Engine message processing

SHAR processes each NATS message in the WORKFLOW stream WORKFLOW.>. The engine contains several message processors, and each of them deal with a specific message type such as state transitions and activity execution.

If an error occurs whilst processing a message, the error type defines whether the step should be retried, the activity aborted, or the workflow terminated.

The following example describes the way SHAR processes an activity. In this case SHAR is running a service task on a client that has previously registered that it can perform task y.

sequenceDiagram
autonumber
participant SHAR
participant NATS
participant Client
NATS--)SHAR: WORKFLOW.x.State.Traversal.Execute
activate SHAR
SHAR--)NATS: WORKFLOW.x.State.Activity.Execute
SHAR--)NATS: WORKFLOW.x.State.Traversal.Complete
note over SHAR: Locate the activity in the WORKFLOW_DEF KV.<br/>This activity is a Service Task type. <br/> Export any variables the task needs.
note over SHAR: Store state snapshot in</br>WORKFLOW_VARSTATE KV.
SHAR--)NATS: WORKFLOW.x.State.Job.Execute.ServiceTask.y
deactivate SHAR
NATS--)Client: WORKFLOW.x.State.Job.Execute.ServiceTask.y
activate Client
note over Client: The client performs any processing<br/>using the provided workflow variables<br/>and returns result variables.
Client--)NATS: WORKFLOW.x.State.Job.Complete.ServiceTask
deactivate Client
NATS--)SHAR: WORKFLOW.x.State.Job.Complete.ServiceTask
activate SHAR
note over SHAR: The Service Task completed successfully. <br/> Merge any variables back in the workflow state</br>from the WORKFLOW_VARSTATE KV.
SHAR--)NATS: WORKFLOW.x.State.Activity.Complete
deactivate SHAR
NATS--)SHAR: WORKFLOW.x.State.Activity.Complete
activate SHAR
note over SHAR: Locate the next traversals in the WORKFLOW_DEF KV.
loop For each traversal
opt If traversal condition is met
SHAR--)NATS: WORKFLOW.x.State.Traversal.Execute
end
end
deactivate SHAR

NATS messages

NATS messages are used to trigger state-machine activities and transitions as seen above.

The same NATS messages are used by the engine for performing housekeeping tasks such as clearing up the key/value store.

NATS Message boundaries

It is to be expected that the engine’s host may terminate abruptly during execution. The engine seeks to mitigate the effects of this by starting each critical piece of functionality using a message.

If the engine terminates during execution of a critical section, then the triggering message will be resent to another SHAR instance to be re-processed.

SHAR has been written in such a way that the follow-on NATS message is sent before the previous NATS message is acknowledged. This ensures that the workflow stays live even during NATS down, or SHAR termination.