readable-stream

readable-stream

Stability: 1 - Experimental

An exploration of a new kind of readable streams for Node.js

This is an abstract class designed to be extended. It also provides a wrap method that you can use to provide the simpler readable API for streams that have the "readable stream" interface of Node 0.8 and before.

Usage

var Readable = require('readable-stream');
var r = new Readable();

r.read = function(n) {
  // your magic goes here.
  // return n bytes, or null if there is nothing to be read.
  // if you return null, then you MUST emit 'readable' at some
  // point in the future if there are bytes available, or 'end'
  // if you are not going to have any more data.
  //
  // You MUST NOT emit either 'end' or 'readable' before
  // returning from this function, but you MAY emit 'end' or
  // 'readable' in process.nextTick().
};

r.on('end', function() {
  // no more bytes will be provided.
});

r.on('readable', function() {
  // now is the time to call read() again.
});

Justification

Writable streams in node are very straightforward to use and extend. The write method either returns true if the bytes could be completely handled and another write should be performed, or false if you would like the user to back off a bit, in which case a drain event at some point in the future will let them continue writing. The end() method lets the user indicate that no more bytes will be written. That's pretty much the entire required interface for writing.

However, readable streams in Node 0.8 and before are rather complicated.

1. The data events start coming right away, no matter what. There is no way to do other actions before consuming data, without handling buffering yourself.
2. If you extend the interface in userland programs, then you must implement pause() and resume() methods, and take care of buffering yourself.

So, while writers only have to implement write(), end(), and drain, readers have to implement (at minimum):

• pause() method
• resume() method
• data event
• end event

If you are using a readable stream, and want to just get the first 10 bytes, make a decision, and then pass the rest off to somewhere else, then you have to handle buffering, pausing, and so on. This is all rather brittle and easy to get wrong for all but the most trivial use cases.

Additionally, this all made the reader.pipe(writer) method unnecessarily complicated and difficult to extend without breaking something. Backpressure and error handling is especially challenging and brittle.

Solution

The reader does not have pause/resume methods. If you want to consume the bytes, you call read(). If bytes are not being consumed, then effectively the stream is in a paused state. It exerts backpressure on upstream connections, doesn't read from files, etc.

If read() returns null, then a future readable event will be fired when there are more bytes ready to be consumed.

This is simpler and conceptually closer to the underlying mechanisms. The resulting pipe() method is much shorter and simpler.

Compatibility

It's not particularly difficult to wrap older-style streams in this new interface, or to wrap this type of stream in the older-style interface.

The Readable class takes an argument which is an old-style stream with data events and pause() and resume() methods, and uses that as the data source. For example:

var r = new Readable(oldReadableStream);

// now you can use r.read(), and it will emit 'readable' events

The Readable class will also automatically convert into an old-style data-emitting stream if any listeners are added to the data event. So, this works fine, though you of course lose a lot of the benefits of the new interface:

var r = new ReadableThing();

r.on('data', function(chunk) {
  // ...
});

// now pause, resume, etc. are patched into place, and r will
// continually call read() until it returns null, emitting the
// returned chunks in 'data' events.

r.on('end', function() {
  // ...
});