Node: cluster: cluster in a module

[vsemozhetbyt]

• Version: 10.6.0 (all?)
• Platform: Windows (all?)
• Subsystem: cluster

Sorry if I completely do not understand some basics.

Today a friend ask me why some code works with worker_threads module well but the similar code works with cluster module weirdly. I am not much proficient in both modules, but, in a quick digging, I've found out some unexpectedness in the cluster work: if cluster.fork() happens in the module code, it forks both module and parent script. Is this an expected and documented behavior? Or is this some basics related self-evident stuff?

Reduced example:

parent.js:

'use strict';

const cluster = require('cluster');

console.log(`${__filename} is master: ${cluster.isMaster}`);

const f = require('./child.js');

f();

child.js:

'use strict';

const cluster = require('cluster');

console.log(`${__filename} is master: ${cluster.isMaster}`);

if (cluster.isMaster) {
  module.exports = function f() {
    cluster.fork();
    console.log('ok');
  };
} else {
  // ...
}

Output:

parent.js is master: true
child.js is master: true
ok
parent.js is master: false
child.js is master: false
parent.js:9
f();
^

TypeError: f is not a function
    at Object.<anonymous> (parent.js:9:1)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
    at startup (internal/bootstrap/node.js:236:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:560:3)

[targos]

Do you expect that the fork only executes child.js?

[vsemozhetbyt]

Yes. Sorry)

[targos]

@vsemozhetbyt It's expected and documented here: https://nodejs.org/dist/latest-v10.x/docs/api/cluster.html#cluster_cluster_settings

The default worker file is process.argv[1]. If you do cluster.settings.exec = __filename from the master in child.js, you'll get the result you want.

[vsemozhetbyt]

Thank you!

[targos]

I should add that cluster.setupMaster({ exec: __filename }) is the recommended way to set the settings.

[vsemozhetbyt]

Additional ref: https://stackoverflow.com/questions/39626750/why-is-node-cluster-fork-forking-the-parent-scope-when-implemented-as-a-module