I had asked in an interview, are there any cases that may force you to use blocking code in a node.js server?
my answer was: I didn't ever need that in any project but I think it may be useful in some tasks that need much CPU processing like Some Image Processing or video generation.
so experts, can you correct that for me, is there any case that a blocking code would be a must?
First off, you have to distinguish between the different types of programs. A server that you expect to be responsive to many different incoming requests has very different needs than a single user program you write to do some file management or fetch some content and insert it in a database.
So, if you're not a multi-user server, you may be able to use synchronous I/O everywhere it's offered (most specifically for file access). For example, I have several scripts that do file management on my hard disk. These scripts don't have any server component and are run automatically in the middle of the night to trim backups, trim log files, etc... These scripts are perfectly OK to use synchronous I/O for pretty much anything.
If, on the other hand, you are a mutli-user server and you need to be responsive to incoming requests that can arrive at any time, then the only two times you can/should use blocking I/O or blocking crypto are at startup time or in some sort of shut-down scenario. For all other code in service of incoming requests, you have to use non-blocking, asynchronous I/O to avoid locking up your server during a request and making it non-responsive to new incoming requests.
If you have time consuming, CPU-intensive operations such as image processing or video generation, then you will want to offload that processing to another thread or process so that your main server thread is not blocked doing that processing. A typical way of handling that would be to create a worker pool of N processes/threads that can be sent jobs to crunch on. Then, you keep your most CPU-intensive work out of the main nodejs thread, allowing it to stay responsive to incoming requests.
so experts, can you correct that for me, is there any case that a blocking code would be a must?
Synchronous (blocking) I/O vastly simplifies server startup as you can do things like read configurations synchronously. You could write that code asynchronously, but then your module interface often end up having to return promises that indicate when it's actually ready and done with its initialization which complicates using the module.
For example, require() is synchronous and this really, really helps make initialization a lot simpler.
The only place I know of in a server where blocking code might be required is if you're trying to write something to disk right before your program exits when it's already in the process of exiting. You get notified of an exit event and if you try to use asynchronous file I/O, then your program will exit before the I/O finishes. In that case, you may need to use synchronous file I/O (which is not a problem in that circumstance).