Drover is native nodejs solution that takes away pain when orchestrating composite application and providers all-in-one point for graceful cluster control

Key features:

• run and manage multiple app processes as simple app
• runtime scale
• graceful reload (helpful for zero-downtime releases)

Using npm:

$ npm i --save drover

Using yarn:

$ yarn add drover

main.js

const { MasterFactory } = require('drover');

const master = MasterFactory.create(
    {
        script: 'path/to/app.js',
    }
);

process.on('SIGINT', async () => {
    await master.gracefulShutdown();
    process.exit(0);
});

master.start().catch(console.error);

app.js

const express = require('express');
const { MessageBus } = require('drover');

const mb = new MessageBus();

mb.on('stop', () => {
    server.close(mb.sendStopped);
});

mb.on('quit', () => {
    mb.sendShuttedDown();
    setTimeout(() => process.exit(0), 50);
});

const app = express().get('/', (req, res) => {
    res.send('hello');
});

const server = app.listen(1999, mb.sendStarted);

• Main context (facade) - facade part of application, which is responsible for launching logic and managing the lifecycle of functional parts of the application
• Functional context (business logic) - functional parts of application that directly contain the business logic
• Master - entity of drover, which takes the role of creating and maintaining a given number of workers. This entity is always located only in main context of application.
• Worker - entity of drover, which assumes a role in the actual up-to-date presentation of state of functional context process on main context side.
• MessageBus - entity of drover, which connects the functional context of application with main context provides bidirectional commands via IPC channel. Strongly associated with Worker and appear as it's representation on side of the functional context.

• in main context we should control worker processes of application. Here we can describe the logic of restarting falling workers, log them or even implement API that will allow us change workers state from the outside process context. E.g. to bring the workers in maintenance mode and suspend the application without stopping the master process itself.

• in main context, we get a guarantee that all workers have successfully completed the start, according to the internal business logic of the functional parts of the application. When you use default cluster module, the only signal for you about raising workers is to establish a IPC channel between worker and master. This is not very convenient and not very informative. Indeed, it does not guarantee that the worker is started up functionally correct up to bootstrap steps.

• in main context, we get access to the direct signaling of functional context, including the ability to correctly stop worker process or perform full controlled restart

• Logic segregation - main context of the application MUST NOT contain business logic and vice versa - functional context of the application MUST NOT contain logic of monitoring and controlling lifecycle of composite parts of the application itself.

To distinguish exit reasons you may import ExitReasons object.

const { ExitReasons } = require('drover');

There are such available classes:

• ExitReasons.ExternalSignal

• ExitReasons.NormalExit

• ExitReasons.AbnormalExit

Underlying process may not start at all, it may fail after some time or it may be killed with signal. Each instance has its own reason-specific payload field. There is a list of reasons:

• ExitReasons.ExternalSignal - worker process was killed by someone or by another process with the signal, the signal name can be found in payload.signal field.

• ExitReasons.NormalExit - worker process has exited with 0 code. payload.code is provided

• ExitReasons.AbnormalExit- worker process has exited with non-zero code. payload.code is provided.

FYI: You can find and run every case listed below in examples.

In this case, the logic in the approach will be absolutely identical to the usual use of the cluster nodejs module. However, in the case of a drover, we get a number of important advantages and convenience of working with a complex application using understandable entities and a transparent interface.

1. Create instance of Master

const master = MasterFactory.create(
    {
        script: 'path/to/app.js', // required
        count: 4,                 // optional
        env: {                    // optional
            PORT: 1934
        },
    }
);

2. Add listener on worker-exit event of Master

master.on('worker-exit', async (reason, workerId) => {
    if (reason instanceof ExitReasons.ExternalSignal || reason instanceof ExitReasons.AbnormalExit) {
        // restart worker if something abnormal happened or external process killed worker by signal
        await master.restartWorkerById(workerId);
    } else {
        // for different cases just hard quit all app
        const { code, signal } = reason.payload;
        // quit method will be described in next section
        await quit(code, signal, true);
    }
});

