Streams de Node.js: todo lo que necesita saber

Programación
Actualización: este artículo ahora es parte de mi libro "Node.js más allá de lo básico".

Lea la versión actualizada de este contenido y más sobre Node en jscomplete.com/node-beyond-basics .

Las transmisiones de Node.js tienen la reputación de ser difíciles de trabajar e incluso más difíciles de entender. Bueno, tengo buenas noticias para ti, ese ya no es el caso.

A lo largo de los años, los desarrolladores crearon muchos paquetes con el único propósito de facilitar el trabajo con transmisiones. Pero en este artículo, me centraré en la API de transmisión nativa de Node.js.

"Las transmisiones son la mejor y más incomprendida idea de Node".

- Dominic Tarr

¿Qué son exactamente las corrientes?

Las secuencias son colecciones de datos, como matrices o cadenas. La diferencia es que las transmisiones pueden no estar disponibles todas a la vez y no tienen que caber en la memoria. Esto hace que las transmisiones sean realmente poderosas cuando se trabaja con grandes cantidades de datos o datos que provienen de una fuente externa, un fragmento a la vez.

Sin embargo, las transmisiones no solo se tratan de trabajar con big data. También nos dan el poder de la componibilidad en nuestro código. Así como podemos componer poderosos comandos de Linux canalizando otros comandos de Linux más pequeños, podemos hacer exactamente lo mismo en Node con streams.

const grep = ... // A stream for the grep output const wc = ... // A stream for the wc input grep.pipe(wc)

Muchos de los módulos integrados en Node implementan la interfaz de transmisión:

La lista anterior tiene algunos ejemplos de objetos nativos de Node.js que también son flujos de escritura y lectura. Algunos de estos objetos son flujos tanto legibles como de escritura, como sockets TCP, zlib y flujos criptográficos.

Observe que los objetos también están estrechamente relacionados. Mientras que una respuesta HTTP es una secuencia legible en el cliente, es una secuencia de escritura en el servidor. Esto se debe a que en el caso de HTTP, básicamente leemos de un objeto ( http.IncomingMessage) y escribimos en el otro ( http.ServerResponse).

También tenga en cuenta cómo las stdiocorrientes ( stdin, stdout, stderr) tienen los tipos de flujos inversos cuando se trata de procesos hijos. Esto permite una manera realmente fácil de canalizar hacia y desde estos flujos desde los flujos del proceso principal stdio.

Un ejemplo práctico de corrientes

La teoría es genial, pero a menudo no es 100% convincente. Veamos un ejemplo que demuestra la diferencia que pueden hacer los flujos en el código cuando se trata del consumo de memoria.

Primero creemos un archivo grande:

const fs = require('fs'); const file = fs.createWriteStream('./big.file'); for(let i=0; i<= 1e6; i++) { file.write('Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.\n'); } file.end();

Mira lo que usé para crear ese gran archivo. ¡Un flujo de escritura!

El fsmódulo se puede usar para leer y escribir en archivos usando una interfaz de flujo. En el ejemplo anterior, lo estamos escribiendo a big.filetravés de un flujo de escritura de 1 millón de líneas con un bucle.

La ejecución del script anterior genera un archivo de aproximadamente 400 MB.

Aquí hay un servidor web de Node simple diseñado para servir exclusivamente a big.file:

const fs = require('fs'); const server = require('http').createServer(); server.on('request', (req, res) => { fs.readFile('./big.file', (err, data) => { if (err) throw err; res.end(data); }); }); server.listen(8000);

Cuando el servidor recibe una solicitud, que va a servir al archivo de gran tamaño utilizando el método asíncrono, fs.readFile. Pero bueno, no es como si estuviéramos bloqueando el ciclo de eventos ni nada. Todo es genial, ¿verdad? ¿Correcto?

Bueno, veamos qué sucede cuando ejecutamos el servidor, nos conectamos a él y monitoreamos la memoria mientras lo hacemos.

Cuando ejecuté el servidor, comenzó con una cantidad normal de memoria, 8.7 MB:

Luego me conecté al servidor. Tenga en cuenta lo que sucedió con la memoria consumida:

Vaya, el consumo de memoria saltó a 434,8 MB.

