meteor-cluster

Meteor Package enabling users to create a Worker Pool on the server to handle heavy jobs.

It can run synchronous and asynchronous tasks from a persitent / in-memory queue.

It can also run recurring and scheduled tasks.

TaskQueue

TaskQueue is both a Mongodb and an in-memory backed job queue.

It enables to add, update, remove jobs consistently between processes.

You can attach event listeners to handle the tasks results / errors

prototype

TaskQueue.addTask({ taskType: String, data: Object, priority: Integer, _id: String, dueDate: Date, inMemory: Boolean })

• taskType is mandatory
• data is mandatory but you can pass an empty object
• priority is mandatory, default is set to 1
• _id is optional
• dueDate is mandatory, default is set to new Date()
• inMemory is optional, default is set to false
  

Event listeners (Master only) :

TaskQueue.addEventListener(eventType: String, callback: function)

• eventType is one of [ 'done', 'error' ]

• callback is a function prototyped as callback({ value: Any, task: Task }), value contains the result / error.
  

  TaskQueue.removeEventListener(eventType: String)
  

• eventType is one of [ 'done', 'error' ]
  

  note : you can only attach one event listener by eventType.
  

In-Memory Queue (Master only) :

TaskQueue.inMemory.findById(_id: String)


TaskQueue.inMemory.removeById(_id: String)


TaskQueue.inMemory.tasks() : returns all in-memory tasks


TaskQueue.inMemory.availableTasks() : returns available in-memory tasks

note on the in-memory / persistent task queue

Both in-memory and persistent tasks are available at the same time, and can be used altogether but :

• in-memory tasks can only be created on the Master (which is because it’s non persistent…)
• in-memory tasks will always be called first over persistent tasks even if their respective priority are greater.
• if you use both in-memory and persistent tasks at the same time, the persistent tasks will be called only when no in-memory tasks are available (may change later).
  
  

Cluster

Cluster is an isomorphic class to handle both the Worker and the Master


on the Master it :

• verifies if jobs are in the queue
• verifies if workers are available, or create them
• dispatches jobs to the workers
• removes the task from the queue once the job is done

• closes the workers when no jobs are available(behavior can be overriden with keepAlive)

  on the Worker it :

• starts the job
• when the job’s done, tell the Master that it’s available and to remove previous task.

prototype

constructor(taskMap: Object, { port: Integer, maxAvailableWorkers: Integer, refreshRate: Integer, inMemoryOnly: Boolean, messageBroker: function, logs: String, keepAlive: String | Integer, autoInitialize: Boolean })

• taskMap: a map of functions associated to a taskType
• maxAvailableWorkers: maximum number of child process (cores), default is set to system maximum
• port: server port for child process servers, default set to 3008
• refreshRate: Worker pool refresh rate (in ms), default set to 1000
• inMemoryOnly: force the cluster to only pull jobs from the in-memory task queue.
• messageBroker is optional, default set to null (see IPC section)
  
• logs: is one of ['all', 'error'], default sets to all : if set to 'error', will only show the errors and warning logs.

• keepAlive: an optional parameter that can be set to either:

  • 'always' to have the system start up the maxAvailableWorkers number of workers immediately and keep them all alive always
  • some Integer value will have the system not shutdown workers until the number passed in milliseconds has passed since last a job was available to be picked up by a worker

  NOTE: default behavior when keepAlive is not set is to only keep alive workers when there are jobs available to be picked up by them.

• autoInitialize: an optional parameter that controls whether the system will automatically initialize the cluster’s polling for jobs when the MasterCluster is created. Default is set to true.

  Cluster.isMaster(): true if this process is the master
  

  Cluster.maxWorkers(): returns the maximum number of workers available at the same time
  

  setRefreshRate(refreshRate: Integer): change the refresh rate on the master

  if the Master process crashes or restarts, all the unfinished jobs will be restarted from the beginning.
  
  Each job is logged when started / finished with the format : ${timestamp}cluster:${taskType}:${taskId}
  

IPC (advanced usage)

Introduced in version 2.0.0, you can communicate between the child processes and the Master.
To do so, you must provide the Master Cluster instance with a messageBroker function.
this function will handle (on the master) all custom messages from the child processes.

the function should be prototype as follow :

messageBroker(respond: function, msg: { status: Int > 1, data: Any })

• respond enables you to answer to a message from a child
  

All communications between the master and a child must be started by the child.
To do so you can use the second parameter passed in all functions provided to the taskMap toggleIPC which is prototyped as follow :

toggleIPC(messageBroker: function, initalize: function): Promise

• messageBroker is prototyped as messageBroker(msg: Any)
• initialize is prototyped as initialize(sendMessageToMaster: function)
  

because toggleIPC returns a promise you must return it (recursively), otherwise the job will be considered done, and the worker Idle.

Not returning it will result in unwanted, non expected behavior.

CPUS allocation

You should not use the default maxAvailableWorkers (cpus allocation number) value.
The default value is set to your system cpus number, but it’s a reference value.
It’s up to you to understand your needs and allocate cpus accordingly.

how can I calculate the maximum number of cpus I can allocate ?

for example, if you’re running on a 8 core machine :

• The app you’re running is using 1 cpu to run.
• You should have a reverse proxy on your server, you should at least save 1 cpu (may be more depending on your traffic).
• the database you’re using is hosted on the same server, you should save 1 cpu for it.
• you’re running an external service such as Redis or Elastic Search, so that’s 1 down.

so you should have maxAvailableWorkers = Cluster.maxWorkers() - 4 === 4

what if I allocated too much CPUS ?

