Memory leak

In case you are facing critical errors for the server like FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory than this is probably caused by memory leak in your app.

Local profiling

To find out root cause of the problem you need to analyze memory allocation on server over time.

Start the app

1. You may use either start or start-prod command to profile your app, but keep in mind that start runs app in dev-environment that leads to higher memory consumption and additional code that may distract you from analyzing core logic of the app. However start is preferred when you need to make code changes and you do not need to get the exact values of memory usage as for the prod-environment.
2. Using @tramvai/cli you can pass a special flag --debug and pass it either to the start or start-prod command
3. You may pass any additional environment variables when calling cli commands as usual. For example, you may pass flag HTTP_CLIENT_CACHE_DISABLED to disable http cache.
4. To pass additional flags to the running nodejs instance you may use env NODE_OPTIONS. E.g., if you want to limit the heap memory for the running server pass NODE_OPTIONS="--max-old-space-size=256"
5. If you are profiling using start-prod with different env values without code changes you can use start-prod command that will reuse previous builds, e.g. tramvai start-prod -t none app --debug

Use the DevTools

After starting the app in debug mode you can open Chrome DevTools to be ready to take some profiling.

You may read more about how to profile memory leaks with Chrome in next links:

• https://medium.com/@basakabhijoy/debugging-and-profiling-memory-leaks-in-nodejs-using-chrome-e8ece4560dba#:~:text=Fire%20up%20the%20chrome%20browser
• https://developer.chrome.com/docs/devtools/memory-problems/#summary

Test requests

Most of the time memory leaks are happens for every request, so you need to do some request.

1. Using browser go to the page that is handled by you app
2. Use any additional tools to make a bunch of requests, e.g. autocannon

What to look

After you've started the app and took memory snapshots or memory allocations look for the data that stays in memory when it shouldn't.

ChildContainer

In context of the tramvai app look for the ChildContainer first as it is created for every request and contains the whole Request DI instance that consumes a lot of memory. It should be collected by the GC after request end, but sometimes some code may prevent it from collecting.

Summary

1. Start the app
2. Start the DevTools
3. Check memory usage on start and take the initial memory snapshot
4. Make some test requests
5. Take another memory snapshot
6. Compare two snapshots and look for the leaks or unusual memory consumption

Example

1. Start app in prod-mode yarn tramvai start-prod travelaviasearch --debug
2. Start Chrome DevTools
3. Take the memory snapshot
4. Execute requests with autocannon -c 20 -d 60 localhost:3000/travel/flights/
5. Click the button Collect garbage
6. Take another memory snapshot
7. Compare two snapshots
8. If you want to test app with other envs stop the server and close the DevTools, then run it again with HTTP_CLIENT_CACHE_DISABLED=true MOCKER_ENABLED=true NODE_OPTIONS="--max-old-space-size=256" yarn tramvai start-prod -t none travelaviasearch