Pipes and Filters

Use the Pipes and Filters architectural style to divide a larger processing task into a sequence of smaller, independent processing steps (Filters) that are connected by channels (Pipes).

Each filter exposes a very simple interface: it receives messages on the inbound pipe, processes the message, and publishes the results to the outbound pipe.

The pipe connects one filter to the next, sending output messages from one filter to the next. Because all component use the same external interface they can be composed into different solutions by connecting the components to different pipes.

We can add new filters, omit existing ones or rearrange them into a new sequence -- all without having to change the filters themselves. The connection between filter and pipe is sometimes called port. In the basic form, each filter component has one input port and one output port.

What about native Node streams?

Node's core Stream module follows this same basic idea. Filters are essentially Transform streams, that you would .pipe() together to form a pipeline.

The aim of this library is to provide a much simpler interface due to having a limited feature set. If you are confident using Node's Stream API you can achieve a similar result.

Usage

Install the pipes-and-filters package.

npm install --save pipes-and-filters

Create, configure and execute a pipeline.

1. Import the `Pipeline` class by requiring the `pipes-and-filters` package.

var Pipeline = require('pipes-and-filters');

2. Create a pipeline, with an optional name.

var pipeline = Pipeline.create('order processing');

3. Create a filter function, that takes an input object and a Node style `next` callback to indicate the filter has completed or errored.

var decrypt = function(input, next) {
  // call a crypto lib's async decrypt function and process the error/result
  crypto.decrypt(input, function(err, decrypted) {
    // raise an error
    if (err) {
  	  return next(error);
    }

    // continue to next filter
    next(null, decrypted);
  }); 
};

4. Register one or more filters.

pipeline.use(decrypt);
pipeline.use(authenticate);
pipeline.use(deDuplicate);

You may optionally provide the context when the function is called.

pipeline.use(foo.filter, foo)

5. Add an `error` handler.

pipeline.once('error', function(err) {
  console.error(err);
});

The pipeline will stop processing on any filter error.

6. Add an `end` handler to be notified when the pipeline has completed.

pipeline.once('end', function(result) {
  console.log('completed', result);
});

7a. Execute the pipeline for a given input.

pipeline.execute(input);

With this style, an error event handler is required. Otherwise the default action on any filter error is to print a stack trace and exit the program.

7b. Execute the pipeline with a Node-style error/result callback.

pipeline.execute(input, function(err, result) {
  if (err) {
    console.error(err);
    return;
  }

  console.log('completed', result);
});

With this style, an error and/or end event handler are not required.

Early exit

You may exit early from a pipeline by passing Pipeline.break to the next callback. This will immediately stop execution and prevent any further filters from being called.

pipeline.use(function(input, next) {
	// exit the pipeline
	next(null, Pipeline.break);
});

For convenience, you may use Pipeline.breakIf passing in a predicate function that returns true to exit early.

Pipeline.breakIf(function(input) {
	return true;  // exit early
});

Note, if you want to be notified of a break in the pipeline you will need to add an event handler to the break event.

pipeline.on('break', function() {
	console.log('Pipeline exited early');
});

Alternatives

Another option is to use the pipeline function from event-stream. This can be combined with map to create a stream from an asynchronous function.

var es = require('event-stream');

es.pipeline(
  es.map(decrypt),
  es.map(authenticate),
  es.map(deDuplicate)
);

Enable objectMode for your streams to make them behave as a stream of objects.

slashdotdash/node-pipes-and-filters