My preso on SimpleWorkFlow from the AWS-DC meetup

1. AWS
Simple WorkFlow
by Nathan McCourtney
May 2nd, 2012
@beaknit

2. "Workflow"?
A workflow management system is a computer system that
manages and defines a series of tasks within an
organization to produce a final outcome or outcomes.
At each stage in the workflow, one individual or group is
responsible for a specific task. Once the task is complete,
the workflow software ensures that the individuals
responsible for the next task are notified and receive the
data they need to execute their stage of the process
http://en.wikipedia.org/wiki/Workflow

3. Amazon's Workflow Model
● A Workflow is the automation of a business process.
● A Domain is a collection of related Workflows.
● Actions are the individual tasks undertaken to carry out
a Workflow.
● Activity Workers are the pieces of code that actually
implement the tasks. Each kind of Worker has its own
Activity Type.
● A Decider implements a Workflow's coordination logic.
- From Jeff Barr's post

4. Workers & Deciders = Actors
From wikipedia:
In computer science, the Actor model is a mathematical
model of concurrent computation that treats "actors" as the
universal primitives of concurrent digital computation: in
response to a message that it receives, an actor can make
local decisions, create more actors, send more messages,
and determine how to respond to the next message
received.

5. Essentially, SWF is a framework for
abstracting out actors and passing
messages between them via queues

6. Act!
Work
Queue
Actor
Messages
1. Fetches its work
2. Drops off its results
Result
Queue

7. AWS Message Passing
● HTTPS
● Long-poll interval of 60 seconds
● Message format is JSON:
{"decisionType": "ScheduleActivityTask",
"scheduleActivityTaskDecisionAttributes":
{"activityType":
{"name": "activityVerify",
"version": "1.0"},
"activityId": "verification-27",
"control": "digital music",
"input": "5634-0056-4367-0923,12/12,437",
"scheduleToCloseTimeout": "900",
"taskList":
{"name": "specialTaskList"},
"scheduleToStartTimeout": "300",
"startToCloseTimeout": "600",
"heartbeatTimeout": "120"}
}
● One decision queue per Workflow
● Many activity queues per Workflow

8. Decider to SWF Comms
SWF
Decider
DTL ATL
1. Decider polls Decision Task List (DTL) for new
decision tasks
2. Decision task returned with entire workflow
execution history to provider context for
decisions
3. Decider executes conditional logic and returns
an activity task back to SimpleWorkFlow which
then deposits in the Activity Task List (ATL)
Note: Multiple deciders can process the same DTL

9. From AWS Ruby SDK
# poll for decision tasks from 'my-task-list'
domain.decision_tasks.poll( 'my-task-list' ) do |task|
# investigate new events and make decisions
task.new_events.each do |event|
case event.event_type
when 'WorkflowExecutionStarted'
task.schedule_activity_task 'do-something' , :input => 'abc xyz'
when 'ActivityTaskCompleted'
task.complete_workflow_execution :result => event.attributes.result
end
end
end # decision task is completed here

10. Worker to SWF Comms
SWF
Worker
ATL DTL
1. Worker polls ATL for appropriate tasks - different
workers can consume different tasks
2. Task completion returned to SWF which makes
the result available to the Decider via the DTL
Note: Multiple workers can process the same ATL

11. From AWS Ruby SDK
# poll 'my-task-list' for activities
domain.activity_tasks.poll( 'my-task-list' ) do |activity_task|
case activity_task.activity_type.name
when 'do-something'
# ...
else
activity_task.fail! :reason => 'unknown activity task type'
end
end

12. AWS example order-processing SWF
Source: SWF Dev Guide

13. Practical Exercise
Nightly restore of a database to QA nodes combined with automated release
migration.

14. Examples

16. Further Reading
● Official Announcement
● Rightscale's blog on their use
● Dev Guide - Basic Concepts
● AWS Ruby SDK