Implementing Workflows

A workflow implementation implements a workflow interface. Each time a new workflow execution is started, a new instance of the workflow implementation object is created. Then, one of the methods (depending on which workflow type has been started) annotated with @WorkflowMethod is invoked. As soon as this method returns the workflow, execution is closed. While workflow execution is open, it can receive calls to signal and query methods. No additional calls to workflow methods are allowed. The workflow object is stateful, so query and signal methods can communicate with the other parts of the workflow through workflow object fields.

Calling Activities

Workflow.newActivityStub returns a client-side stub that implements an activity interface. It takes activity type and activity options as arguments. Activity options are needed only if some of the required timeouts are not specified through the @ActivityMethod annotation.

Calling a method on this interface invokes an activity that implements this method. An activity invocation synchronously blocks until the activity completes, fails, or times out. Even if activity execution takes a few months, the workflow code still sees it as a single synchronous invocation. It doesn’t matter what happens to the processes that host the workflow. The business logic code just sees a single method call.

public class FileProcessingWorkflowImpl implements FileProcessingWorkflow {

    private final FileProcessingActivities activities;

    public FileProcessingWorkflowImpl() {
        this.activities = Workflow.newActivityStub(FileProcessingActivities.class);
    }

    @Override
    public void processFile(Arguments args) {
        String localName = null;
        String processedName = null;
        try {
            localName = activities.download(args.getSourceBucketName(), args.getSourceFilename());
            processedName = activities.processFile(localName);
            activities.upload(args.getTargetBucketName(), args.getTargetFilename(), processedName);
        } finally {
            if (localName != null) { // File was downloaded.
                activities.deleteLocalFile(localName);
            }
            if (processedName != null) { // File was processed.
                activities.deleteLocalFile(processedName);
            }
        }
    }
    ...
}

If different activities need different options, like timeouts or a task list, multiple client-side stubs can be created with different options.

public FileProcessingWorkflowImpl() {
    ActivityOptions options1 = new ActivityOptions.Builder()
             .setTaskList("taskList1")
             .build();
    this.store1 = Workflow.newActivityStub(FileProcessingActivities.class, options1);

    ActivityOptions options2 = new ActivityOptions.Builder()
             .setTaskList("taskList2")
             .build();
    this.store2 = Workflow.newActivityStub(FileProcessingActivities.class, options2);
}

Calling Activities Asynchronously

Sometimes workflows need to perform certain operations in parallel. The Async class static methods allow you to invoke any activity asynchronously. The calls return a Promise result immediately. Promise is similar to both Java Future and CompletionStage. The Promise get blocks until a result is available. It also exposes the thenApply and handle methods. See the Promise JavaDoc for technical details about differences with Future.

To convert a synchronous call:

String localName = activities.download(surceBucket, sourceFile);

To asynchronous style, the method reference is passed to Async.function or Async.procedure followed by activity arguments:

Promise<String> localNamePromise = Async.function(activities::download, surceBucket, sourceFile);

Then to wait synchronously for the result:

String localName = localNamePromise.get();

Here is the above example rewritten to call download and upload in parallel on multiple files:

public void processFile(Arguments args) {
    List<Promise<String>> localNamePromises = new ArrayList<>();
    List<String> processedNames = null;
    try {
        // Download all files in parallel.
        for (String sourceFilename : args.getSourceFilenames()) {
            Promise<String> localName = Async.function(activities::download,
                args.getSourceBucketName(), sourceFilename);
            localNamePromises.add(localName);
        }
        // allOf converts a list of promises to a single promise that contains a list
        // of each promise value.
        Promise<List<String>> localNamesPromise = Promise.allOf(localNamePromises);

        // All code until the next line wasn't blocking.
        // The promise get is a blocking call.
        List<String> localNames = localNamesPromise.get();
        processedNames = activities.processFiles(localNames);

        // Upload all results in parallel.
        List<Promise<Void>> uploadedList = new ArrayList<>();
        for (String processedName : processedNames) {
            Promise<Void> uploaded = Async.procedure(activities::upload,
                args.getTargetBucketName(), args.getTargetFilename(), processedName);
            uploadedList.add(uploaded);
        }
        // Wait for all uploads to complete.
        Promise<?> allUploaded = Promise.allOf(uploadedList);
        allUploaded.get(); // blocks until all promises are ready.
    } finally {
        for (Promise<String> localNamePromise : localNamePromises) {
            // Skip files that haven't completed downloading.
            if (localNamePromise.isCompleted()) {
                activities.deleteLocalFile(localNamePromise.get());
            }
        }
        if (processedNames != null) {
            for (String processedName : processedNames) {
                activities.deleteLocalFile(processedName);
            }
        }
    }
}

Child Workflows

Besides activities, a workflow can also orchestrate other workflows.

