Node.js control flow using EventEmitter
Recently, I read yet another blog post complaining about how concurrency is hard in Node.js. As is often the case, the examples highlighting the problems are leaving a lot to be desired.
Since I’ve written quite a bit of Node.js, I’d like to present an example of concurrency style in Node.js that isn’t a “strawman” often touted as the bane of Node.js’ existence.
EventEmitter
If you’ve been around Node.js for a little bit, you might have heard something along the lines “everything is a stream”, or “everything is an event emitter.” There is a reason for that.
Let’s take the example from the blog post I mentioned:
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| function dostuff(callback) { | |
| task1(function(x) { | |
| task2(x, function(y) { | |
| task3(y, function(z) { | |
| if (z < 0) { | |
| callback(0); | |
| } else { | |
| callback(z); | |
| }); | |
| }); | |
| }); | |
| } |
If you find yourself doing the above, you’re either in a rush, or should stop and rethink your design. Here’s how to accomplish a similar thing using continuation passing:
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| var dostuffEmitter = new EventEmitter(); | |
| dostuffEmitter.on('task1', function (callback) { | |
| dostuffEmitter.emit('task2', x, callback); | |
| }); | |
| dostuffEmitter.on('task2', function (x, callback) { | |
| dostuffEmitter.emit('task3', y, callback); | |
| }); | |
| dostuffEmitter.on('task3', function (y, callback) { | |
| dostuffEmitter.emit('finish', z, callback); | |
| }); | |
| dostuffEmitter.on('finish', function (z, callback) { | |
| if (z < 0) { | |
| callback(0); | |
| } else { | |
| callback(z); | |
| } | |
| }); |
Why is this “better”? Well, for one, adding error handling is straightforward:
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| var dostuffEmitter = new EventEmitter(); | |
| dostuffEmitter.on('task1', function (callback) { | |
| if (error) { | |
| dostuffEmitter.emit('error', error); | |
| return; | |
| } | |
| dostuffEmitter.emit('task2', x, callback); | |
| }); | |
| dostuffEmitter.on('task2', function (x, callback) { | |
| if (error) { | |
| dostuffEmitter.emit('error', error); | |
| return; | |
| } | |
| dostuffEmitter.emit('task3', y, callback); | |
| }); | |
| dostuffEmitter.on('task3', function (y, callback) { | |
| if (error) { | |
| dostuffEmitter.emit('error', error); | |
| return; | |
| } | |
| dostuffEmitter.emit('finish', z, callback); | |
| }); | |
| dostuffEmitter.on('finish', function (z, callback) { | |
| if (z < 0) { | |
| callback(0); | |
| } else { | |
| callback(z); | |
| } | |
| }); |
We can add telemetry like so:
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| dostuffEmitter.on('task2', function (x, callback) { | |
| if (error) { | |
| dostuffEmitter.emit('error', error); | |
| return; | |
| } | |
| dostuffEmitter.emit('telemetry', someMetricHere); | |
| dostuffEmitter.emit('task3', y, callback); | |
| }); |
We can alter control flow without rewriting the “callback hell”:
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| dostuffEmitter.on('task1', function (callback) { | |
| dostuffEmitter.emit('task3', x, callback); | |
| }); | |
| dostuffEmitter.on('task2', function (x, callback) { | |
| dostuffEmitter.emit('finish', y, callback); | |
| }); | |
| dostuffEmitter.on('task3', function (y, callback) { | |
| dostuffEmitter.emit('task2', z, callback); | |
| }); |
In general, the EventEmitter will be a better idea. I hope you’ll find some of these hints interesting and find writing control flow in Node.js a bit more friendly.
No Motherships 