Básicamente, big.fileguardamos todo el contenido en la memoria antes de escribirlo en el objeto de respuesta. Esto es muy ineficaz.

El objeto de respuesta HTTP ( resen el código anterior) también es un flujo de escritura. Esto significa que si tenemos un flujo legible que representa el contenido de big.file, podemos simplemente canalizar esos dos entre sí y lograr prácticamente el mismo resultado sin consumir ~ 400 MB de memoria.

El fsmódulo de Node puede darnos una secuencia legible para cualquier archivo usando el createReadStreammétodo. Podemos canalizar eso al objeto de respuesta:

const fs = require('fs'); const server = require('http').createServer(); server.on('request', (req, res) => { const src = fs.createReadStream('./big.file'); src.pipe(res); }); server.listen(8000);

Ahora, cuando te conectas a este servidor, sucede algo mágico (mira el consumo de memoria):

¿Qué esta pasando?

Cuando un cliente solicita ese archivo grande, lo transmitimos un fragmento a la vez, lo que significa que no lo almacenamos en la memoria. El uso de memoria creció en unos 25 MB y eso es todo.

Puede llevar este ejemplo al límite. Regenere el big.filecon cinco millones de líneas en lugar de solo un millón, lo que llevaría el archivo a más de 2 GB, y eso es en realidad más grande que el límite de búfer predeterminado en Node.

Si intenta servir ese archivo usando fs.readFile, simplemente no puede, de forma predeterminada (puede cambiar los límites). Pero con fs.createReadStream, no hay ningún problema en la transmisión de 2 GB de datos al solicitante y, lo mejor de todo, el uso de la memoria del proceso será aproximadamente el mismo.

¿Listo para aprender transmisiones ahora?

Este artículo es una reseña de parte de mi curso Pluralsight sobre Node.js. Allí cubro contenido similar en formato de video.

Streams 101

Hay cuatro tipos de transmisiones fundamentales en Node.js: transmisiones legibles, grabables, dúplex y transformadas.

  • Una secuencia legible es una abstracción de una fuente desde la que se pueden consumir datos. Un ejemplo de eso es el fs.createReadStreammétodo.
  • Un flujo de escritura es una abstracción de un destino en el que se pueden escribir datos. Un ejemplo de eso es el fs.createWriteStreammétodo.
  • A duplex streams is both Readable and Writable. An example of that is a TCP socket.
  • A transform stream is basically a duplex stream that can be used to modify or transform the data as it is written and read. An example of that is the zlib.createGzip stream to compress the data using gzip. You can think of a transform stream as a function where the input is the writable stream part and the output is readable stream part. You might also hear transform streams referred to as “through streams.”

All streams are instances of EventEmitter. They emit events that can be used to read and write data. However, we can consume streams data in a simpler way using the pipe method.

The pipe method

Here’s the magic line that you need to remember:

readableSrc.pipe(writableDest)

In this simple line, we’re piping the output of a readable stream — the source of data, as the input of a writable stream — the destination. The source has to be a readable stream and the destination has to be a writable one. Of course, they can both be duplex/transform streams as well. In fact, if we’re piping into a duplex stream, we can chain pipe calls just like we do in Linux:

readableSrc .pipe(transformStream1) .pipe(transformStream2) .pipe(finalWrtitableDest)

The pipe method returns the destination stream, which enabled us to do the chaining above. For streams a (readable), b and c (duplex), and d (writable), we can:

a.pipe(b).pipe(c).pipe(d) # Which is equivalent to: a.pipe(b) b.pipe(c) c.pipe(d) # Which, in Linux, is equivalent to: $ a | b | c | d

The pipe method is the easiest way to consume streams. It’s generally recommended to either use the pipe method or consume streams with events, but avoid mixing these two. Usually when you’re using the pipe method you don’t need to use events, but if you need to consume the streams in more custom ways, events would be the way to go.

Stream events

Beside reading from a readable stream source and writing to a writable destination, the pipe method automatically manages a few things along the way. For example, it handles errors, end-of-files, and the cases when one stream is slower or faster than the other.

