Angular SSR - PM2 cluster support

Which @angular/* package(s) are relevant/related to the feature request?

platform-server

Description

Currently to scale Angular SSR beyond 1 core if you use PM2 you must have nginx in front with configured upstreams on multiple ports. PM2 configuration is also complicated with increment_var setting for port. Any change to a number of instances requires changing nginx configuration.

Proposed solution

Angular SSR should be able to run in PM2 cluster mode for easy scaling

Alternatives considered

/

Regardless of using clustering or not. It's highly recommended to place a load balancer like Nginx in front of your Node.js application. Direct traffic should generally never reach the Node.js server itself.

PM2 and Cluster Mode

When using PM2 in cluster mode, or utilizing the standard Node.js cluster module, all worker processes typically share the same network port. The primary process (or PM2) acts as a load balancer and manages the distribution of incoming connections to these workers. Why do you want to have different ports?

I tried PM2 clustering with Angular SSR, and it worked right away without needing any changes.

Clustering Implementation Example without PM2

Implementing basic clustering is also straightforward using the native Node.js cluster API. The following code snippet demonstrates a primary process forking workers and ensuring a dead worker is replaced:

if (cluster.isPrimary) {
  // Primary/Master process logic
  console.log(`Master process ${process.pid} is running`);

  // Fork workers based on CPU count
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  // Auto-restart failed workers
  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died. Spawning a new one.`);
    cluster.fork(); // Spawn a replacement worker
  });
} else {
  // Worker process logic (listens for requests)
  const port = 4000;
  app.listen(port, (error) => {
    if (error) {
      throw error;
    }
    console.log(`Node Express server listening on http://localhost:${port} (Worker ${process.pid})`);
  });
}

@alan-agius4

@alan-agius4 I fully agree nginx should be in front. I believe it's not stated in documentation.

Why do you want to have different ports?

I don't understand the question. I don't want to have multiple ports but I need to have them because cluster mode is not supported and in fork mode each instance must have it's own port.

I tried PM2 clustering with Angular SSR, and it worked right away without needing any changes.

I tried few months ago and it would just hang on start. Also I found references on the internet that it's not supported.
Are you sure it works for you? If yes I will test again.

@hheexx

@hheexx, yes I worked when using the latest version to scaffold the project.

I tried now with my project that uses stock server.ts and it does not work.
Application enters restart loop with status errored end no log output.

What is your PM2 config? I tried again and it just works out of the box.

$ ng new ssr-cluster --ssr
$ cd ssr-cluster
$ ng b

ecosystem.config.js

module.exports = {
  apps: [
    {
      name: "cluster",
      script: "./dist/ssr-cluster/server/server.mjs",
      instances: "5", 
      exec_mode: "cluster",
    },
  ],
};

$ npx pm2 start ecosystem.config.js
[PM2] Spawning PM2 daemon with pm2_home=/xxxxxxx/.pm2
[PM2] PM2 Successfully daemonized
[PM2][WARN] Applications cluster not running, starting...
[PM2] App [cluster] launched (5 instances)
┌────┬────────────────────┬──────────┬──────┬───────────┬──────────┬──────────┐
│ id │ name               │ mode     │ ↺    │ status    │ cpu      │ memory   │
├────┼────────────────────┼──────────┼──────┼───────────┼──────────┼──────────┤
│ 0  │ cluster            │ cluster  │ 0    │ online    │ 0%       │ 65.5mb   │
│ 1  │ cluster            │ cluster  │ 0    │ online    │ 0%       │ 63.0mb   │
│ 2  │ cluster            │ cluster  │ 0    │ online    │ 0%       │ 57.5mb   │
│ 3  │ cluster            │ cluster  │ 0    │ online    │ 0%       │ 53.9mb   │
│ 4  │ cluster            │ cluster  │ 0    │ online    │ 0%       │ 51.8mb   │
└────┴────────────────────┴──────────┴──────┴───────────┴──────────┴──────────┘

$ curl http://localhost:4000 -I
HTTP/1.1 200 OK
X-Powered-By: Express
content-length: 21576
content-type: text/html;charset=UTF-8
etag: "e80dbacaf06858bd2e5f221570e342a7e7b88cff271b023f2f7dc55b232a4012"
Date: Fri, 17 Oct 2025 15:21:40 GMT
Connection: keep-alive
Keep-Alive: timeout=5

Which version of Angular CLI are using to generate the project? (i.e. the globally installed version). Maybe that’s outdated?

Now I tried with ng new project and it works like you said. yey...

For some reason it does not work with my project that also uses latest angular and stock server.ts.
Looks like something from inside angular project is preventing it to ran and that's strange....

Check that you have the following condition:

angular-cli/packages/schematics/angular/ssr/files/application-builder/server.ts.template

Line 54 in b1d6d2f

if (isMainModule(import.meta.url) || process.env['pm_id']) {

Closing since in an ng new project clustering works out of the box.

Found it!
It's not Angular related.

It was about permissions on a logs folder.
In fork mode logs are written by pm2 process, in cluster mode it's run by child process.
On my server those are not same users.

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Angular SSR - PM2 cluster support #31387

Which @angular/* package(s) are relevant/related to the feature request?

Description

Proposed solution

Alternatives considered

PM2 and Cluster Mode

Clustering Implementation Example without PM2

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Angular SSR - PM2 cluster support #31387

Description

Which @angular/* package(s) are relevant/related to the feature request?

Description

Proposed solution

Alternatives considered

Activity

alan-agius4 commented on Oct 15, 2025

PM2 and Cluster Mode

Clustering Implementation Example without PM2

hheexx commented on Oct 15, 2025

alan-agius4 commented on Oct 15, 2025

hheexx commented on Oct 17, 2025

alan-agius4 commented on Oct 17, 2025

alan-agius4 commented on Oct 17, 2025

hheexx commented on Oct 17, 2025

alan-agius4 commented on Oct 17, 2025

hheexx commented on Oct 20, 2025

angular-automatic-lock-bot commented on Nov 20, 2025

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