Signals in Java

Signal methods can only be defined inside Workflows Interfaces and are methods annotated with the @SignalMethod annotation, for example:

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

    @SignalMethod
    void updateGreeting(String greeting);

    @SignalMethod
    void exit();
}

A Workflow interface can define any number of signal methods.

Note that the @SignalMethod interface has a name parameter which can be used to set the signal type. If not specified, the signal type defaults to the name of the method.

The following example shows how signals can be used to update the Workflow state. You can use the Workflow.await to block the current Workflow execution until the provided unblock condition is evaluated to true. In our case, the unblocking condition is evaluated to true when we receive a signal that updates the greeting to something different from the current greeting. This workflow completes when the greeting becomes "Bye".

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

    @SignalMethod
    void updateGreeting(String greeting);
}

public class HelloWorldImpl implements HelloWorld {
    private final Logger workflowLogger = Workflow.getLogger(HelloWorldImpl.class);
    private String greeting;

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

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