CODEWITHSUNDEEP
node.js
DancoD
DancoD

Reputation: 9

Async callback blocks NodeJS

I have a server-client based NODE.JS application.

server.js

...
socket.on('message', function(message) {
    if(message.code == 103)
    {
        process_some_data()
    }
    else
    {
        console.log("UNKNOWN MESSAGE");
    }
});
...

client.js

.. sending responses back to server.js

the process_some_data() function takes about 4 seconds to complete, and when i have just one client it is not a problem, but if i have 10, they all choke and wait till the the last finishes.

I found out that the entire socket event waits till he finishes the current job, for example if i comment process_some_data(), it will not be frozen

I have tried 2 tweaks but the didn't worked :

...
socket.on('message', function(message) {
    if(message.code == 103)
    {
        setTimeout(function() {
            process_some_data();

            console.log("FINISH");
        }, 1)
    }
    else
    {
        console.log("UNKNOWN MESSAGE");
    }
});
...

And even used http://caolan.github.io/async/ ,but no use :

...
socket.on('message', function(message) {
    if(message.code == 103)
    {
        // Array to hold async tasks
        var asyncTasks = [];

        async.series([
            setTimeout(function() {

                process_some_data();

                console.log("FINISH");
            }, 1)
        ], function (err, results) {
            console.log(results);
        });
    }
    else
    {
        console.log("UNKNOWN MESSAGE");
    }
});
...

How to make this ASYNC? Really need this.

Thank you.

Upvotes: 0

Views: 212

Answers (1)

salezica
salezica

Reputation: 76929

You need multiple processes to solve this with Javascript, because Javascript engines are single-threaded.

What?

When it comes to handling I/O events, such as reading a socket, writing to a file or waiting for a signal, Javascript engines give the appearance of doing multiple things at the same time.

They are actually not: it's just that, under most conditions, processing these events takes so little computation, and the events themselves occur with so much time in between (a microsecond is an eternity for a CPU), that the engine can just process them one after another with plenty of time to spare.

In human time-scale, it looks like the engine is doing a lot of stuff in parallel, but it's just working serially at great speed.

No matter how you schedule your code to run, using setTimeout or Promise, it will still block other events from being processed during the time it's actively computing. Long-running computations (in the scale of seconds, instead of milliseconds) expose the single-threaded nature of the engine: it cannot actually do multiple things at the same time.

Multiple processes

Your computer, however, probably has multiple CPU cores. Unlike the Javascript engine, your hardware is capable of tackling multiple tasks at the same time, at least 1 per core. Even with a single core, your operating system can solve the problem if you run multiple processes.

Since a single Javascript process is single-threaded, you need multiple Javascript processes for this. An easy and time-proven architecture to solve your problem is this:

  • One Javascript program, running in one process, reads from the socket. Instead of calling process_some_data(), however, it puts all incoming messages in a queue.

  • This program then sends items from the queue to another Javascript program, running in a different process, that performs the computation using another CPU core. There may be multiple copies of this second process. In a modern computer, it makes sense to have twice as many active processes as you have CPU cores.

A simple approach for Node is to write an HTTP server, using express, that runs the computationally-intensive task. The main program can then use HTTP to delegate tasks to the workers, while still being able to read from the socket.

This is a good article on the topic of multi-processing with Node, using the cluster API.

Upvotes: 1

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