However, streams can also be consumed with events directly. Here’s the simplified event-equivalent code of what the pipe method mainly does to read and write data:

# readable.pipe(writable) readable.on('data', (chunk) => { writable.write(chunk); }); readable.on('end', () => { writable.end(); });

Here’s a list of the important events and functions that can be used with readable and writable streams:

The events and functions are somehow related because they are usually used together.

The most important events on a readable stream are:

  • The data event, which is emitted whenever the stream passes a chunk of data to the consumer
  • The end event, which is emitted when there is no more data to be consumed from the stream.

The most important events on a writable stream are:

  • The drain event, which is a signal that the writable stream can receive more data.
  • The finish event, which is emitted when all data has been flushed to the underlying system.

Events and functions can be combined to make for a custom and optimized use of streams. To consume a readable stream, we can use the pipe/unpipe methods, or the read/unshift/resume methods. To consume a writable stream, we can make it the destination of pipe/unpipe, or just write to it with the write method and call the end method when we’re done.

Paused and Flowing Modes of Readable Streams

Readable streams have two main modes that affect the way we can consume them:

  • They can be either in the paused mode
  • Or in the flowing mode

Those modes are sometimes referred to as pull and push modes.

All readable streams start in the paused mode by default but they can be easily switched to flowing and back to paused when needed. Sometimes, the switching happens automatically.

When a readable stream is in the paused mode, we can use the read() method to read from the stream on demand, however, for a readable stream in the flowing mode, the data is continuously flowing and we have to listen to events to consume it.

In the flowing mode, data can actually be lost if no consumers are available to handle it. This is why, when we have a readable stream in flowing mode, we need a data event handler. In fact, just adding a data event handler switches a paused stream into flowing mode and removing the data event handler switches the stream back to paused mode. Some of this is done for backward compatibility with the older Node streams interface.

To manually switch between these two stream modes, you can use the resume() and pause() methods.

When consuming readable streams using the pipe method, we don’t have to worry about these modes as pipe manages them automatically.

Implementing Streams

When we talk about streams in Node.js, there are two main different tasks:

  • The task of implementing the streams.
  • The task of consuming them.

So far we’ve been talking about only consuming streams. Let’s implement some!

Stream implementers are usually the ones who require the stream module.

Implementing a Writable Stream

To implement a writable stream, we need to to use the Writable constructor from the stream module.

const { Writable } = require('stream');

We can implement a writable stream in many ways. We can, for example, extend the Writable constructor if we want

class myWritableStream extends Writable { }

However, I prefer the simpler constructor approach. We just create an object from the Writable constructor and pass it a number of options. The only required option is a write function which exposes the chunk of data to be written.

const { Writable } = require('stream'); const outStream = new Writable({ write(chunk, encoding, callback) { console.log(chunk.toString()); callback(); } }); process.stdin.pipe(outStream);

This write method takes three arguments.

  • The chunk is usually a buffer unless we configure the stream differently.
  • The encoding argument is needed in that case, but usually we can ignore it.
  • The callback is a function that we need to call after we’re done processing the data chunk. It’s what signals whether the write was successful or not. To signal a failure, call the callback with an error object.

In outStream, we simply console.log the chunk as a string and call the callback after that without an error to indicate success. This is a very simple and probably not so useful echo stream. It will echo back anything it receives.

To consume this stream, we can simply use it with process.stdin, which is a readable stream, so we can just pipe process.stdin into our outStream.

When we run the code above, anything we type into process.stdin will be echoed back using the outStreamconsole.log line.

This is not a very useful stream to implement because it’s actually already implemented and built-in. This is very much equivalent to process.stdout. We can just pipe stdin into stdout and we’ll get the exact same echo feature with this single line:

process.stdin.pipe(process.stdout);

Implement a Readable Stream

To implement a readable stream, we require the Readable interface, and construct an object from it, and implement a read() method in the stream’s configuration parameter:

const { Readable } = require('stream'); const inStream = new Readable({ read() {} });

There is a simple way to implement readable streams. We can just directly push the data that we want the consumers to consume.

const { Readable } = require('stream'); const inStream = new Readable({ read() {} }); inStream.push('ABCDEFGHIJKLM'); inStream.push('NOPQRSTUVWXYZ'); inStream.push(null); // No more data inStream.pipe(process.stdout);

When we push a null object, that means we want to signal that the stream does not have any more data.

