How to have heavy processing operations done in node.js

Question

I have a heavy data processing operation that I need to get done per 10-12 simulatenous request. I have read that for higher level of concurrency Node.js is a good platform and it achieves it by having an non blocking event loop.

What I know is that for having things like querying a database, I can spawn off an event to a separate process (like mongod, mysqld) and then have a callback which will handle the result from that process. Fair enough.

But what if I want to have a heavy piece of computation to be done within a callback. Won't it block other request until the code in that callback is executed completely. For example I want to process an high resolution image and code I have is in Javascript itself (no separate process to do image processing).

The way I think of implementing is like

get_image_from_db(image_id, callback(imageBitMap) {
    heavy_operation(imageBitMap); // Can take 5 seconds.
});

Will that heavy_operation stop node from taking in any request for those 5 seconds. Or am I thinking the wrong way to do such task. Please guide, I am JS newbie.

UPDATE

Or can it be like I could process partial image and make the event loop go back to take in other callbacks and return to processing that partial image. (something like prioritising events).

Answer 1

Yes it will block it, as the callback functions are executed in the main loop. It is only the asynchronously called functions which do not block the loop. It is my understanding that if you want the image processing to execute asynchronously, you will have to use a separate processes to do it.

Note that you can write your own asynchronous process to handle it. To start you could read the answers to How to write asynchronous functions for Node.js.

UPDATE

how do i create a non-blocking asynchronous function in node.js? may also be worth reading. This question is actually referenced in the other one I linked, but I thought I'd include it here to for simplicity.