FlushWritable

A Writable stream that flushes before emitting finish.

Sponsored by Leadnomics.

What it is

Node.js's Streams API is a fantastic tool, but has a nagging shortcoming: while the Transform stream implements a _flush method that is called before its final events are fired, the Writable stream does not. So if you're buffering rows to be INSERTed into a SQL table rather than slowly writing one at a time, or you're buffering bytes for a transfer to S3, there is no way of flushing those buffers to the target data store before the finish event is emitted.

FlushWritable is a drop-in replacement for stream.Writable that implements a _flush call that behaves exactly how Transform._flush does. It's called with a callback, waits for the callback to be called, and then fires finish (or error if an error was passed). No additional execution after the finish event, no implementing nonstandard event types, no chaining a shell Transform stream before the Writable to hijack its _flush call. And it's fully futureproof against the Node.js team actually adding a _flush method to the native stream.Writable in a later version of Node, so you don't have to worry about your code breaking on upgrade.

How does it work?

It's pretty simple. Writable is an EventEmitter. FlushWritable extends Writable and overrides EventEmitter.emit in its own prototype, listening for a request that finish be emitted. When that comes in, it blocks that event from emitting, and calls _flush if it's defined.

The callback it passes to _flush will trigger finish to actually be emitted. If that callback is called with a truthy first argument, error is emitted instead. All other events pass right through and are emitted as expected. If a future version of node adds a Writable.prototype._flush method, the whole thing short-circuits and native functionality takes over.

Installation

In your project folder, type:

npm install flushwritable --save

Usage

Just extend FlushWritable instead of stream.Writable in your write stream, and feel free to define a _flush(cb) function!

var FlushWritable = require('flushwritable'),
    util = require('util');

function MyWriteStream(opts) {
    FlushWritable.call(this, opts);
    this._buffer = [];
}
util.inherits(MyWriteStream, FlushWritable);

MyWriteStream.prototype._flush = function(cb) {
	writeBufferSomewhere(this._buffer, cb);
};

MyWriteStream.prototype._write = function(data, encoding, cb) {
	this._buffer.push(data);
	cb();
};

License

FlushWritable is distributed under the MIT license.

Credits

FlushWritable was created by Tom Frost at Leadnomics in 2014.