To consume this simple readable stream, we can simply pipe it into the writable stream process.stdout.

When we run the code above, we’ll be reading all the data from inStream and echoing it to the standard out. Very simple, but also not very efficient.

We’re basically pushing all the data in the stream before piping it to process.stdout. The much better way is to push data on demand, when a consumer asks for it. We can do that by implementing the read() method in the configuration object:

const inStream = new Readable({ read(size) { // there is a demand on the data... Someone wants to read it. } });

When the read method is called on a readable stream, the implementation can push partial data to the queue. For example, we can push one letter at a time, starting with character code 65 (which represents A), and incrementing that on every push:

const inStream = new Readable({ read(size) { this.push(String.fromCharCode(this.currentCharCode++)); if (this.currentCharCode > 90) { this.push(null); } } }); inStream.currentCharCode = 65; inStream.pipe(process.stdout);

While the consumer is reading a readable stream, the read method will continue to fire, and we’ll push more letters. We need to stop this cycle somewhere, and that’s why an if statement to push null when the currentCharCode is greater than 90 (which represents Z).

This code is equivalent to the simpler one we started with but now we’re pushing data on demand when the consumer asks for it. You should always do that.

Implementing Duplex/Transform Streams

With Duplex streams, we can implement both readable and writable streams with the same object. It’s as if we inherit from both interfaces.

Here’s an example duplex stream that combines the two writable and readable examples implemented above:

const { Duplex } = require('stream'); const inoutStream = new Duplex({ write(chunk, encoding, callback) { console.log(chunk.toString()); callback(); }, read(size) { this.push(String.fromCharCode(this.currentCharCode++)); if (this.currentCharCode > 90) { this.push(null); } } }); inoutStream.currentCharCode = 65; process.stdin.pipe(inoutStream).pipe(process.stdout);

By combining the methods, we can use this duplex stream to read the letters from A to Z and we can also use it for its echo feature. We pipe the readable stdin stream into this duplex stream to use the echo feature and we pipe the duplex stream itself into the writable stdout stream to see the letters A through Z.

It’s important to understand that the readable and writable sides of a duplex stream operate completely independently from one another. This is merely a grouping of two features into an object.

A transform stream is the more interesting duplex stream because its output is computed from its input.

For a transform stream, we don’t have to implement the read or write methods, we only need to implement a transform method, which combines both of them. It has the signature of the write method and we can use it to push data as well.

Here’s a simple transform stream which echoes back anything you type into it after transforming it to upper case format:

const { Transform } = require('stream'); const upperCaseTr = new Transform({ transform(chunk, encoding, callback) { this.push(chunk.toString().toUpperCase()); callback(); } }); process.stdin.pipe(upperCaseTr).pipe(process.stdout);

In this transform stream, which we’re consuming exactly like the previous duplex stream example, we only implemented a transform() method. In that method, we convert the chunk into its upper case version and then push that version as the readable part.

Streams Object Mode

By default, streams expect Buffer/String values. There is an objectMode flag that we can set to have the stream accept any JavaScript object.

Here’s a simple example to demonstrate that. The following combination of transform streams makes for a feature to map a string of comma-separated values into a JavaScript object. So “a,b,c,d” becomes {a: b, c: d}.

const { Transform } = require('stream'); const commaSplitter = new Transform({ readableObjectMode: true, transform(chunk, encoding, callback) { this.push(chunk.toString().trim().split(',')); callback(); } }); const arrayToObject = new Transform({ readableObjectMode: true, writableObjectMode: true, transform(chunk, encoding, callback) { const obj = {}; for(let i=0; i < chunk.length; i+=2) { obj[chunk[i]] = chunk[i+1]; } this.push(obj); callback(); } }); const objectToString = new Transform({ writableObjectMode: true, transform(chunk, encoding, callback) { this.push(JSON.stringify(chunk) + '\n'); callback(); } }); process.stdin .pipe(commaSplitter) .pipe(arrayToObject) .pipe(objectToString) .pipe(process.stdout)

We pass the input string (for example, “a,b,c,d”) through commaSplitter which pushes an array as its readable data ([“a”, “b”, “c”, “d”]). Adding the readableObjectMode flag on that stream is necessary because we’re pushing an object there, not a string.