Workflow.newChildWorkflowStub returns a client-side stub that implements a child workflow interface. It takes a child workflow type and optional child workflow options as arguments. Workflow options may be needed to override the timeouts and task list if they differ from the ones defined in the @WorkflowMethod annotation or parent workflow.

The first call to the child workflow stub must always be to a method annotated with @WorkflowMethod. Similar to activities, a call can be made synchronous or asynchronous by using Async#function or Async#procedure. The synchronous call blocks until a child workflow completes. The asynchronous call returns a Promise that can be used to wait for the completion. After an async call returns the stub, it can be used to send signals to the child by calling methods annotated with @SignalMethod. Querying a child workflow by calling methods annotated with @QueryMethod from within workflow code is not supported. However, queries can be done from activities using the provided WorkflowClient stub.

public interface GreetingChild {
    @WorkflowMethod
    String composeGreeting(String greeting, String name);
}

public static class GreetingWorkflowImpl implements GreetingWorkflow {

    @Override
    public String getGreeting(String name) {
        GreetingChild child = Workflow.newChildWorkflowStub(GreetingChild.class);

        // This is a blocking call that returns only after child has completed.
        return child.composeGreeting("Hello", name );
    }
}

Running two children in parallel:

public static class GreetingWorkflowImpl implements GreetingWorkflow {

    @Override
    public String getGreeting(String name) {

        // Workflows are stateful, so a new stub must be created for each new child.
        GreetingChild child1 = Workflow.newChildWorkflowStub(GreetingChild.class);
        Promise<String> greeting1 = Async.function(child1::composeGreeting, "Hello", name);

        // Both children will run concurrently.
        GreetingChild child2 = Workflow.newChildWorkflowStub(GreetingChild.class);
        Promise<String> greeting2 = Async.function(child2::composeGreeting, "Bye", name);

        // Do something else here.
        ...
        return "First: " + greeting1.get() + ", second: " + greeting2.get();
    }
}

To send a signal to a child, call a method annotated with @SignalMethod:

public interface GreetingChild {
    @WorkflowMethod
    String composeGreeting(String greeting, String name);

    @SignalMethod
    void updateName(String name);
}

public static class GreetingWorkflowImpl implements GreetingWorkflow {

    @Override
    public String getGreeting(String name) {
        GreetingChild child = Workflow.newChildWorkflowStub(GreetingChild.class);
        Promise<String> greeting = Async.function(child::composeGreeting, "Hello", name);
        child.updateName("Cadence");
        return greeting.get();
    }
}

Calling methods annotated with @QueryMethod is not allowed from within workflow code.

Workflow Implementation Constraints

Cadence uses the Microsoft Azure Event Sourcing pattern to recover the state of a workflow object including its threads and local variable values. In essence, every time a workflow state has to be restored, its code is re-executed from the beginning. When replaying, side effects (such as activity invocations) are ignored because they are already recorded in the workflow event history. When writing workflow logic, the replay is not visible, so the code should be written since it executes only once. This design puts the following constraints on the workflow implementation:

Always do the following in activities:

Workflow method arguments and return values are serializable to a byte array using the provided DataConverter interface. The default implementation uses JSON serializer, but you can use any alternative serialization mechanism.

The values passed to workflows through invocation parameters or returned through a result value are recorded in the execution history. The entire execution history is transferred from the Cadence service to workflow workers with every event that the workflow logic needs to process. A large execution history can thus adversely impact the performance of your workflow. Therefore, be mindful of the amount of data that you transfer via activity invocation parameters or return values. Otherwise, no additional limitations exist on activity implementations.