javascript - Coordinating parallel execution in node.js

The event-driven programming model of node.js makes it somewhat tricky to coordinate the program flow.

Simple sequential execution gets turned into nested callbacks, which is easy enough (though a bit convoluted to write down).

But how about parallel execution? Say you have three tasks A,B,C that can run in parallel and when they are done, you want to send their results to task D.

With a fork/join model this would be

• fork A


• fork B


• fork C


• join A,B,C, run D

How do I write that in node.js ? Are there any best practices or cookbooks? Do I have to hand-roll a solution every time, or is there some library with helpers for this?

Answer

Nothing is truly parallel in node.js since it is single threaded. However, multiple events can be scheduled and run in a sequence you can't determine beforehand. And some things like database access are actually "parallel" in that the database queries themselves are run in separate threads but are re-integrated into the event stream when completed.

So, how do you schedule a callback on multiple event handlers? Well, this is one common technique used in animations in browser side javascript: use a variable to track the completion.

This sounds like a hack and it is, and it sounds potentially messy leaving a bunch of global variables around doing the tracking and in a lesser language it would be. But in javascript we can use closures:

function fork (async_calls, shared_callback) {
  var counter = async_calls.length;
  var callback = function () {

    counter --;
    if (counter == 0) {
      shared_callback()
    }
  }

  for (var i=0;i    async_calls[i](callback);
  }
}


// usage:
fork([A,B,C],D);

In the example above we keep the code simple by assuming the async and callback functions require no arguments. You can of course modify the code to pass arguments to the async functions and have the callback function accumulate results and pass it to the shared_callback function.

---

Additional answer:

Actually, even as is, that fork() function can already pass arguments to the async functions using a closure:

fork([
  function(callback){ A(1,2,callback) },
  function(callback){ B(1,callback) },
  function(callback){ C(1,2,callback) }
],D);

the only thing left to do is to accumulate the results from A,B,C and pass them on to D.

---

Even more additional answer:

I couldn't resist. Kept thinking about this during breakfast. Here's an implementation of fork() that accumulates results (usually passed as arguments to the callback function):

function fork (async_calls, shared_callback) {
  var counter = async_calls.length;

  var all_results = [];
  function makeCallback (index) {
    return function () {
      counter --;
      var results = [];
      // we use the arguments object here because some callbacks 
      // in Node pass in multiple arguments as result.
      for (var i=0;i        results.push(arguments[i]);
      }

      all_results[index] = results;
      if (counter == 0) {
        shared_callback(all_results);
      }
    }
  }

  for (var i=0;i    async_calls[i](makeCallback(i));
  }

}

That was easy enough. This makes fork() fairly general purpose and can be used to synchronize multiple non-homogeneous events.

Example usage in Node.js:

// Read 3 files in parallel and process them together:

function A (c){ fs.readFile('file1',c) };

function B (c){ fs.readFile('file2',c) };
function C (c){ fs.readFile('file3',c) };
function D (result) {
  file1data = result[0][1];
  file2data = result[1][1];
  file3data = result[2][1];

  // process the files together here
}


fork([A,B,C],D);

---

Update

This code was written before the existence of libraries like async.js or the various promise based libraries. I'd like to believe that async.js was inspired by this but I don't have any proof of it. Anyway.. if you're thinking of doing this today take a look at async.js or promises. Just consider the answer above a good explanation/illustration of how things like async.parallel work.

For completeness sake the following is how you'd do it with async.parallel:

var async = require('async');

async.parallel([A,B,C],D);

Note that async.parallel works exactly the same as the fork function we implemented above. The main difference is it passes an error as the first argument to D and the callback as the second argument as per node.js convention.

Using promises, we'd write it as follows:

// Assuming A, B & C return a promise instead of accepting a callback

Promise.all([A,B,C]).then(D);