# Signals

Signals provide a mechanism to send data directly to a running workflow. Previously, you had two options for passing data to the workflow implementation:

  • Via start parameters
  • As return values from activities

With start parameters, we could only pass in values before workflow execution began.

Return values from activities allowed us to pass information to a running workflow, but this approach comes with its own complications. One major drawback is reliance on polling. This means that the data needs to be stored in a third-party location until it's ready to be picked up by the activity. Further, the lifecycle of this activity requires management, and the activity requires manual restart if it fails before acquiring the data.

Signals, on the other hand, provide a fully asynchronous and durable mechanism for providing data to a running workflow. When a signal is received for a running workflow, Cadence persists the event and the payload in the workflow history. The workflow can then process the signal at any time afterwards without the risk of losing the information. The workflow also has the option to stop execution by blocking on a signal channel.

# Implement Signal Handler in Workflow

See the below example from sample (opens new window).

public interface HelloWorld {
    @WorkflowMethod
    void sayHello(String name);

    @SignalMethod
    void updateGreeting(String greeting);
}

public static class HelloWorldImpl implements HelloWorld {

    private String greeting = "Hello";

    @Override
    public void sayHello(String name) {
        int count = 0;
        while (!"Bye".equals(greeting)) {
            logger.info(++count + ": " + greeting + " " + name + "!");
            String oldGreeting = greeting;
            Workflow.await(() -> !Objects.equals(greeting, oldGreeting));
        }
        logger.info(++count + ": " + greeting + " " + name + "!");
    }

    @Override
    public void updateGreeting(String greeting) {
        this.greeting = greeting;
    }
}

The workflow interface now has a new method annotated with @SignalMethod. It is a callback method that is invoked every time a new signal of "HelloWorldupdateGreeting" is delivered to a workflow. The workflow interface can have only one @WorkflowMethod which is a main function of the workflow and as many signal methods as needed.

The updated workflow implementation demonstrates a few important Cadence concepts. The first is that workflow is stateful and can have fields of any complex type. Another is that the Workflow.await function that blocks until the function it receives as a parameter evaluates to true. The condition is going to be evaluated only on workflow state changes, so it is not a busy wait in traditional sense.

# Signal From Command Line

cadence: docker run --network=host --rm ubercadence/cli:master --do test-domain workflow start  --workflow_id "HelloSignal" --tasklist HelloWorldTaskList --workflow_type HelloWorld::sayHello --execution_timeout 3600 --input \"World\"
Started Workflow Id: HelloSignal, run Id: 6fa204cb-f478-469a-9432-78060b83b6cd

Program output:

16:53:56.120 [workflow-root] INFO  c.u.c.samples.hello.GettingStarted - 1: Hello World!

Let's send a signal using CLI

cadence: docker run --network=host --rm ubercadence/cli:master --do test-domain workflow signal --workflow_id "HelloSignal" --name "HelloWorld::updateGreeting" --input \"Hi\"
Signal workflow succeeded.

Program output:

16:53:56.120 [workflow-root] INFO  c.u.c.samples.hello.GettingStarted - 1: Hello World!
16:54:57.901 [workflow-root] INFO  c.u.c.samples.hello.GettingStarted - 2: Hi World!

Try sending the same signal with the same input again. Note that the output doesn't change. This happens because the await condition doesn't unblock when it sees the same value. But a new greeting unblocks it:

cadence: docker run --network=host --rm ubercadence/cli:master --do test-domain workflow signal --workflow_id "HelloSignal" --name "HelloWorld::updateGreeting" --input \"Welcome\"
Signal workflow succeeded.

Program output:

16:53:56.120 [workflow-root] INFO  c.u.c.samples.hello.GettingStarted - 1: Hello World!
16:54:57.901 [workflow-root] INFO  c.u.c.samples.hello.GettingStarted - 2: Hi World!
16:56:24.400 [workflow-root] INFO  c.u.c.samples.hello.GettingStarted - 3: Welcome World!

Now shut down the worker and send the same signal again:

cadence: docker run --network=host --rm ubercadence/cli:master --do test-domain workflow signal --workflow_id "HelloSignal" --name "HelloWorld::updateGreeting" --input \"Welcome\"
Signal workflow succeeded.

Note that sending signals as well as starting workflows does not need a worker running. The requests are queued inside the Cadence service.

Now bring the worker back. Note that it doesn't log anything besides the standard startup messages. This occurs because it ignores the queued signal that contains the same input as the current value of greeting. Note that the restart of the worker didn't affect the workflow execution. It is still blocked on the same line of code as before the failure. This is the most important feature of Cadence. The workflow code doesn't need to deal with worker failures at all. Its state is fully recovered to its current state that includes all the local variables and threads.

Let's look at the line where the workflow is blocked:

> docker run --network=host --rm ubercadence/cli:master --do test-domain workflow stack --workflow_id "Hello2"
Query result:
"workflow-root: (BLOCKED on await)
com.uber.cadence.internal.sync.SyncDecisionContext.await(SyncDecisionContext.java:546)
com.uber.cadence.internal.sync.WorkflowInternal.await(WorkflowInternal.java:243)
com.uber.cadence.workflow.Workflow.await(Workflow.java:611)
com.uber.cadence.samples.hello.GettingStarted$HelloWorldImpl.sayHello(GettingStarted.java:32)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)"

Yes, indeed the workflow is blocked on await. This feature works for any open workflow, greatly simplifying troubleshooting in production. Let's complete the workflow by sending a signal with a "Bye" greeting:

16:58:22.962 [workflow-root] INFO  c.u.c.samples.hello.GettingStarted - 4: Bye World!

Note that the value of the count variable was not lost during the restart.

Also note that while a single worker instance is used for this walkthrough, any real production deployment has multiple worker instances running. So any worker failure or restart does not delay any workflow execution because it is just migrated to any other available worker.

# SignalWithStart From Command Line

You may not know if a workflow is running and can accept a signal. The signalWithStart feature allows you to send a signal to the current workflow instance if one exists or to create a new run and then send the signal. SignalWithStartWorkflow therefore doesn't take a run ID as a parameter.

Learn more from the --help manual:

docker run --network=host --rm ubercadence/cli:master --do test-domain workflow signalwithstart -h
NAME:
   cadence workflow signalwithstart - signal the current open workflow if exists, or attempt to start a new run based on IDResuePolicy and signals it

USAGE:
   cadence workflow signalwithstart [command options] [arguments...]
...
...
...

# Signal from user/applicaiton code

You may want to signal workflows without running the command line.

The WorkflowClient (opens new window) API allows you to send signal (or SignalWithStartWorkflow) from outside of the workflow to send a signal to the current workflow.

Note that when using newWorkflowStub to signal a workflow, you MUST NOT passing WorkflowOptions.

The WorkflowStub (opens new window) with WorkflowOptions is only for starting workflows.

The WorkflowStub (opens new window) without WorkflowOptions is for signal or query