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Amazon Simple Workflow Service
Developer Guide (API Version 2012-01-25)

Subscription Workflow Tutorial Part 2: Implementing the Workflow

Up until now, our code has been pretty generic. This is the part where we begin to really define what our workflow does, and what activities we'll need to implement it.

Designing the Workflow

If you recall, the initial idea for this workflow consisted of the following steps:

  1. Get a subscription address (email or SMS) from the user.

  2. Create an SNS topic and subscribe the provided endpoints to the topic.

  3. Wait for the user to confirm the subscription.

  4. If the user confirms, publish a congratulatory message to the topic.

We can think of each step in our workflow as an activity that it must perform. Our workflow is responsible for scheduling each activity at the appropriate time, and coordinating data transfer between activities.

For this workflow, we'll create a separate activity for each of these steps, naming them descriptively:

  1. get_contact_activity

  2. subscribe_topic_activity

  3. wait_for_confirmation_activity

  4. send_result_activity

These activities will be executed in order, and data from each step will be used in the subsequent step.

We could design our application so that all of the code exists in one source file, but this runs contrary to the way that Amazon SWF was designed. It is designed for workflows that can span the entire Internet in scope, so let's at least break the application up into two separate executables:

  • swf_sns_workflow.rb - Contains the workflow and workflow starter.

  • swf_sns_activities.rb - Contains the activities and activities starter.

The workflow and activity implementations can be run in separate windows, separate computers, or even different parts of the world. Because Amazon SWF is keeping track of the details of your workflows and activities, your workflow can coordinate scheduling and data transfer of your activities no matter where they are running.

Setting up our Workflow Code

We'll begin by creating a file called swf_sns_workflow.rb. In this file, declare a class called SampleWorkflow. Here is the class declaration and its constructor, the initialize method.

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require_relative 'utils.rb' # SampleWorkflow - the main workflow for the SWF/SNS Sample # # See the file called `README.md` for a description of what this file does. class SampleWorkflow attr_accessor :name def initialize(task_list) # the domain to look for decision tasks in. @domain = init_domain # the task list is used to poll for decision tasks. @task_list = task_list # The list of activities to run, in order. These name/version hashes can be # passed directly to AWS::SimpleWorkflow::DecisionTask#schedule_activity_task. @activity_list = [ { :name => 'get_contact_activity', :version => 'v1' }, { :name => 'subscribe_topic_activity', :version => 'v1' }, { :name => 'wait_for_confirmation_activity', :version => 'v1' }, { :name => 'send_result_activity', :version => 'v1' }, ].reverse! # reverse the order... we're treating this like a stack. register_workflow end

As you can see, we are keeping the following class instance data:

  • domain - The domain name retrieved from init_domain in utils.rb.

  • task_list - The task list passed in to initialize.

  • activity_list - The activity list, which has the names and versions of the activities we'll run.

The domain name, activity name, and activity version are enough for Amazon SWF to positively identify an activity type, so that is all of the data we need to keep about our activities in order to schedule them.

The task list will be used by the workflow's decider code to poll for decision tasks and schedule activities.

At the end of this function, we call a method we haven't yet defined: register_workflow. We'll define this method next.

Registering the Workflow

To use a workflow type, we must first register it. Like an activity type, a workflow type is identified by its domain, name, and version. Also, like both domains and activity types, you cannot re-register an existing workflow type. If you need to change anything about a workflow type, you must provide it with a new version, which essentially creates a new type.

Here is the code for register_workflow, which is used to either retrieve the existing workflow type we registered on a previous run or to register the workflow if it has not yet been registered.

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# Registers the workflow def register_workflow workflow_name = 'swf-sns-workflow' @workflow_type = nil # a default value... workflow_version = '1' # Check to see if this workflow type already exists. If so, use it. @domain.workflow_types.each do | a | if (a.name == workflow_name) && (a.version == workflow_version) @workflow_type = a end end if @workflow_type.nil? options = { :default_child_policy => :terminate, :default_task_start_to_close_timeout => 3600, :default_execution_start_to_close_timeout => 24 * 3600 } puts "registering workflow: #{workflow_name}, #{workflow_version}, #{options.inspect}" @workflow_type = @domain.workflow_types.register(workflow_name, workflow_version, options) end puts "** registered workflow: #{workflow_name}" end

First, we check to see if the workflow name and version is already registered by iterating through the domain's workflow_types collection. If we find a match, we'll use the workflow type that was already registered.

