Introduction to Amazon Simple Workflow Service (Amazon SWF)

Used to build applications that coordinate work across distributed components. In it, a task represents a logical unit of work performed by a component of application.
Coordinating tasks across application requires managing
- inter-task dependencies
- scheduling
- concurrency
SWF gives control over
implementing and coordinating tasks
tracking their progress
maintaining their state.
required to implement workers to perform tasks
workers can run on EC2, or on user’s premises.
can create long-running tasks
- SWF
- stores tasks
- assigns them to workers when they are ready
- monitors their progress
- maintains their state, including details on their completion.

Workflows

used to coordinate and manage execution of activities which
run asynchronously
across multiple computing devices
feature both sequential and parallel processing
when designing a workflow, analyze application to identify its component tasks,
Tasks in SWF are ‘activities’.
workflow’s coordination logic determines order in which activities are executed.

Workflow Domains

It scopes SWF resources within AWS account.
a domain for all components of a workflow is needed like workflow type and activity types
workflows in different domains cannot interact with one another.

Workflow History

A detailed, complete, and consistent record of every event occurring since workflow execution started.
An event represents a discrete change in workflow execution’s state
Events can be
- scheduled and completed activities
- task timeouts
- signals

Actors

Workflow Starter
- is any application that can initiate workflow executions.
Deciders
- logic that coordinates tasks in a workflow
- It schedules activity tasks and provides input data to activity workers
- It also processes events arriving while workflow is in progress and closes workflow when objective has been completed.
Activity Worker
- a single computer process (or thread) performing activity tasks in workflow.
- different activity workers process tasks of different activity types
- multiple activity workers can process same type of task.

Amazon SWF Tasks

three types of tasks – Activity tasks, AWS Lambda tasks, and Decision tasks.
Activity Task –commands an activity worker to perform its function. Has information that activity worker needs to perform its function.
AWS Lamda –same as an activity task, but executes Lambda function instead of SWF activity.
Decision Task –commands a decider. contains current workflow history.

Task Lists

organize various tasks associated with a workflow
similar to dynamic queues
when a task is scheduled in SWF, specify a queue (task list) to put it in
During polling SWF for a task, determine which queue (task list) to get task from.
Task lists helps to route tasks to workers as per use case.
They are dynamic and no need to register a task list or explicitly create it

Long Polling

Deciders and activity workers communicate with SWF using long polling
decider or activity worker periodically
- initiates communication with SWF
- notify SWF of its availability to accept a task
- specify task list to get tasks from
Works well for high-volume task processing.
Deciders and activity workers can manage their own capacity.

Workflow Execution Closure

Once we start a workflow execution, it is open.
open workflow execution can be closed as
- completed
- canceled
- failed
- timed out
can also be continued as a new execution, or terminated.
decider, person administering workflow, or Amazon SWF can close a workflow execution.

Lifecycle of a Workflow Execution

SWF interacts with actors by assigning them appropriate tasks either activity or decision tasks.
Starting from beginning of a workflow execution to its completion,
Below, lifecycle of order processing workflow execution from components that act on it.