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How to extend the state machine
===============================

This article explains how to extend the state machine code that underlies flow execution. It is intended for Corda
contributors.

How to add suspending operations
--------------------------------

To add a suspending operation for a simple request-response type function that perhaps involves some external IO we can
use the internal ``FlowAsyncOperation`` interface.

.. container:: codeset

    .. literalinclude:: ../../core/src/main/kotlin/net/corda/core/internal/FlowAsyncOperation.kt
        :language: kotlin
        :start-after: DOCSTART FlowAsyncOperation
        :end-before: DOCEND FlowAsyncOperation

Let's imagine we want to add a suspending operation that takes two integers and returns their sum. To do this we
implement ``FlowAsyncOperation``:

.. container:: codeset

    .. literalinclude:: ../../docs/source/example-code/src/main/kotlin/net/corda/docs/tutorial/flowstatemachines/TutorialFlowAsyncOperation.kt
        :language: kotlin
        :start-after: DOCSTART SummingOperation
        :end-before: DOCEND SummingOperation

    .. literalinclude:: ../../docs/source/example-code/src/main/java/net/corda/docs/java/tutorial/flowstatemachines/SummingOperation.java
        :language: java
        :start-after: DOCSTART SummingOperation
        :end-before: DOCEND SummingOperation

As we can see the constructor of ``SummingOperation`` takes the two numbers, and the ``execute`` function simply returns
a future that is immediately completed by the result of summing the numbers. Note how we don't use ``@Suspendable`` on
``execute``, this is because we'll never suspend inside this function, the suspension will happen before we're calling
it.

Note also how the input numbers are stored in the class as fields. This is important, because in the flow's checkpoint
we'll store an instance of this class whenever we're suspending on such an operation. If the node fails or restarts
while the operation is underway this class will be deserialized from the checkpoint and ``execute`` will be called
again.

Now we can use the internal function ``executeAsync`` to execute this operation from a flow.

.. container:: codeset

    .. literalinclude:: ../../core/src/main/kotlin/net/corda/core/internal/FlowAsyncOperation.kt
        :language: kotlin
        :start-after: DOCSTART executeAsync
        :end-before: DOCEND executeAsync

It simply takes a ``FlowAsyncOperation`` and an optional flag we don't care about for now. We can use this function in a
flow:

.. container:: codeset

    .. literalinclude:: ../../docs/source/example-code/src/main/kotlin/net/corda/docs/tutorial/flowstatemachines/TutorialFlowAsyncOperation.kt
        :language: kotlin
        :start-after: DOCSTART ExampleSummingFlow
        :end-before: DOCEND ExampleSummingFlow

    .. literalinclude:: ../../docs/source/example-code/src/main/java/net/corda/docs/java/tutorial/flowstatemachines/ExampleSummingFlow.java
        :language: java
        :start-after: DOCSTART ExampleSummingFlow
        :end-before: DOCEND ExampleSummingFlow

That's it! Obviously this is a mostly useless example, but this is the basic code structure one could extend for heavier
computations/other IO. For example the function could call into a ``CordaService`` or something similar. One thing to
note is that the operation executed in ``execute`` must be redoable(= "idempotent") in case the node fails before the
next checkpoint is committed.

How to test
-----------

The recommended way to test flows and the state machine is using the Driver DSL. This ensures that you will test your
flow with a full node.

.. container:: codeset

    .. literalinclude:: ../../docs/source/example-code/src/integration-test/kotlin/net/corda/docs/TutorialFlowAsyncOperationTest.kt
        :language: kotlin
        :start-after: DOCSTART summingWorks
        :end-before: DOCEND summingWorks

    .. literalinclude:: ../../docs/source/example-code/src/integration-test/java/net/corda/docs/java/TutorialFlowAsyncOperationTest.java
        :language: java
        :start-after: DOCSTART summingWorks
        :end-before: DOCEND summingWorks

The above will spin up a node and run our example flow.

