How to use signal to pause and resume workflow ?

[lantian789]

There is some sample code snippet of how to use signal in some relevant documentation:

1. https://cadenceworkflow.io/docs/java-client/signals/#implement-signal-handler-in-workflow
2. https://pkg.go.dev/go.uber.org/[email protected]/workflow#hdr-Signals

It 's still very hard for me to know how to use this great feature to stop and resume the workflow, I really appreciate if someone could provide a complete demo code to help me master this wonderful technique. Thank you so much !

[longquanzheng]

Key idea:

1. Use a signal to modify a status variable to indicate whether or not it's paused
2. Use workflow.Go to run a different goroutine(don't use native go to start, which would cause non-deterministic issues)
3. Use workflow.Await to wait for the condition before executing any step you want to check

Sample code in Golang client:

func samplePauseResumeWorkflow(ctx workflow.Context) error {
                // Use a signal to modify a status variable to indicate whether or not it's paused 
		s := workflow.NewSelector(ctx)
		paused := false
		s.AddReceive(workflow.GetSignalChannel(ctx, "operation"), func(c workflow.Channel, ok bool) {
			if ok{
				c.Receive(ctx, &paused)
			}
		})
		
		// Use `workflow.Go` to run a different goroutine(don't use native go to start, which would cause non-deterministic issues)
		workflow.Go(ctx, func(ctx workflow.Context) {
			for {
				s.Select(ctx)
			}	
		})
		
                // Use `workflow.Await` to wait for the condition before executing any step you want to check
		workflow.Await(ctx, func() bool {
			return paused != true
		})
		workflow.ExecuteActivity(ctx, activityA)
		
		workflow.Await(ctx, func() bool {
			return paused != true
		})
		workflow.ExecuteActivity(ctx, activityB)
		....
                 ....
}

[lantian789]

what is the purpose of doing "Use workflow.Go to run a different goroutine(don't use native go to start, which would cause non-deterministic issues)" ?

[longquanzheng]

That’s the similar idea of go func () … to start a gorotine to run something concurrently without blocking the next line of code.

Without that, we can’t have the next line running and the code the in the forloop forever.

[longquanzheng]

Yeah that’s the right way to send signal

[lantian789]

I thought Await function will block the execution of the code until paused is set to false.

[Groxx]

await will block the calling goroutine, yes. that's why the select is happening in another goroutine - that isn't blocked by await, so it can react to the signal.