Metrics not being computed when the application is forked into a child process

[quixote15]

I created a new Nestjs project, Installed the lib and then copy pasted the example in the README of a counter. That worked fine.

However, forking the nestjs application into a sub-process and trying to increment the counter does not updates the metric value.

Even tough I increment the metric inside an infinite loop

let = 0; 
while (true) {
    console.log('incrementing prometheus counter in background... ' + i++);
    service.increment();
    await setTimeout(500);
  }

The metric endpoint returns zero

I've created repo which replicates this issue:

https://github.com/quixote15/nestjs-prometheus-fork-issue

Can anybody help me with this?

[willsoto]

Is this something that works using prom-client directly?

[quixote15]

@willsoto

It could be something related to prom-client since it is a dependency of this lib but I'm just using the methods exported by nestjs-prometheus directly as you can see here

Maybe prom-client does not support child process ?

[willsoto]

Before I look into anything here, you will need to confirm the behavior of both prom-client and NestJS when it comes to child processes.

Given this library is a simple thin wrapper around prom-client, it's unlikely the issue would be here. But before I do any work to figure it out, you will need to confirm that the underlying libraries support child processes as you'd expect and then we could say with certainty that something in this library was preventing that from working.

[quixote15]

Looking into prom-client repo I found that they are trying to add support to this

siimon/prom-client#403
siimon/prom-client#401 (comment)

So yeah, definetly not something in this library

[willsoto]

Great! Thanks for looking into it. If they ever get support added and it still doesn't work, please feel free to re-open this issue.