 KiB
ReStructuredText
79 lines
4.4 KiB
ReStructuredText
Flow Hospital
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=============
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Overview
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--------
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The **flow hospital** refers to a built-in node service that manages flows that have encountered an error.
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This service is responsible for recording, tracking, diagnosing, recovering and retrying. It determines whether errored flows should be retried
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from their previous checkpoints or have their errors propagate. Flows may be recoverable under certain scenarios (eg. manual intervention
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may be required to install a missing contract JAR version). For a given errored flow, the flow hospital service determines the next course of
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action towards recovery and retry.
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.. note:: The flow hospital will never terminate a flow, but will propagate its error back to the state machine, and ultimately, end user code to handle
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if it ultimately proves impossible to resolve automatically.
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This concept is analogous to *exception management handling* associated with enterprise workflow software, or
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*retry queues/stores* in enterprise messaging middleware for recovering from failure to deliver a message.
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Functionality
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-------------
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Flow hospital functionality is enabled by default in |release|. No explicit configuration settings are required.
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There are two aspects to the flow hospital:
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- run-time behaviour in the node upon failure, including retry and recovery transitions and policies.
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- visualisation of failed flows in the Explorer UI.
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.. _flow-hospital-runtime:
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Run-time behaviour
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~~~~~~~~~~~~~~~~~~
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Specifically, there are two main ways a flow is hospitalized:
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1. A counterparty invokes a flow on your node that isn’t installed (i.e. missing CorDapp):
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this will cause the flow session initialisation mechanism to trigger a ``ClassNotFoundException``.
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If this happens, the session initiation attempt is kept in the hospital for observation and will retry if you restart the node.
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Corrective action requires installing the correct CorDapp in the node's "cordapps" directory.
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.. warning:: There is currently no retry API. If you don’t want to install the cordapp, you should be able to call `killFlow` with the UUID
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associated with the failing flow in the node's log messages.
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2. Once started, if a flow experiences an error, the following failure scenarios are handled:
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* SQLException mentioning a deadlock*:
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if this happens, the flow will retry. If it retries more than once, a back off delay is applied to try and reduce contention.
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Current policy means these types of failed flows will retry forever (unless explicitly killed). No intervention required.
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* Database constraint violation:
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this scenario may occur due to natural contention between racing flows as Corda delegates handling using the database's optimistic concurrency control.
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As the likelihood of re-occurrence should be low, the flow will actually error and fail if it experiences this at the same point more than 3 times. No intervention required.
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* Finality Flow handling - Corda 3.x (old style) ``FinalityFlow`` and Corda 4.x ``ReceiveFinalityFlow`` handling:
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if on the receive side of the finality flow, any error will result in the flow being kept in for observation to allow the cause of the
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error to be rectified (so that the transaction isn’t lost if, for example, associated contract JARs are missing).
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Intervention is expected to be “rectify error, perhaps uploading attachment, and restart node” (or alternatively reject and call `killFlow`).
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* `FlowTimeoutException`:
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this is used internally by the notary client flow when talking to an HA notary. It’s used to cause the client to try and talk to a different
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member of the notary cluster if it doesn't hear back from the original member it sent the request to within a “reasonable” time.
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The time is hard to document as the notary members, if actually alive, will inform the requester of the ETA of a response.
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This can occur an infinite number of times. i.e. we never give up notarising. No intervention required.
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Futures
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-------
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The flow hospital will be extended in the following areas:
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- Human Computer Interaction (HCI) with MQ integration <ref design/CID>
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- Addition of Public APIs (and CRaSH utility functions) to trigger retries
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- Improved back-off and retry policies
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- Improved Explorer visualization operational controls (eg. ability to select and retry or terminate a failed flow).
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- Tighter integration with Corda Enterprise monitoring and management tooling
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