# AWS Flow Framework for Java Replay Behavior
<a name="programming-replay"></a>

This topic discusses examples of replay behavior, using the examples in the [What is the AWS Flow Framework for Java?](welcome.md) section. Both [synchronous](#programming-replay-synchronous) and [asynchronous](#programming-replay-asynchronous) scenarios are discussed.

## Example 1: Synchronous Replay
<a name="programming-replay-synchronous"></a>

For an example of how replay works in a synchronous workflow, modify the [HelloWorldWorkflow](getting-started-example-helloworldworkflow.md) workflow and activity implementations by adding `println` calls within their respective implementations, as follows:

```
public class GreeterWorkflowImpl implements GreeterWorkflow {
...
   public void greet() {
      System.out.println("greet executes");
      Promise<String> name = operations.getName();
      System.out.println("client.getName returns");
      Promise<String> greeting = operations.getGreeting(name);
      System.out.println("client.greeting returns");
      operations.say(greeting);
      System.out.println("client.say returns");
   }
}
**************
public class GreeterActivitiesImpl implements GreeterActivities {
   public String getName() {
      System.out.println("activity.getName completes");
      return "World";
   }

   public String getGreeting(String name) {
      System.out.println("activity.getGreeting completes");
      return "Hello " + name + "!";
   }

   public void say(String what) {
      System.out.println(what);
   }
}
```

For details about the code, see [HelloWorldWorkflow Application](getting-started-example-helloworldworkflow.md). The following is an edited version of the output, with comments that indicate the start of each replay episode.

```
//Episode 1
greet executes
client.getName returns
client.greeting returns
client.say returns

activity.getName completes
//Episode 2
greet executes
client.getName returns
client.greeting returns
client.say returns

activity.getGreeting completes
//Episode 3
greet executes
client.getName returns
client.greeting returns
client.say returns

Hello World! //say completes
//Episode 4
greet executes
client.getName returns
client.greeting returns
client.say returns
```

The replay process for this example works as follows:
+ The first episode schedules the `getName` activity task, which has no dependencies.
+ The second episode schedules the `getGreeting` activity task, which depends on `getName`.
+ The third episode schedules the `say` activity task, which depends on `getGreeting`.
+ The final episode schedules no additional tasks and finds no uncompleted activities, which terminates the workflow execution. 

**Note**  
The three activities client methods are called once for each episode. However, only one of those calls results in an activity task, so each task is performed only once.

## Example 2: Asynchronous Replay
<a name="programming-replay-asynchronous"></a>

Similarly to the [synchronous replay example](#programming-replay-synchronous), you can modify [HelloWorldWorkflowAsync Application](getting-started-example-helloworldworkflowasync.md) to see how an asynchronous replay works. It produces the following output:

```
//Episode 1
greet executes
client.name returns
workflow.getGreeting returns
client.say returns

activity.getName completes
//Episode 2
greet executes
client.name returns
workflow.getGreeting returns
client.say returns
workflow.getGreeting completes

Hello World! //say completes
//Episode 3
greet executes
client.name returns
workflow.getGreeting returns
client.say returns
workflow.getGreeting completes
```

HelloWorldAsync uses three replay episodes because there are only two activities. The `getGreeting` activity was replaced by the *getGreeting* asynchronous workflow method, which doesn't initiate a replay episode when it completes.

The first episode doesn't call `getGreeting`, because it depends on the completion of the *name* activity. However, after *getName* completes, replay calls *getGreeting* once for each succeeding episode.

## See Also
<a name="see-also"></a>
+ [AWS Flow Framework Basic Concepts: Distributed Execution](awsflow-basics-distributed-execution.md)