The Affect Of Back-Pressure When Piping Data Into Multiple Writable Streams In Node.js

high-water.js

// Require node modules.
var http = require( "http" );
var fileSystem = require( "fs" );
var stream = require( "stream" );

// Create an instance of our http server.
var httpServer = http.createServer(
	function handleRequest( request, response ) {

		// Create our file-input stream.
		var hankhill = fileSystem.createReadStream( "./hank-hill.png" );

		// Pipe it into a simple transform stream.
		// --
		// NOTE: Our transform stream is not being piped into anything else. However,
		// this time, the highWaterMark is sufficiently high to buffer the entire file
		// before causing back-pressure. As such, the source stream never needs to be
		// paused.
		hankhill.pipe(
			new stream.PassThrough({
				highWaterMark: ( 200 * 1024 )
			})
		);

		// ALSO pipe it into the response stream.
		hankhill.pipe( response );

	}
);

httpServer.listen( 8080 );

console.log( "Server running on port 8080" );

test.js

// Require node modules.
var http = require( "http" );
var fileSystem = require( "fs" );
var stream = require( "stream" );

// Create an instance of our http server.
var httpServer = http.createServer(
	function handleRequest( request, response ) {

		// Create our file-input stream.
		var hankhill = fileSystem.createReadStream( "./hank-hill.png" );

		// Pipe it into a simple transform stream.
		// --
		// NOTE: Our transform stream is not being piped into anything else. As such,
		// its internal buffers MAY fill up (depending on the highWaterMark setting and
		// the size of the file being streamed), creating backpressure and subsequently
		// pausing the file-input stream.
		hankhill.pipe( new stream.PassThrough() );

		// ALSO pipe it into the response stream.
		hankhill.pipe( response );

	}
);

httpServer.listen( 8080 );

console.log( "Server running on port 8080" );

test2.js

// Require node modules.
var http = require( "http" );
var fileSystem = require( "fs" );
var stream = require( "stream" );

// Create an instance of our http server.
var httpServer = http.createServer(
	function handleRequest( request, response ) {

		// Create our file-input stream.
		var hankhill = fileSystem.createReadStream( "./hank-hill.png" );

		// Pipe it into a simple transform stream.
		// --
		// NOTE: This time, our passthrough / transform stream is piping into a file-
		// output stream. As such, the passthrough buffers can drain and the transform
		// stream doesn't have to pause the file-input stream (at least not indefinitely).
		hankhill.pipe(
			new stream.PassThrough().pipe(
				fileSystem.createWriteStream( "./copy.png" )
			)
		);

		// ALSO pipe it into the response stream.
		hankhill.pipe( response );

	}
);

httpServer.listen( 8080 );

console.log( "Server running on port 8080" );