We then take the array and pipe it into the arrayToObject stream. We need a writableObjectMode flag to make that stream accept an object. It’ll also push an object (the input array mapped into an object) and that’s why we also needed the readableObjectMode flag there as well. The last objectToString stream accepts an object but pushes out a string, and that’s why we only needed a writableObjectMode flag there. The readable part is a normal string (the stringified object).

Node’s built-in transform streams

Node has a few very useful built-in transform streams. Namely, the zlib and crypto streams.

Here’s an example that uses the zlib.createGzip() stream combined with the fs readable/writable streams to create a file-compression script:

const fs = require('fs'); const zlib = require('zlib'); const file = process.argv[2]; fs.createReadStream(file) .pipe(zlib.createGzip()) .pipe(fs.createWriteStream(file + '.gz'));

You can use this script to gzip any file you pass as the argument. We’re piping a readable stream for that file into the zlib built-in transform stream and then into a writable stream for the new gzipped file. Simple.

The cool thing about using pipes is that we can actually combine them with events if we need to. Say, for example, I want the user to see a progress indicator while the script is working and a “Done” message when the script is done. Since the pipe method returns the destination stream, we can chain the registration of events handlers as well:

const fs = require('fs'); const zlib = require('zlib'); const file = process.argv[2]; fs.createReadStream(file) .pipe(zlib.createGzip()) .on('data', () => process.stdout.write('.')) .pipe(fs.createWriteStream(file + '.zz')) .on('finish', () => console.log('Done'));

So with the pipe method, we get to easily consume streams, but we can still further customize our interaction with those streams using events where needed.

What’s great about the pipe method though is that we can use it to compose our program piece by piece, in a much readable way. For example, instead of listening to the data event above, we can simply create a transform stream to report progress, and replace the .on() call with another .pipe() call:

const fs = require('fs'); const zlib = require('zlib'); const file = process.argv[2]; const { Transform } = require('stream'); const reportProgress = new Transform({ transform(chunk, encoding, callback) { process.stdout.write('.'); callback(null, chunk); } }); fs.createReadStream(file) .pipe(zlib.createGzip()) .pipe(reportProgress) .pipe(fs.createWriteStream(file + '.zz')) .on('finish', () => console.log('Done'));

This reportProgress stream is a simple pass-through stream, but it reports the progress to standard out as well. Note how I used the second argument in the callback() function to push the data inside the transform() method. This is equivalent to pushing the data first.

The applications of combining streams are endless. For example, if we need to encrypt the file before or after we gzip it, all we need to do is pipe another transform stream in that exact order that we needed. We can use Node’s crypto module for that:

const crypto = require('crypto'); // ... fs.createReadStream(file) .pipe(zlib.createGzip()) .pipe(crypto.createCipher('aes192', 'a_secret')) .pipe(reportProgress) .pipe(fs.createWriteStream(file + '.zz')) .on('finish', () => console.log('Done'));

The script above compresses and then encrypts the passed file and only those who have the secret can use the outputted file. We can’t unzip this file with the normal unzip utilities because it’s encrypted.

To actually be able to unzip anything zipped with the script above, we need to use the opposite streams for crypto and zlib in a reverse order, which is simple:

fs.createReadStream(file) .pipe(crypto.createDecipher('aes192', 'a_secret')) .pipe(zlib.createGunzip()) .pipe(reportProgress) .pipe(fs.createWriteStream(file.slice(0, -3))) .on('finish', () => console.log('Done'));

Assuming the passed file is the compressed version, the code above will create a read stream from that, pipe it into the crypto createDecipher() stream (using the same secret), pipe the output of that into the zlib createGunzip() stream, and then write things out back to a file without the extension part.

That’s all I have for this topic. Thanks for reading! Until next time!

Learning React or Node? Checkout my books:

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