Node.js process doesn't exit when run under pm2

Issue

I have a node.js script that runs and exits fine in console, but it doesn’t exit unless I call process.exit() in pm2. PM2 config is:

        {
            name: "worker",
            script: "./worker.js",
            restart_delay: 60000,
            out_file: "/tmp/worker.log",
            error_file: "/tmp/worker_err.log"
        },

I’ve installed why-is-node-running to see what keeps the process running in 10 seconds after the expected exit and the output is:



There are 9 handle(s) keeping the process running

# TLSWRAP
node:internal/async_hooks:200

# TLSWRAP
node:internal/async_hooks:200

# ZLIB
node:internal/async_hooks:200                                                 
/Users/r/code/app/node_modules/decompress-response/index.js:43          - const decompressStream = isBrotli ? zlib.createBrotliDecompress() : zlib.createUnzip();
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:586
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:768
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:786

# TLSWRAP
node:internal/async_hooks:200

# ZLIB
node:internal/async_hooks:200                                                 
/Users/r/code/app/node_modules/decompress-response/index.js:43          - const decompressStream = isBrotli ? zlib.createBrotliDecompress() : zlib.createUnzip();
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:586
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:768
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:786

# TLSWRAP
node:internal/async_hooks:200

# ZLIB
node:internal/async_hooks:200                                                 
/Users/r/code/app/node_modules/decompress-response/index.js:43          - const decompressStream = isBrotli ? zlib.createBrotliDecompress() : zlib.createUnzip();
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:586
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:768
file:///Users/r/code/app/node_modules/got/dist/source/core/index.js:786

# TLSWRAP
node:internal/async_hooks:200

# Timeout
node:internal/async_hooks:200            
node:internal/async_hooks:468            
node:internal/timers:162                 
node:internal/timers:196                 
file:///Users/r/code/app/worker.js:65
node:internal/process/task_queues:94     

Why doesn’t node exit? How do I further debug this?

PS: Sorry for a large paste

UPDATE

I’ve managed to reproduce this in a comically small 2-liner:

import got from "got";
await got.post('https://anty-api.com/browser_profiles', {form: {a: 123}}).json();

The above code throws as expected when run form console, yet keeps running forever when called by pm2.

UPDATE 2

It does reproduce with an empty app file too.

Solution

I think this is just the way pm2 works. You can expect that, when running under pm2, the node process will continue to run forever, (whether your app is responsible for pending async event sources or not) unless you either crash or do something to explicitly terminate it such as process.exit().

As you’ve discovered, this has nothing to do with any code in your app.js. Even an empty app.js exhibits this behaviour. This is a fundamental design aspect of pm2. It wraps your program and it’s the wrapper that is keep the node process alive.

This is because pm2 runs your program (in forked mode, as opposed to cluster mode) by launching a node process that runs ProcessContainerFork.js (the wrapper). This module establishes and maintains a connection to pm2’s managing process (a.k.a "god daemon") and loads your app’s main module with require('module')._load(...). The communication channel will always count as an event source that keeps the actual node process alive.

Even if your program does nothing, the status of your program will be "online". Even if your program reaches the state where, had it been launched directly, node would have exited, the state is still "online" in this case because of the wrapper.

This leaves the designers of pm2 with the challenge of trying to know if your program is no longer responsible for any events (in which case node would normally exit). pm2 doesn’t have the feature to distinguish between reasons node is being kept alive due to code you wrote in your app.js vs reasons node is being kept alive due to the infrastructure established by ProcessContainerFork.js. One could certainly imagine that pm2 could use async_hooks to keep track of event sources originating from your app rather than from ProcessContainerFork.js (much like how why-is-node-running does), and then tearing down properly when it reaches this state. Perhaps pm2 chooses not to do this to avoid the performance penalty associated with async hooks? Perhaps an app that exits on purpose but is intended to be restarted seems too much like a cron job? I’m speculating yours is not the primary use case for pm2. I suppose you could make a feature request and see what the pm2 authors have to say about it.

I think this means if you want to gracefully exit and have pm2 restart your program, you’ll need to call process.exit to do so. You won’t be able to rely on node knowing that there are no more event sources because pm2 is responsible for some of them. You will, of course, have to ensure that all your relevant pending promises or timers have resolved before calling process.exit because that will immediately terminate the process without waiting for pending things to happen.

Answered By – Wyck

Answer Checked By – Timothy Miller (AngularFixing Admin)

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