You can’t allocate more than your maximum system cpu number.

You still can outrange the theoretical maximum process number :

in such case your overall system should be slowed down because some of the processes execution will be deferred.
It will drastically reduce the multi-core performance gain.

examples

basic usage

  import { Meteor } from 'meteor/meteor'
  import { Cluster, TaskQueue } from 'meteor/nschwarz:cluster'
  const taskMap = {
    'TEST': job => console.log(`testing ${job._id} at position ${job.data.position}`),
    'SYNC': (job) => console.log("this is a synchronous task"),
    'ASYNC': (job) => new Promise((resolve, reject) => Meteor.setTimeout(() => {
      console.log("this is an asynchronous task")
      resolve()
    }, job.data.timeout))
  }
  function onJobsDone({ value, task }) {
    console.log('do something with the result')
  }
  function onJobsError({ value, task }) {
    console.log('do something with the error')
  }
  function syncTask() {
    return TaskQueue.addTask({ taskType: 'SYNC', data: {}})
  }
  function asyncTask() {
    return TaskQueue.addTask({ taskType: 'ASYNC', data: { timeout: 5000 }, priority: 6 })
  }
  function inMemoryTask(priority, position) {
   return TaskQueue.addTask({ taskType: 'TEST', priority, data: { position }, inMemory: true })
  }
  function persistentTask(priority, position) {
   return TaskQueue.addTask({ taskType: 'TEST', priority, data: { position }, inMemory: false })
  }
  const cluster = new Cluster(taskMap)
  Meteor.startup(() => {
    if (Cluster.isMaster()) {
      TaskQueue.addEventListener('done', onJobsDone)
      TaskQueue.addEventListener('error', onJobsError)
      syncTask()
      asyncTask()
      inMemoryTask(8, 1)
      inMemoryTask(1, 2)
      persistentTask(8, 1)
      persistentTask(1, 2)
    }
  })

scheduled task example : run a task in ten minutes

  import { add } from 'date-fns/date' // external library to handle date objects
  const dueDate = add(new Date(), { minutes: 10 })
  TaskQueue.addTask({ taskType: 'sometype', priority: 1, data: {}, dueDate })

scheduled task example : run a recurring task every ten minutes

  import { add } from 'date-fns/date' // external library to handle date objects
  function recurringTask(job) {
    // do something
    const dueDate = add(new Date(), { minutes: 10 })
    TaskQueue.addTask({ taskType: 'recurringTask', priority: 1, data: {}, dueDate })
  }
  const taskMap = {
    recurringTask
  }

simple IPC example (advanced usage)

function ipcPingTest(job, toggleIPC) {
  return toggleIPC(
    (msg) => {
      console.log(msg)
      return 'result you eventually want to pass to the master'
    }, (smtm) => smtm({ status: 4, data: 'ping' })
  )
}
const taskMap = {
  ipcPingTest
}
function messageBroker(respond, msg) {
  if (msg.data === 'ping') {
    respond('pong')
  }
}
const cluster = new Cluster(taskMap, { messageBroker })

multiple IPC example (advanced usage)

function ipcPingTest(job, toggleIPC) {
  return toggleIPC(
    (msg) => {
      console.log(msg)
      return toggleIPC(
        (msg) => console.log(msg),
        (smtm) => smtm({ status: 4, data: 'ping' })
      )
    }, (smtm) => smtm({ status: 4, data: 'ping' }))
}
const taskMap = {
  ipcPingTest
}
function messageBroker(respond, msg) {
  if (msg.data === 'ping') {
    respond('pong')
  }
}
const cluster = new Cluster(taskMap, { messageBroker })

common mistakes and good practices

secure your imports

Because the worker will only work on tasks, you should remove the unnecessary imports to avoid resources consumption and longer startup time.

As a good practice you should put all your Master imports logic in the same file, and import it only on the master.

What I mean by “Master imports Logic” is :

• all your publications
• all your REST endpoints declarations
• graphql server and such…
• SSR / front related code

It could be summarized as such :

// in your entry file
if (Cluster.isMaster()) {
  import './MasterImports.js'
}
// ...rest of your cluster logic

recurring tasks

Because recurring tasks are created “recursively”, there will always be a task in the queue.

If the server is restarted, it will start the recurring task because it’s still in the queue.

Be sure to remove all recurring task on the master before starting others, or secure the insert.

Otherwise you will have multiple identical recurring tasks running at the same time.

You can either do :

Meteor.startup(() => {
  if (Cluster.isMaster()) {
    TaskQueue.remove({ taskType: 'recurringTask' })
  }  
})

or at task initialization :

  const recurringTaskExists = TaskQueue.findOne({ taskType: 'recurringTask' }) !== undefined
  if (!recurringTaskExists) {
    TaskQueue.addtask({ taskType: 'recurringTask', priority: 1, data: {}, dueDate })
  }

task uniqueness

If you want to be sure to have unique tasks, you should set a unique Id with TaskQueue.addTask.

A good model could be : ${taskType}${associated_Model_ID}

multiple apps

There’s no way right now to know from which app the task is started (may change later) :

you should only run the Cluster on one of the app to avoid other apps to run a task which is not included in its taskMap.

You can still use the TaskQueue in all the apps of course.

If your apps have different domain names / configurations (for the mailer for example), you should pass these through the data field.

For example if you’re using Meteor.absoluteUrl or such in a task it will have the value associated with the app running the Cluster.