Retrieve named send signal channel

Community Support
1

I’m trying to find a way to retrieve and use a SendChannel from within a workflow (several function calls deep). Temporal has:

  • workflow.NewNamedChannel, which returns a Channel
  • workflow.GetSignalChannel, which returns a ReceiveChannel (not a Channel or a SendChannel)
  • workflow.SignalExternalWorkflow, which can send a signal to an external workflow

What seems to be missing is a way to retrieve a SendChannel or Channel for use within a workflow. That would be there if either:

  • workflow.GetSignalChannel returned a Channel, instead of a ReceiveChannel or
  • workflow.SignalWorkflow existed to do what SignalExternalWorkflow does, but within the same workflow.

What am I missing here?

2

Signals are, by definition, external to a workflow. So it doesn’t make sense to send them from within. What is a specific use case you are trying to solve?

3

I have a DSL-based workflow that uses the concept of “state” and “action”, something like (in Go):

jumpSignalChan := workflow.NewNamedChannel(ctx, "jump")
ctx = workflow.WithValue(ctx, "jumpChannel", jumpSignalChan)

selector := workflow.NewNamedSelector(ctx, "jump")
selector.AddReceive(jumpSignalChan, func(c workflow.ReceiveChannel, more bool) {
	c.Receive(ctx, &nextStateCode)
	jumpRequested = true
})

for stateCode = StartState; stateCode != EndState; stateCode = nextStateCode {
	state = states[stateCode]
	....
	for _, action := range state.Actions {
		workflow.Go(ctx, func(c workflow.Context) {
			nextStateCode, ns, err = action.Body.Eval(ctx, ns, defn, action, nil, workflowID)
			actionFinished = true
		})

		workflow.Await(ctx, func() bool {
			return jumpRequested || actionFinished
		})

		if jumpRequested {
			jumpRequested = false
			break
		}
	}
	..
	if nextStateCode == "" && state.Next != "" {
		nextStateCode = state.Next
	}

So the idea is: evaluate all of the actions in the current state, but if one of them executes a “jump” action, move to the state requested by the jump. It acts like a “goto”. Any of the (possibly recursive) calls to Eval could generate a jump.

4

You can have a goroutine that forwards requests from the signal channel to the jump channel if needed.

5

How would I communicate with that Go routine from, say, a call to action.Body.Eval, above? I (mistakenly?) thought that that’s what channels were for?

6

What is Eval? If it is part of the workflow, it sends to the jump channel directly.

7

Yes, Eval is part of the workflow. What I’m not understanding is how to get the SendChannel from a workflow module function call. Or should I put it into the workflow context, as my example shows? Or do I need to pass it into the functions that need it? I’m looking for the equivalent of workflow.GetSignalChannel but for SendChannels (or Channels) as opposed to ReceiveChannels.

8

I would pass it as a constructor argument to the other objects in your DSL.

Workflow is code. So, you use the same techniques you would normally use to pass dependencies.

9

Okay, that’ll work. Thanks!