A Writable stream that flushes before emitting finish.
Sponsored by Leadnomics.
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.
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.
In your project folder, type:
npm install flushwritable --save
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();
};FlushWritable is distributed under the MIT license.
FlushWritable was created by Tom Frost at Leadnomics in 2014.