minionpool

About

minionpool allows you to concurrently process any tasks you need to (similar to a worker pool). And it's very simple to use.

Installing it

The npm package is called minionpool.

How it works

First things first. To make use of this, you just have to provide a few callbacks, then instantiate a MinionPool and start() it. A MinionPool has Minions, that process tasks. Tasks are provided by a task source, which is called to get one task at a time, one for each minion started, and one for each minion that finishes a previous task.

Both the minions and the task source can keep a state, which is useful to keep database connections for example.

Polling for tasks vs Injecting tasks

You can either make the pool poll for tasks from the task source, or inject tasks asynchronously "from the outside". You can also find an example of polling from tasks in the mysql example, and one that injects tasks in the rabbitmq example. See below.

Quick Example

Main code

var minionpoolMod = require('minionpool');

var minionPool = new minionpoolMod.MinionPool(options);
minionPool.start();

Configuring the pool

Let's see now what can be defined inside the options that we're passing to the MinionPool constructor:

var options = {

  // A name for your minions pool, useful for debugging.
  name: 'test',

  // Optional. When 'true', some messages are written to console.
  debug: true,

  // How many minions to run concurrently.
  concurrency: 5,

  // Optional. Uses the given Function to log messages.
  logger: console.log,

  // Called to initialize a 'task source'. It should call the callback with
  // an initial 'state' (like db connections, file descriptors, etc). See below.
  // The state will be passed when calling the next property.
  taskSourceStart: function(callback) {
    callback(state);
  },

  // Polling for tasks: The task source produce 'tasks', that are assigned to
  // minions. A task source receives its state from 'taskSourceStart' first, and
  // then from whatever it returns on subsequen calls (see below). Also, it
  // should call the callback with the new task (or 'undefined' when none is found).
  //
  // If you are not going to poll for tasks, set this one to undefined, and call
  // MinionPool.injectTask() to inject the tasks into the pool, they will be 
  // assigned to free minions (or rescheduled if none is available at the time,
  // be careful not to produce too many tasks).
  taskSourceNext: function(state, callback) {
    var task = ...;
    callback(task);
    return state;
  },

  // Called to do any cleanup for the task generator once it runs out of tasks.
  // Receives the 'state', doesn't have to pass any arguments to the callback.
  taskSourceEnd: function(state, callback) {
    callback();
  },

  // The actual code that works on a task. Should call the 'callback' when
  // done, passing the new state.
  minionTaskHandler: function(task, state, callback) {
    callback(state);
  },

  // Called to initialize each one of the minions. Returns the initial state for
  // each one of them.
  minionStart: function(callback) {
    callback(state);
  },

  // Called to cleanup any needed stuff for each minion.
  minionEnd: function(state, callback) {
    callback();
  },

  // Called when the pool has finished all the available work.
  poolEnd: function() {
    process.exit(0);
  }
};

Example using MySQL (will process all rows in a given table).

See this.

Example using RabbitMQ

See this.

Example using the ArrayMinionPool

See this.

Into the details: Lifecycle (Polling)

• A pool starts.
• All minions in the pool are started. For each minion, minionStart is called.
• After all the minions are started, the task source is started, by calling taskSourceStart.
• After the task source starts one task per minion is requested by calling taskSourceNext.
• When a minion finishes processing a task (minionTaskHandler), a new one will be assigned to it.
• When a minion finishes a task, but the task source does not have a task for it, that minion will become idle, and shutdown, by calling minionEnd.
• When all the minions have shutdowned, the task source will shutdown too, by calling taskSourceEnd.
• When the task source shuts down, the pool will shut down, and poolEnd will be called.

Into the details: Lifecycle (Injected tasks)

• A pool starts.
• All minions in the pool are started. For each minion, minionStart is called.
• After all the minions are started, the task source is started, by calling taskSourceStart.
• Someone calls MinionPool.injectTask from the outside, a free minion is picked up and the task is assigned to it. Otherwise, the task is rescheduled internally until a free minion is available.
• The pool only shuts down when MinionPool.end() is called. Then all minions are finished.
• When all the minions have shutdowned, the task source will shutdown too, by calling taskSourceEnd.
• When the task source shuts down, the pool will shut down, and poolEnd will be called.