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Overseer

Overview

This module oversees the creation and maintenance of the Vigil process cluster. It spins off child processes which each perform a specific role (server, monitor, etc). Each worker runs in a domain. When the worker encounters an uncaught exception, it is allowed to gracefully die, and a new fork is created here to replace it.


Source-through

First, require the cluster module, and the list of well defined worker roles.

cluster = require 'cluster'
{roles} = require './config'

workers = {}

This sets the configuration for cluster.fork().

The new settings are effective immediately and permanently, they cannot be changed later on.

overseerConfig =
    exec:   require.resolve('./workers')
    args:   []
    silent: false

cluster.setupMaster overseerConfig

The resurrectionHandler is called when a worker dies. We figure out it's role form worker.id and fork() a new worker of that role to replace the dead one.

resurrectionHandler = (worker, code, signal) ->
    workerRole = 'runner'
    workerId = worker.id
    console.log "Worker {ID: #{workerId}, ROLE: #{workerRole}} has died. Restarting..."
    workers[workerRole] = cluster.fork role: workerRole

A callback to ensure graceful death of the master process.

masterExitHandler = ->
    console.log "\nVigil is quitting."

We can set environment variables for the child while forking. Here, we set the role environment variable so the worker knows what it's supposed to do. We loop through roles creating a worker for each, and storing their reference in the hash table workers.

start = ->
    workers[role] = cluster.fork {role} for role in roles

    cluster.on 'exit', resurrectionHandler

    process.on 'SIGINT', ->
        process.exit 1

    process.on 'exit', masterExitHandler

    return {process, cluster}

if require.main is module
    start()
else
    module.exports = start