corda/docs/source/contributing-flow-state-machines.rst

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.

../../core/src/main/kotlin/net/corda/core/internal/FlowAsyncOperation.kt

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

../../docs/source/example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/flowstatemachines/TutorialFlowAsyncOperation.kt

../../docs/source/example-code/src/main/java/net/corda/docs/java/tutorial/flowstatemachines/SummingOperation.java

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.

../../core/src/main/kotlin/net/corda/core/internal/FlowAsyncOperation.kt

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

../../docs/source/example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/flowstatemachines/TutorialFlowAsyncOperation.kt

../../docs/source/example-code/src/main/java/net/corda/docs/java/tutorial/flowstatemachines/ExampleSummingFlow.java

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.

../../docs/source/example-code/src/integration-test/kotlin/net/corda/docs/TutorialFlowAsyncOperationTest.kt

../../docs/source/example-code/src/integration-test/java/net/corda/docs/java/TutorialFlowAsyncOperationTest.java

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:

../../docs/source/example-code/src/main/kotlin/net/corda/docs/kotlin/tutorial/flowstatemachines/TutorialFlowAsyncOperation.kt

../../docs/source/example-code/src/main/java/net/corda/docs/java/tutorial/flowstatemachines/SummingOperationThrowing.java

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:

[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.