If we don't find a match, then a new workflow type is registered (by calling register on the same workflow_types collection that we were searching for the workflow in) with the name 'swf-sns-workflow', version '1', and the following options.

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options = { :default_child_policy => :terminate, :default_task_start_to_close_timeout => 3600, :default_execution_start_to_close_timeout => 24 * 3600 }

Options passed in during registration are used to set default behavior for our workflow type, so we don't need to set these values every time we start a new workflow execution.

Here, we just set some timeout values: the maximum time it can take from the time a task starts to when it closes (one hour), and the maximum time it can take for the workflow execution to complete (24 hours). If either of these times are exceeded, the task or workflow will timeout.

For more information about timeout values, see Amazon SWF Timeout Types .

Polling for Decisions

At the heart of every workflow execution there is a decider. The decider's responsibility is for managing the execution of the workflow itself. The decider receives decision tasks and responds to them, either by scheduling new activities, cancelling and restarting activities, or by setting the state of the workflow execution as complete, cancelled, or failed.

The decider uses the workflow execution's task list name to receive decision tasks to respond to. To poll for decision tasks, call poll on the domain's decision_tasks collection to loop over available decision tasks. You can then check for new events in the decision task by iterating over its new_events collection.

The returned events are AWS::SimpleWorkflow::HistoryEvent objects, and you can get the type of the event by using the returned event's event_type member. For a list and description of history event types, see HistoryEvent in the Amazon Simple Workflow Service API Reference.

Here is the beginning of the decision task poller's logic. A new method in our workflow class called poll_for_decisions.

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def poll_for_decisions # first, poll for decision tasks... @domain.decision_tasks.poll(@task_list) do | task | task.new_events.each do | event | case event.event_type

We'll now branch the execution of our decider based on the event_type that is received. The first one we are likely to receive is WorkflowExecutionStarted. When this event is received, it means that Amazon SWF is signaling to your decider that it should begin the workflow execution. We'll begin by scheduling the first activity by calling schedule_activity_task on the task we received while polling.

We'll pass it the first activity we declared in our activity list, which, because we reversed the list so we can use it like a stack, occupies the last position on the list. The "activities" we defined are just maps consisting of a name and version number, but this is all that Amazon SWF needs to identify the activity for scheduling, assuming that the activity has already been registered.

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when 'WorkflowExecutionStarted' # schedule the last activity on the (reversed, remember?) list to # begin the workflow. puts "** scheduling activity task: #{@activity_list.last[:name]}" task.schedule_activity_task( @activity_list.last, { :task_list => "#{@task_list}-activities" } )

When we schedule an activity, Amazon SWF sends an activity task to the activity task list that we pass in while scheduling it, signaling the task to begin. We'll deal with activity tasks in Subscription Workflow Tutorial Part 3: Implementing the Activities, but it is worth noting that we don't execute the task here. We only tell Amazon SWF that it should be scheduled.

The next activity that we'll need to address is the ActivityTaskCompleted event, which occurs when Amazon SWF has received an activity completed response from an activity task.

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when 'ActivityTaskCompleted' # we are running the activities in strict sequential order, and # using the results of the previous activity as input for the next # activity. last_activity = @activity_list.pop if(@activity_list.empty?) puts "!! All activities complete! Sending complete_workflow_execution..." task.complete_workflow_execution return true; else # schedule the next activity, passing any results from the # previous activity. Results will be received in the activity # task. puts "** scheduling activity task: #{@activity_list.last[:name]}" if event.attributes.has_key?('result') task.schedule_activity_task( @activity_list.last, { :input => event.attributes[:result], :task_list => "#{@task_list}-activities" } ) else task.schedule_activity_task( @activity_list.last, { :task_list => "#{@task_list}-activities" } ) end end

Since we are executing our tasks in a linear fashion, and only one activity is executing at once, we'll take this opportunity to pop the completed task from the activity_list stack. If this results in an empty list, then we know that our workflow is complete. In this case, we signal to Amazon SWF that our workflow is complete by calling complete_workflow_execution on the task.

