Next-i18next: Add memory leak load testing

[isaachinman]

As per #450, we need load tests to ensure heap size is stable over hundreds or thousands of requests.

[dfvalero]

Hi! How can I help with this?

[isaachinman]

We need to find a way to:

1. Run an npm script, passing in an --inspect flag, or similar, holding this process open for later assertion
2. Immediately assert against the memory usage of the process
3. Run artillery against a localhost port
4. Assert again against the memory usage of the process
5. Close the process

[lucasfeliciano]

What I did a while back to profile our application when we had memory leaks in the past was to use https://www.npmjs.com/package/heapdump library.

I took two approaches:

1. Take a snapshot of memory when the server starts and one when there is a memory leak

// Start memory reporting block
  if (process.env.NODE_ENV === 'production') {
    heapdump.writeSnapshot(`initial-${Date.now()}.heapsnapshot`)

     memwatch.on('leak', function(info) {
      heapdump.writeSnapshot(`leak-${Date.now()}.heapsnapshot`)

       console.log('-- Memory leak')
      console.log(info)
    })
  }
  // End memory reporting block

Then I would get the snapshot from the server and load on chrome inspector to analyze it.
If I'm not wrong, I think this approach didn't give me anything valuable. So I took a different approach.

1. I took a snapshot every 30 minutes, and with that, I had more data to work with:

  // Start memory reporting block
  if (process.env.NODE_ENV === 'production') {
    const initialHeapDumpName = `${Date.now()}.initial.heapsnapshot`
    console.log(`>>> Initial snapshot created with name ${initialHeapDumpName}`)
    heapdump.writeSnapshot(initialHeapDumpName)

    // Take a memory snapshot every 30 minutes
    setInterval(function writeHeapSnapshot() {
      const partialHeapDumpName = `${Date.now()}.interval.heapsnapshot`
      heapdump.writeSnapshot(
        `>>> Partial snapshot created with name ${partialHeapDumpName}`
      )
    }, 1000 * 60 * 30)
  }
  // End memory reporting block

But be aware that while the server is writing the snapshot it blocks the event loop and make your server slow until it finishes the process

[isaachinman]

@lucasfeliciano Nice - interesting stuff. Depending on how we launch the process, it should be as simple as using process.memoryUsage().heapUsed, though, right?

I wonder if there is a util/process-runner that measures heap size over time and allows you to perform assertions. That'd be a really useful package if anyone wants to create and maintain it...

[lucasfeliciano]

@isaachinman If I'm not wrong process.memoryUsage().heapUsed gives us the total amount of memory.

heapdump.writeSnapshot() gives us a snapshot file to load on chrome inspector.

So we can analyze in detail what objects are in the heap.

It is pretty useful!
Check it out: https://developers.google.com/web/tools/chrome-devtools/memory-problems/

[isaachinman]

@lucasfeliciano I'm fully aware of how to analyse via the inspector, but the point is that we need a programmatic way to do this so that it can be run as a blocking step in CI. Is that possible with heap snapshots?

[lucasfeliciano]

We can set a listener using memwatch pacakge which I also used on the snippet above but forgot to mention.

memwatch.on('leak', (info) => {
  console.error('Memory leak detected:\n', info);
});

[isaachinman]

@lucasfeliciano Interesting! Does memwatch only inspect the process that launched it, or is it also aware of all child processes?