AWS Flow Framework for Java
Developer Guide (API Version 2012-01-25)
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Workflow and Activity Contracts

Java interfaces are used to declare the signatures of workflows and activities. The interface forms the contract between the implementation of the workflow (or activity) and the client of that workflow (or activity). For example, a workflow type MyWorkflow is defined using an interface that is annotated with the @Workflow annotation:

@Workflow
@WorkflowRegistrationOptions(
   defaultExecutionStartToCloseTimeoutSeconds = 60,
   defaultTaskStartToCloseTimeoutSeconds = 10)
public interface MyWorkflow
{
    @Execute(version = "1.0")
    void startMyWF(int a, String b);

    @Signal
    void signal1(int a, int b, String c);

    @GetState
    MyWorkflowState getState();
} 

The contract has no implementation-specific settings. This use of implementation-neutral contracts allows clients to be decoupled from the implementation and hence provides the flexibility to change the implementation details without breaking the client. Conversely, you may also change the client without necessitating changes to the workflow or activity being consumed. For example, the client may be modified to call an activity asynchronously using promises (Promise<T>) without requiring a change to the activity implementation. Similarly, the activity implementation may be changed so that it is completed asynchronously, for example, by a person sending an email—without requiring the clients of the activity to be changed.

In the example above, the workflow interface MyWorkflow contains a method, startMyWF, for starting a new execution. This method is annotated with the @Execute annotation and must have a return type of void or Promise<>. In a given workflow interface, at most one method can be annotated with this annotation. This method is the entry point of the workflow logic, and the framework calls this method to execute the workflow logic when a decision task is received.

The workflow interface also defines the signals that may be sent to the workflow. The signal method gets invoked when a signal with a matching name is received by the workflow execution. For example, the MyWorkflow interface declares a signal method, signal1, annotated with the @Signal annotation.

The @Signal annotation is required on signal methods. The return type of a signal method must be void. A workflow interface may have zero or more signal methods defined in it. You may declare a workflow interface without an @Execute method and some @Signal methods to generate clients that cannot start their execution but can send signals to running executions.

Methods annotated with @Execute and @Signal annotations may have any number of parameters of any type other than Promise<T> or its derivatives. This allows you to pass strongly typed inputs to a workflow execution at start and while it is running. The return type of the @Execute method must be void or Promise<>.

Additionally, you may also declare a method in the workflow interface to report the latest state of a workflow execution, for instance, the getState method in the previous example. This state is not the entire application state of the workflow. The intended use of this feature is to allow you to store up to 32 KB of data to indicate the latest status of the execution. For example, in an order processing workflow, you may store a string that indicates that the order has been received, processed, or canceled. This method is called by the framework every time a decision task is completed to get the latest state. The state is stored in Amazon Simple Workflow Service (Amazon SWF) and can be retrieved using the generated external client. This allows you to check the latest state of a workflow execution. Methods annotated with @GetState must not take any arguments and must not have a void return type. You can return any type, which fits your needs, from this method. In the above example, an object of MyWorkflowState (see definition below) is returned by the method that is used to store a string state and a numeric percent complete. The method is expected to perform read-only access of the workflow implementation object and is invoked synchronously, which disallows use of any asynchronous operations like calling methods annotated with @Asynchronous. At most one method in a workflow interface can be annotated with @GetState annotation.

public class MyWorkflowState {
   public String status;
   public int percentComplete;
} 

Similarly, a set of activities are defined using an interface annotated with @Activities annotation. Each method in the interface corresponds to an activity—for example:

@Activities(version = "1.0")
@ActivityRegistrationOptions(
     defaultTaskScheduleToStartTimeoutSeconds = 300,
     defaultTaskStartToCloseTimeoutSeconds = 3600)
public interface MyActivities {
    // Overrides values from annotation found on the interface
    @ActivityRegistrationOptions(description = "This is a sample activity",
         defaultTaskScheduleToStartTimeoutSeconds = 100,
                                                                                                                                             defaultTaskStartToCloseTimeoutSeconds = 60)
    int activity1();

    void activity2(int a);
} 

The interface allows you to group together a set of related activities. You can define any number of activities within an activities interface, and you can define as many activities interfaces as you want. Similar to @Execute and @Signal methods, activity methods can take any number of arguments of any type other than Promise<T> or its derivatives. The return type of an activity must not be Promise<T> or its derivatives.