How to debug issues
-------------------

Let's assume we made a mistake in our summing operation:

.. container:: codeset

    .. literalinclude:: ../../docs/source/example-code/src/main/kotlin/net/corda/docs/tutorial/flowstatemachines/TutorialFlowAsyncOperation.kt
        :language: kotlin
        :start-after: DOCSTART SummingOperationThrowing
        :end-before: DOCEND SummingOperationThrowing

    .. literalinclude:: ../../docs/source/example-code/src/main/java/net/corda/docs/java/tutorial/flowstatemachines/SummingOperationThrowing.java
        :language: java
        :start-after: DOCSTART SummingOperationThrowing
        :end-before: DOCEND SummingOperationThrowing

The operation now throws a rude exception. If we modify the example flow to use this and run the same test we will get
a lot of logs about the error condition (as we are in dev mode). The interesting bit looks like this:

.. parsed-literal::
    [WARN ] 18:38:52,613 [Node thread-1] (DumpHistoryOnErrorInterceptor.kt:39) interceptors.DumpHistoryOnErrorInterceptor.executeTransition - Flow [03ab886e-3fd3-4667-b944-ab6a3b1f90a7] errored, dumping all transitions:

     --- Transition of flow [03ab886e-3fd3-4667-b944-ab6a3b1f90a7] ---
      Timestamp: 2018-06-01T17:38:52.426Z
      Event: DoRemainingWork
      Actions:
        CreateTransaction
        PersistCheckpoint(id=[03ab886e-3fd3-4667-b944-ab6a3b1f90a7], checkpoint=Checkpoint(invocationContext=InvocationContext(origin=RPC(actor=Actor(id=Id(value=aliceUser), serviceId=AuthServiceId(value=NODE_CONFIG), owningLegalIdentity=O=Alice Corp, L=Madrid, C=ES)), trace=Trace(invocationId=26bcf0c3-f1d8-4098-a52d-3780f4095b7a, timestamp: 2018-06-01T17:38:52.234Z, entityType: Invocation, sessionId=393d1175-3bb1-4eb1-bff0-6ba317851260, timestamp: 2018-06-01T17:38:52.169Z, entityType: Session), actor=Actor(id=Id(value=aliceUser), serviceId=AuthServiceId(value=NODE_CONFIG), owningLegalIdentity=O=Alice Corp, L=Madrid, C=ES), externalTrace=null, impersonatedActor=null), ourIdentity=O=Alice Corp, L=Madrid, C=ES, sessions={}, subFlowStack=[Inlined(flowClass=class net.corda.docs.tutorial.flowstatemachines.ExampleSummingFlow, subFlowVersion=CorDappFlow(platformVersion=1, corDappName=net.corda.docs-c6816652-f975-4fb2-aa09-ef1dddea19b3, corDappHash=F4012397D8CF97926B5998E046DBCE16D497318BB87DCED66313912D4B303BB7))], flowState=Unstarted(flowStart=Explicit, frozenFlowLogic=74BA62EC5821EBD4FC4CBE129843F9ED6509DB37E6E3C8F85E3F7A8D84083500), errorState=Clean, numberOfSuspends=0, deduplicationSeed=03ab886e-3fd3-4667-b944-ab6a3b1f90a7))
        PersistDeduplicationFacts(deduplicationHandlers=[net.corda.node.internal.FlowStarterImpl$startFlow$startFlowEvent$1@69326343])
        CommitTransaction
        AcknowledgeMessages(deduplicationHandlers=[net.corda.node.internal.FlowStarterImpl$startFlow$startFlowEvent$1@69326343])
        SignalFlowHasStarted(flowId=[03ab886e-3fd3-4667-b944-ab6a3b1f90a7])
        CreateTransaction
      Continuation: Resume(result=null)
      Diff between previous and next state:
    isAnyCheckpointPersisted:
        false
        true
    pendingDeduplicationHandlers:
        [net.corda.node.internal.FlowStarterImpl$startFlow$startFlowEvent$1@69326343]
        []
    isFlowResumed:
        false
        true


