AWS Developer Blog

New Sample Simple Workflow

by Norm Johanson | on | in .NET | Permalink | Comments |  Share

When you install the SDK from our website, many samples are installed inside Visual Studio, including the Express editions of Visual Studio. Look in the New Project Wizard, where you’ll find samples showing off many of the AWS services.

 

We recently added a new sample that shows off using Amazon Simple Workflow Service (SWF) with the .NET SDK. The sample is under AWS -> App Services section and is called AWS Simple Workflow Image Processing Sample. The sample shows how to use SWF to monitor images coming from S3 and to generate thumbnails of various sizes. In a real-world scenario, this would most likely be done with multiple processes monitoring SWF for decision and activity tasks. This sample is set up as WPF app hosting virtual consoles, each representing an individual process to make it easier to run the sample.

 

The virtual console on the top is the process that chooses an image to generate thumbnails for and starts the workflow execution.

// Snippet from StartWorkflowExecutionProcessor.cs that starts the workflow execution

swfClient.StartWorkflowExecution(new StartWorkflowExecutionRequest
{
    // Serialize input to a string
    Input = Utils.SerializeToJSON(input),
    //Unique identifier for the execution
    WorkflowId = DateTime.Now.Ticks.ToString(),
    Domain = Constants.ImageProcessingDomain,
    WorkflowType = new WorkflowType
    {
        Name = Constants.ImageProcessingWorkflow,
        Version = Constants.ImageProcessingWorkflowVersion
    }
});

 

The virtual console in the bottom left monitors SWF for decision tasks. When it gets a decision task, it looks at the workflow’s history and sees what activities have been completed to figure out which thumbnail hasn’t be created yet. If one of the thumbnail sizes hasn’t been created yet, it schedules an activity to create the next thumbnail sizes. If all the thumbnails have been created, it completes the workflow.

// Snippet from ImageProcessWorkflow.cs that polls for decision tasks and decides what decisions to make.

void PollAndDecide()
{
    this._console.WriteLine("Image Process Workflow Started");
    while (!_cancellationToken.IsCancellationRequested)
    {
        DecisionTask task = Poll();
        if (!string.IsNullOrEmpty(task.TaskToken))
        {
            // Create the next set of decisions based on the current state and
            // the execution history
            List decisions = Decide(task);

            // Complete the task with the new set of decisions
            CompleteTask(task.TaskToken, decisions);
        }
    }
}

 

The virtual console in the bottom right monitors SWF for activity tasks to perform. The activity task will have input from the decider process that tells what image to create a thumbnail for and what size of thumbnail.

// Snippet from ImageActivityWorker.cs showing the main loop for the worker that polls for tasks and processes them.

void PollAndProcessTasks()
{
    this._console.WriteLine("Image Activity Worker Started");
    while (!_cancellationToken.IsCancellationRequested)
    {
        ActivityTask task = Poll();
        if (!string.IsNullOrEmpty(task.TaskToken))
        {
            ActivityState activityState = ProcessTask(task.Input);
            CompleteTask(task.TaskToken, activityState);
        }
    }
}