Throttled Transform Stream

A NodeJS transform stream which runs a limited number of transformations per second, in parallel, and preserves input order. Optimised for API queries.

npm install throttled-transform-stream --save

Usage

import ThrottledTransform from 'throttled-transform-stream';

class MyThrottledTransformStream extends ThrottledTransform {
	constructor() {
		super({queriesPerSecond: 35, objectMode: true});
	}

	_throttledTransform(data, encoding, done) {
		// asynchronous operation
		done(null, data);
	}

	// optional
	_throttledFlush(done) {
		// finish stuff, if required
		done();
	}
}

Alternatively, you can use the following shortcut function:

import ThrottledTransform from 'throttled-transform-stream';

const MyThrottledTransformStream = ThrottledTransform.create((data, encoding, done) => {
	// long-running, asynchronous operation
	done(null, data);
});

## Documentation

All classes extending the ThrottledTransform class must implement the method _throttledTransform.
They may implement _throttledFlush, although this is not required.

### API

ThrottledTransform.create(transform, flush = function(done) { done(); }, defaultOptions = {})

• transform <Function> The _transform function of the stream. See below for more details
• flush <Function> The _flush function of the stream. See below for more details
• defaultOptions <Object> Default options for the class constructor (queriesPerSecond, all options of stream.Transform)

API for extending ThrottledTransform

The constructor of the ThrottledTransform class accepts all options accepted by stream.Transform. In addition, it accepts the queriesPerSecond property, which sets the maximum number of transformations per second.
The rate limitation follows a bucket model: queriesPerSecond transformations are started simultaneously. The limitation code then waits until a full second has passed since the first transformation was started, and then starts the next batch of queriesPerSecond transformations.

All classes extending ThrottledTransform must implement the _throttledTransform method, and may implement the _throttledFlush method.

ThrottledTransform._throttledTransform(chunk, encoding, callback)

• chunk <Buffer> | <String> The chunk to be transformed.
• encoding <String> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value - 'buffer', ignore it in this case.
• callback <Function> A callback function to be called after the supplied chunk has been processed. The first argument passed to the callback must be an error which has occurred during transformation, or null. The second argument is the result. The stream will stop processing transforms and emit an error event instantly if the error passed to the callback function was not null.

Please note that, as opposed to traditional NodeJS transform streams, you MUST NOT call this.push directly. Emit values through the callback function instead.
You must not call the callback more than once.

ThrottledTransform._throttledFlush(callback)

• callback <Function> A callback function to be called when the stream has finished flushing.

ThrottledTransform implementations may implement the transform._flush() method. This will be called when there is no more written data to be consumed, but before the 'end' event is emitted signaling the end of the Readable stream.

ThrottledTransform._skipThrottle(data, encoding)

• data <Buffer> | <String> The chunk to be transformed
• encoding <String> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value - 'buffer', ignore it in this case.

This function is executed for every value written to the stream before the _throttledTransform function is executed with the same value. If this function returns a truthy value it will be emitted immediately, and the execution of _throttledTransform will be skipped. This functionality can be useful for implementing caches or similar mechanisms. If this method is not implemented, it will default to return false.

Gotchas and caveats

• Calling this.push() will result in unexpected behaviour. Push results by calling done(null, result).
• Calling done() more than once will result in unexpected behaviour
• By design, you cannot push multiple results from a single transform