     --- Transition of flow [03ab886e-3fd3-4667-b944-ab6a3b1f90a7] ---
      Timestamp: 2018-06-01T17:38:52.487Z
      Event: Suspend(ioRequest=ExecuteAsyncOperation(operation=net.corda.docs.tutorial.flowstatemachines.SummingOperationThrowing@40f4c23d), maySkipCheckpoint=false, fiber=15EC69204562BB396846768169AD4A339569D97AE841D805C230C513A8BA5BDE, )
      Actions:
        PersistCheckpoint(id=[03ab886e-3fd3-4667-b944-ab6a3b1f90a7], checkpoint=Checkpoint(invocationContext=InvocationContext(origin=RPC(actor=Actor(id=Id(value=aliceUser), serviceId=AuthServiceId(value=NODE_CONFIG), owningLegalIdentity=O=Alice Corp, L=Madrid, C=ES)), trace=Trace(invocationId=26bcf0c3-f1d8-4098-a52d-3780f4095b7a, timestamp: 2018-06-01T17:38:52.234Z, entityType: Invocation, sessionId=393d1175-3bb1-4eb1-bff0-6ba317851260, timestamp: 2018-06-01T17:38:52.169Z, entityType: Session), actor=Actor(id=Id(value=aliceUser), serviceId=AuthServiceId(value=NODE_CONFIG), owningLegalIdentity=O=Alice Corp, L=Madrid, C=ES), externalTrace=null, impersonatedActor=null), ourIdentity=O=Alice Corp, L=Madrid, C=ES, sessions={}, subFlowStack=[Inlined(flowClass=class net.corda.docs.tutorial.flowstatemachines.ExampleSummingFlow, subFlowVersion=CorDappFlow(platformVersion=1, corDappName=net.corda.docs-c6816652-f975-4fb2-aa09-ef1dddea19b3, corDappHash=F4012397D8CF97926B5998E046DBCE16D497318BB87DCED66313912D4B303BB7))], flowState=Started(flowIORequest=ExecuteAsyncOperation(operation=net.corda.docs.tutorial.flowstatemachines.SummingOperationThrowing@40f4c23d), frozenFiber=15EC69204562BB396846768169AD4A339569D97AE841D805C230C513A8BA5BDE), errorState=Clean, numberOfSuspends=1, deduplicationSeed=03ab886e-3fd3-4667-b944-ab6a3b1f90a7))
        PersistDeduplicationFacts(deduplicationHandlers=[])
        CommitTransaction
        AcknowledgeMessages(deduplicationHandlers=[])
        ScheduleEvent(event=DoRemainingWork)
      Continuation: ProcessEvents
      Diff between previous and next state:
    checkpoint.numberOfSuspends:
        0
        1
    checkpoint.flowState:
        Unstarted(flowStart=Explicit, frozenFlowLogic=74BA62EC5821EBD4FC4CBE129843F9ED6509DB37E6E3C8F85E3F7A8D84083500)
        Started(flowIORequest=ExecuteAsyncOperation(operation=net.corda.docs.tutorial.flowstatemachines.SummingOperationThrowing@40f4c23d), frozenFiber=15EC69204562BB396846768169AD4A339569D97AE841D805C230C513A8BA5BDE)
    isFlowResumed:
        true
        false


     --- Transition of flow [03ab886e-3fd3-4667-b944-ab6a3b1f90a7] ---
      Timestamp: 2018-06-01T17:38:52.549Z
      Event: DoRemainingWork
      Actions:
        ExecuteAsyncOperation(operation=net.corda.docs.tutorial.flowstatemachines.SummingOperationThrowing@40f4c23d)
      Continuation: ProcessEvents
      Diff between previous and intended state:
    null
      Diff between previous and next state:
    checkpoint.errorState:
        Clean
        Errored(errors=[FlowError(errorId=-8704604242619505379, exception=java.lang.IllegalStateException: You shouldn't be calling me)], propagatedIndex=0, propagating=false)


     --- Transition of flow [03ab886e-3fd3-4667-b944-ab6a3b1f90a7] ---
      Timestamp: 2018-06-01T17:38:52.555Z
      Event: DoRemainingWork
      Actions:

      Continuation: ProcessEvents
      Diff between previous and next state:
    null

     --- Transition of flow [03ab886e-3fd3-4667-b944-ab6a3b1f90a7] ---
      Timestamp: 2018-06-01T17:38:52.556Z
      Event: StartErrorPropagation
      Actions:
        ScheduleEvent(event=DoRemainingWork)
      Continuation: ProcessEvents
      Diff between previous and next state:
    checkpoint.errorState.propagating:
        false
        true