3. Handle terminating of main context process

const quit = async (code, signal, force = false) => {
    try {
        if (force) {
            // in case of emergency stop we just hard quit all worker processes
            await master.hardShutdown();
        } else {
            // for default app exit we do it in more graceful way
            await master.gracefulShutdown();
        }
    } catch (err) {
        // your stop-failed handler logic here
        console.error(err);
    }

    setTimeout(() => process.exit(0), 0);
};

// handle main process SIGINT (default signal in Unix when "ctrl+c" terminal interruption happened)
process.on('SIGINT', quit);

4. Start application. This part will fork functional context (app.js) and run 4 instances of it.

const run = async () => {
    try {
        await master.start();
        // right here we have a guarantee that all 4 app instances
        // already did their business logic (raised connects, started servers, etc)
    } catch (err) {
        // your start-failed handler logic here
        console.error(err);
        return;
    }

    // primitive health-check from master every 2s
    setInterval(() => {
        master.getWorkersStatuses().forEach((v, i) => {
            console.log(`[app-${i}]: ${statusMap[v]}`);
        });
        console.log('---');
    }, 2000);
};

run().catch(console.error);

1. Create instance of MessageBus:

const mb = new MessageBus();

2. Setup listeners on stop and quit events:

mb.on('stop', () => {
    server.close(mb.sendStopped);
});

mb.on('quit', () => {
    mb.sendShuttedDown();
    setTimeout(() => process.exit(0), 50);
});

3. Send start signal to master when all bootstrap part is done:

const app = express().get('/', (req, res) => {
    res.send(RESPONSE);
});

const server = app.listen(PORT, mb.sendStarted);

For that example above we will expect next output in console repeated every 2s:

[app-0]: STARTED
[app-1]: STARTED
[app-2]: STARTED
[app-3]: STARTED
---

In this case, the logic will same as if you are using several cluster modules within one main context without loss of ease of management.

Most parts of context building are similar to previous example, but with little different points.

Here we instantiate N different masters.

const fooMaster = MasterFactory.create(
    {
        script: 'path/to/app-foo.js',
        count: 2,
        env: {
            PORT: 2100,
            RESPONSE: 'multi-app-cluster'
        }
    }
);

const barMaster = MasterFactory.create(
    {
        script: 'path/to/app-bar.js',
        count: 2,
        env: {
            PORT: 2101,
            RESPONSE: 'multi-app-cluster'
        }
    }
);

const bazMaster = MasterFactory.create(
    {
        script: 'path/to/app-baz.js',
        count: 4,
        env: {
            PORT: 2102,
            RESPONSE: 'multi-app-cluster'
        }
    }
);

After that we subscribe listeners on worker-exit event and start all masters. You can start them parallel with Promise.all, or consistently one by one. It totally depends on your business logic.

If you don't need cluster multi-instances of your app, you still can run you application with drover. Most parts of context building are similar to first example with little difference in master's option count when you instantiate it.

const master = MasterFactory.create(
    {
        script: 'path/to/app.js',
        count: 1
    }
);

In this case you still got advantages of graceful reloads with zero-downtime of your app. When reload begins - one more instance of app will be added right before previous one shut down.

drover covered with MIT license, so it's free to use for any kind of your private or commercial projects without any restrictions and obligations to be open-sourced.

pm2 has programmatic flow, but it is still just API to pm2 demon process and it brings some usage restrictions. With drover you've got more options, so flexibility for your business logic raises a lot.

pm2 uses "let if fail" concept, but drover gives you control instead. You've got not just exits as fact, but you can manage different ExitReasons and handle each case according to your needs.

drover module use debug module for this this purpose.

Just run your app with DEBUG env var like example below:

DEBUG=drover:* node main.js

Sample output for simple-app start:

And changes with SIGINT signal to main.js process triggered by Ctrl+C:

As you see, you have transparent access to all events, state changes and errors described behaviour even via IPC communication.