In the event that the list still has entries, we'll schedule the next activity on the list (again, in the last position). This time, however, we'll look to see if the previous activity returned any result data to Amazon SWF upon completion, which is provided to the workflow in the event's attributes, in the optional result key. If the activity generated a result, we'll pass it as the input option to the next scheduled activity, along with the activity task list.

By retrieving the result values of completed activities, and by setting the input values of scheduled activities, we can pass data from one activity to the next, or we can use data from an activity to change behavior in our decider based on the results from an activity.

For the purposes of this tutorial, these two event types are the most important in defining the behavior of our workflow. However, an activity can generate events other than ActivityTaskCompleted. We'll wrap up our decider code by providing demonstration handler code for the ActivityTaskTimedOut and ActivityTaskFailed events, and for the WorkflowExecutionCompleted event, which will be generated when Amazon SWF processes the complete_workflow_execution call that we make when we run out of activities to run.

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when 'ActivityTaskTimedOut' puts "!! Failing workflow execution! (timed out activity)" task.fail_workflow_execution return false when 'ActivityTaskFailed' puts "!! Failing workflow execution! (failed activity)" task.fail_workflow_execution return false when 'WorkflowExecutionCompleted' puts "## Yesss, workflow execution completed!" task.workflow_execution.terminate return false end end end end

Starting the Workflow Execution

Before any decision tasks will be generated for the workflow to poll for, we need to start the workflow execution.

To start the workflow execution, call start_execution on your registered workflow type (AWS::SimpleWorkflow::WorkflowType). We'll define a small wrapper around this to make use of the workflow_type instance member that we retrieved in the class constructor.

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def start_execution workflow_execution = @workflow_type.start_execution( { :task_list => @task_list } ) poll_for_decisions end end

Once the workflow is executing, decision events will begin to appear on the workflow's task list, which is passed as a workflow execution option in start_execution.

Unlike options that are provided when the workflow type is registered, options that are passed to start_execution are not considered to be part of the workflow type. You are free to change these per workflow execution without changing the workflow's version.

Since we'd like the workflow to begin executing when we run the file, add some code that instantiates the class and then calls the start_execution method that we just defined.

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if __FILE__ == $0 require 'securerandom' # Use a different task list name every time we start a new workflow execution. # # This avoids issues if our pollers re-start before SWF considers them closed, # causing the pollers to get events from previously-run executions. task_list = SecureRandom.uuid # Let the user start the activity worker first... puts "" puts "Amazon SWF Example" puts "------------------" puts "" puts "Start the activity worker, preferably in a separate command-line window, with" puts "the following command:" puts "" puts "> ruby swf_sns_activities.rb #{task_list}-activities" puts "" puts "You can copy & paste it if you like, just don't copy the '>' character." puts "" puts "Press return when you're ready..." i = gets # Now, start the workflow. puts "Starting workflow execution." sample_workflow = SampleWorkflow.new(task_list) sample_workflow.start_execution end

To avoid any task list naming conflicts, we'll use SecureRandom.uuid to generate a random UUID that we can use as the task list name, guaranteeing that a different task list name is used for each workflow execution.

Note

Task lists are used to record events about a workflow execution, so if you use the same task list for multiple executions of the same workflow type, you might get events that were generated during a previous execution, especially if you are running them in near succession to each other, which is often the case when trying out new code or running tests.

To avoid the issue of having to deal with artifacts from previous executions, we can use a new task list for each execution, specifying it when we begin the workflow execution.

There is also a bit of code here to provide instructions for the person running it (probably you), and to provide the "activity" version of the task list. The decider uses this task list name to schedule activities for the workflow, and the activities implementation will listen for activity events on this task list name to know when to begin the scheduled activities and to provide updates about the activity execution.

The code also waits for the user to start running the activities starter before it starts the workflow execution, so the activities starter will be ready to respond when activity tasks begin appearing on the provided task list.

Next Steps

We've completed the workflow implementation. Next, we'll define the activities and an activities starter, in Subscription Workflow Tutorial Part 3: Implementing the Activities.