Node.js logging part 1 - log down the process ID and thread ID

Published on: 23th Nov 2024

Updated on: 21th Jul 2025

Overview

Life can be complicated if you are developing a system that supports many user features. When everything works properly, you will have a feeling like you are invisible. But, when unexpected things happened and the program crashes, you will need to find out the cause of the failure and then solve the issue. We will have to identify which line crashed and gather as much as possible on the runtime value. Node.js stack dump provides some hints on what has happened, at which line it crashed and most of the time this is sufficient to fix the issue.

If you are running your system with multiple instances and each instance has multiple threads, then, the troubleshooting will become complex. This is because Node.js provides the stack dump of the crashing line without the process ID and the thread ID. This makes it difficult to guess or imagine the runtime value and what has happened.

A simple program to explain the process ID and thread ID

const w = require('worker_threads');

if (w.isMainThread) {
    // running on the main thread
    console.log(`main:pid=${process.pid},thread id=${w.threadId}`);

    for (let i = 0; i < 3; i++) {
        const w2 = new w.Worker(__filename);
        w2.my_id = i + 1;
        w2.on('exit', () => {
            console.log('thread exit', w2.my_id);
        });
    }

    setTimeout(() => {
        const w2 = new w.Worker(__filename);
        w2.my_id = 4;
        w2.on('exit', () => {
            console.log('thread exit', w2.my_id);
        });
    }, 1000);

    setTimeout(() => {
        const w2 = new w.Worker(__filename);
        w2.my_id = 5;
        w2.on('exit', () => {
            console.log('thread exit', w2.my_id);
        });
    }, 2000);
}
else {
    // running on a worker thread
    console.log(`worker:pid=${process.pid},thread id=${w.threadId}`);
}

Here's the output:

main:pid=9788,thread id=0
worker:pid=9788,thread id=1
thread exit 1
worker:pid=9788,thread id=2
thread exit 2
worker:pid=9788,thread id=3
thread exit 3
worker:pid=9788,thread id=4
thread exit 4
worker:pid=9788,thread id=5
thread exit 5

Explanations

The process ID (process.pid)
- It is a value assigned by the operating system.
- In the syslog on Linux, it has the information about a process start time and crashing time. It will give you some hints at what time it crashed.
The worker thread ID (threadId)
- It is a running value assigned by Node.js and the main thread ID is always zero (0).
- Logging this value is helpful especially when you are passing some data across threads. With this information, you will be able to tell if the data has been passed to the correct thread. For example, the worker thread passess the data to the main thread and there is a chance that you accidentally passing the data to the same thread. As a result, the program is not working as expected. Another example is that, some data has been cached in thread zero and another script that runs on thread one wants to access these cached data but forget it is on thread one.

Conclusion

Logging the running process is important and it is very helpful in troubleshooting.

Logging part 1 - log down the process ID and thread ID.
Logging part 2 - log down the runtime performance.
Logging part 3 - log destination

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Author

Lau Hon Wan, software developer.