     --- Transition of flow [03ab886e-3fd3-4667-b944-ab6a3b1f90a7] ---
      Timestamp: 2018-06-01T17:38:52.606Z
      Event: DoRemainingWork
      Actions:
        PropagateErrors(errorMessages=[ErrorSessionMessage(flowException=null, errorId=-8704604242619505379)], sessions=[], senderUUID=861f07d6-4b8f-42bd-9b52-5152812db2ba)
        CreateTransaction
        RemoveCheckpoint(id=[03ab886e-3fd3-4667-b944-ab6a3b1f90a7])
        PersistDeduplicationFacts(deduplicationHandlers=[])
        ReleaseSoftLocks(uuid=03ab886e-3fd3-4667-b944-ab6a3b1f90a7)
        CommitTransaction
        AcknowledgeMessages(deduplicationHandlers=[])
        RemoveSessionBindings(sessionIds=[])
        RemoveFlow(flowId=[03ab886e-3fd3-4667-b944-ab6a3b1f90a7], removalReason=ErrorFinish(flowErrors=[FlowError(errorId=-8704604242619505379, exception=java.lang.IllegalStateException: You shouldn't be calling me)]), lastState=StateMachineState(checkpoint=Checkpoint(invocationContext=InvocationContext(origin=RPC(actor=Actor(id=Id(value=aliceUser), serviceId=AuthServiceId(value=NODE_CONFIG), owningLegalIdentity=O=Alice Corp, L=Madrid, C=ES)), trace=Trace(invocationId=26bcf0c3-f1d8-4098-a52d-3780f4095b7a, timestamp: 2018-06-01T17:38:52.234Z, entityType: Invocation, sessionId=393d1175-3bb1-4eb1-bff0-6ba317851260, timestamp: 2018-06-01T17:38:52.169Z, entityType: Session), actor=Actor(id=Id(value=aliceUser), serviceId=AuthServiceId(value=NODE_CONFIG), owningLegalIdentity=O=Alice Corp, L=Madrid, C=ES), externalTrace=null, impersonatedActor=null), ourIdentity=O=Alice Corp, L=Madrid, C=ES, sessions={}, subFlowStack=[Inlined(flowClass=class net.corda.docs.tutorial.flowstatemachines.ExampleSummingFlow, subFlowVersion=CorDappFlow(platformVersion=1, corDappName=net.corda.docs-c6816652-f975-4fb2-aa09-ef1dddea19b3, corDappHash=F4012397D8CF97926B5998E046DBCE16D497318BB87DCED66313912D4B303BB7))], flowState=Started(flowIORequest=ExecuteAsyncOperation(operation=net.corda.docs.tutorial.flowstatemachines.SummingOperationThrowing@40f4c23d), frozenFiber=15EC69204562BB396846768169AD4A339569D97AE841D805C230C513A8BA5BDE), errorState=Errored(errors=[FlowError(errorId=-8704604242619505379, exception=java.lang.IllegalStateException: You shouldn't be calling me)], propagatedIndex=1, propagating=true), numberOfSuspends=1, deduplicationSeed=03ab886e-3fd3-4667-b944-ab6a3b1f90a7), flowLogic=net.corda.docs.tutorial.flowstatemachines.ExampleSummingFlow@600b0c6c, pendingDeduplicationHandlers=[], isFlowResumed=false, isTransactionTracked=false, isAnyCheckpointPersisted=true, isStartIdempotent=false, isRemoved=true, senderUUID=861f07d6-4b8f-42bd-9b52-5152812db2ba))
      Continuation: Abort
      Diff between previous and next state:
    checkpoint.errorState.propagatedIndex:
        0
        1
    isRemoved:
        false
        true

Whoa that's a lot of stuff. Now we get a glimpse into the bowels of the flow state machine. As we can see the flow did
quite a few things, even though the flow code looks simple.

What we can see here is the different transitions the flow's state machine went through that led up to the error
condition. For each transition we see what *Event* triggered the transition, what *Action* s were taken as a consequence,
and how the internal *State* of the state machine was modified in the process. It also prints the transition's
*Continuation*, which indicates how the flow should proceed after the transition.

For example in the first transition we can see that the triggering event was a ``DoRemainingWork``, this is a generic
event that instructs the state machine to check its own state to see whether there's any work left to do, and does it if
there's any.

In this case the work involves persisting a checkpoint together with some deduplication data in a database transaction,
then acknowledging any triggering messages, signalling that the flow has started, and creating a fresh database
transaction, to be used by user code.

The continuation is a ``Resume``, which instructs the state machine to hand control to user code. The state change is
a simple update of bookkeeping data.

In other words the first transition concerns the initialization of the flow, which includes the creation of the
checkpoint.

The next transition is the suspension of our summing operation, triggered by the ``Suspend`` event. As we can see in
this transition we aren't doing any work related to the summation yet, we're merely persisting the checkpoint that
indicates that we want to do the summation. Had we added a ``toString`` method to our ``SummingOperationThrowing`` we
would see a nicer message.

The next transition is the faulty one, as we can see it was also triggered by a ``DoRemainingWork``, and executed our
operation. We can see that there are two state "diff"s printed, one that would've happened had the transition succeeded,
and one that actually happened, which marked the flow's state as errored. The rest of the transitions involve error
propagation (triggered by the ``FlowHospital``) and notification of failure, which ultimately raises the exception on
the RPC ``resultFuture``.