decider.md

Deciders

A decider listens for requests from Amazon for workflow events, and funnel the requests through the ordered workflow tasks.

The deciders require you to create a configured AWS SWF client with the aws-sdk. You also need to create your workflow pipeline. Once you have these created, you can start a decider:

const swf = require('swiffer-decider');
const AWS = require('aws-sdk');
var swfClient = new AWS.SWF({ /* config values */ });
var workflowPipeline = swf.createSerialPipeline([ /* tasks */ ]);

var decider = swf.createDecider(workflowPipeline, swfClient, {
  domain: 'My Domain',
  identity: 'Unique Decider ID',
  taskList: name: 'workflow event channel name'
});

When the decider is ready to begin processing events, you can start it:

decider.start();

Decider Setup

The decider creation method, createDecider takes 3 arguments:

• pipeline - the top-level pipeline of tasks.
• swfClient - the AWS sdk SWF object, created by the a call to new require('aws-sdk').SWF(config)
• config - configuration options for the decider.
  • identity - the name by which the decider declares itself to SWF. This allows for tracing the workflow history to the decider that handled the workflow history.
  • taskList - an object with a name set to the task list that the decider polls for decision tasks. Can be a string, in which case the object will be created for you.
  • domain - the name of the domain containing the task list.

Decider Events

On Poll Response

When the AWS request returns with the decision (or lack of a decision), the decision object is sent to the poll event listeners:

decider.on('poll', function (config) {
  var identity = config.identity,
  var taskListName = config.taskList.name;
});

On Decision Event Processing

When a decider begins processing a decision event, the decider event is fired, with the decision event as the event object.

decider.on('decider', function (evt) {
  console.log(`Processing decision event ${JSON.stringify(evt)}`);
});

See the (swf api docs) for the contents of the event object.

On Internal Errors

If the decider generates an internal error, the error event is fired, with the exception as the event object.

decider.on('error', function (evt) {
  console.error(`${evt.message}\n${evt.stack}`);
});

On Computing Decision Actions

When the actions to run based on a workflow event are computed, the list of actions are sent to the actions event listeners.

decider.on('actions', function (actions) {
  console.log(`Sending list of actions: ${JSON.stringify(actions)}`);
});

On Decision Response

When the list of decisions is ready to be sent to SWF, the list is sent to the decisions event listeners.

decider.on('decisions', function (decision) {
  console.log(`Sending list of decision: ${JSON.stringify(decision)}`);
});

On Workflow Failures

If a workflow encounters an issue that causes it to be failed, a failure event is sent to the failure event listeners.

decider.on('failure', function (err) {
  console.log(`Workflow failed: ${evt.message}`);
});

Details on SWF and Deciders

Under the covers, deciders perform a "long poll" HTTP request to the Amazon servers. The request can wait up to 60 seconds, before the request returns with either no event data, or a decision event.

The request to listen for events passes a specific task list name. When a workflow starts, it is assigned to a task list, either by the registered workflow type, or by the workflow start request. The deciders listening to that task list name will receive the decision event.

Currently, the decider API allows for only one workflow to run on a single task list. If you need to support multiple workflows, each one needs to listen on its own decider, with its own dedicated task list.

Additionally, the registered workflow types, activity types, and task lists are segregated from each other with a domain. Workflows running on one domain do not send events to other domains.