swfP

A framework for building applications on AWS SWF using Promises

swfP makes it easier to build decision and activity workers on AWS SWF by using Promises (via Bluebird) in two ways:

• Workflow definition

  A decider is implemented as a Promise chain. By treating primitives such as activities, timers and child workflows as Promises, it becomes easy to implement parallel and serial execution with .all() and .then()

• Activity execution

  An activity's implementation may return a Promise. This allows an activity to declare success or failure based upon fulfillment or rejection.

Usage

Install with:

npm install --save swfp

Decider

A decider is implemented as a module, which exports a single function. When called, this function should return a Promise, like so:

module.exports = (input, schedule) =>
  schedule.Promise
    .all([
  	  schedule.activity('firstUpper', input),
	  schedule.activity('restLower', input)
    ])
    .then(parts => schedule.activity('concat', parts));

The decider function accepts parameters input (which refers to the original workflow input) and schedule, which exposes the following properties:

• Promise Refers to Bluebird instead of the ES6 implementation

• activity(name, input, options) Starts an activity named name with input parameter input. Additional, optional SWF attributes may be specified with options.

  Returns a Promise which is resolved with the returned result of the Activity when it completed, or rejects with the error when it fails.

• timer(name, seconds) Starts a Timer with name.

  Returns a Promise which resolves to null after seconds have elasped.

• childWorkflow(name, input, options) Starts an activity named name with input parameter input. Additional, optional SWF attributes may be specified with options.

  Returns a Promise which is resolved with the returned result of the Workflow when it completed, or rejects with the error when it fails.

• signal(name) Returns a Promise which is resolved to the value given when a signal named name was called on the current Workflow execution

You can then start a worker for this decider by invoking the following command:

swfp-decider --file=./decider.js --domain=myDomain --taskList=myTasks

Activity Worker

An activity worker exports an object containing activity implementations, like so:

module.exports = {
	firstUpper: str => str[0].toUpperCase(),
	restLower:  str => str.substring(1).toLowerCase(),
	concat:     str => str.join('')
};

Each exported function accepts a single parameter, which is the input value that was supplied to the activity's invocation when it was executed by the Workflow.

Each exported function may do one of the following:

• Return a non-Promise value to mark the execution as successful
• Throw an Error to mark the execution as failed
• Return a Promise to mark the execution as succesful or failed when the Promise is settled

You can then start a worker for these activites by invoking the following command:

swfp-worker --file=./tasks.js --domain=myDomain --taskList=myTasks

Caveats and Notes

• The Promise chain that a Decider exports only gives the illusion of a persistent state; the Decision poller destroys the object at the end of each decision execution.
• If you use any Promises within the Decider outside of those described here, they must resolve before the next tick, or the decider will hang.
• If you need to execute long-running code, implement it as an Activity instead.