Aliran Node.js: Semua yang perlu anda ketahui

Kemas kini: Artikel ini kini menjadi sebahagian daripada buku saya "Node.js Beyond The Basics".

Baca versi kandungan yang dikemas kini dan lebih lanjut mengenai Node di jscomplete.com/node-beyond-basics .

Aliran Node.js mempunyai reputasi kerana sukar dikerjakan, dan lebih sukar difahami. Baiklah saya ada berita baik untuk anda - itu tidak lagi berlaku.

Selama bertahun-tahun, pembangun membuat banyak pakej di luar sana dengan tujuan untuk menjadikan kerja dengan aliran lebih mudah. Tetapi dalam artikel ini, saya akan memberi tumpuan kepada API aliran Node.js asli.

"Aliran adalah idea terbaik dan paling salah difahami Node."

- Dominic Tarr

Apa sebenarnya aliran?

Strim adalah koleksi data - seperti tatasusunan atau rentetan. Perbezaannya adalah bahawa aliran mungkin tidak tersedia sekaligus, dan aliran tersebut tidak harus sesuai dengan memori. Jenama ini bersiaran benar-benar kuat apabila bekerja dengan jumlah data yang besar, atau data yang datang dari sumber yang satu luar sebahagian pada satu masa.

Walau bagaimanapun, aliran bukan sahaja berfungsi dengan data besar Mereka juga memberi kita kekuatan komposisi dalam kod kita. Sama seperti kita dapat menyusun perintah linux yang kuat dengan memasukkan perintah Linux lain yang lebih kecil, kita juga dapat melakukan hal yang sama di Node dengan aliran.

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

Sebilangan besar modul terbina dalam Node melaksanakan antara muka streaming:

Senarai di atas mempunyai beberapa contoh untuk objek Node.js asli yang juga aliran yang boleh dibaca dan boleh ditulis. Sebilangan objek ini adalah aliran yang boleh dibaca dan boleh ditulis, seperti soket TCP, aliran zlib dan crypto.

Perhatikan bahawa objek juga berkait rapat. Walaupun tindak balas HTTP adalah aliran yang dapat dibaca pada pelanggan, ia merupakan aliran yang boleh ditulis di pelayan. Ini kerana dalam kes HTTP, kita pada dasarnya membaca dari satu objek ( http.IncomingMessage) dan menulis ke yang lain ( http.ServerResponse).

Perhatikan juga bagaimana stdiosungai ( stdin, stdout, stderr) mempunyai jenis aliran songsang apabila ia datang kepada proses kanak-kanak. Ini memungkinkan cara yang sangat mudah untuk membuat paip ke dan dari aliran ini dari stdioaliran proses utama .

Contoh praktikal aliran

Teori itu hebat, tetapi selalunya tidak 100% meyakinkan. Mari kita lihat contoh yang menunjukkan perbezaan aliran yang dapat dibuat dalam kod ketika berkaitan dengan penggunaan memori.

Mari buat fail besar terlebih dahulu:

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();

Lihat apa yang saya gunakan untuk membuat fail besar itu. Aliran yang boleh ditulis!

The fsmodul boleh digunakan untuk membaca dari dan tulis ke fail menggunakan antara muka sungai. Dalam contoh di atas, kami menuliskannya big.filemelalui aliran yang dapat ditulis 1 juta baris dengan gelung.

Menjalankan skrip di atas menghasilkan fail sekitar ~ 400 MB.

Berikut adalah pelayan web Node ringkas yang direka untuk melayani secara eksklusif 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);

Apabila pelayan mendapat permintaan, ia akan melayani fail besar menggunakan kaedah tak segerak , fs.readFile. Tapi hei, bukan seperti kita menyekat gelung acara atau apa sahaja. Setiap perkara hebat, bukan? Betul?

Baiklah, mari kita lihat apa yang berlaku semasa kita menjalankan pelayan, menyambungkannya, dan memantau memori semasa melakukannya.

Semasa saya menjalankan pelayan, ia bermula dengan jumlah memori biasa, 8.7 MB:

Kemudian saya menyambung ke pelayan. Perhatikan apa yang berlaku pada memori yang habis:

Wow - penggunaan memori melonjak menjadi 434.8 MB.

Kami pada dasarnya meletakkan keseluruhan big.filekandungan dalam memori sebelum kami menuliskannya ke objek respons. Ini sangat tidak cekap.

Objek respons HTTP ( resdalam kod di atas) juga merupakan aliran yang boleh ditulis. Ini bermaksud jika kita mempunyai aliran yang dapat dibaca yang mewakili isi kandungannya big.file, kita hanya dapat menghubungkan kedua-duanya satu sama lain dan mencapai hasil yang hampir sama tanpa memakan memori ~ 400 MB.

fsModul Node dapat memberi kita aliran yang dapat dibaca untuk sebarang fail menggunakan createReadStreamkaedah tersebut. Kita boleh memasukkannya ke objek tindak balas:

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);

Sekarang apabila anda menyambung ke pelayan ini, perkara ajaib berlaku (lihat penggunaan memori):

Apa yang sedang berlaku?

Apabila pelanggan meminta fail besar itu, kami mengalirkannya satu bagian sekaligus, yang bermaksud kami sama sekali tidak menyimpannya dalam memori. Penggunaan memori meningkat sekitar 25 MB dan itu sahaja.

Anda boleh mendorong contoh ini ke hadnya. Buat semula big.filedengan lima juta baris dan bukan hanya satu juta, yang akan membawa fail ke lebih dari 2 GB, dan itu sebenarnya lebih besar daripada had penyangga lalai di Node.

Sekiranya anda cuba memberikan fail tersebut menggunakan fs.readFile, anda tidak boleh, secara lalai (anda boleh mengubah hadnya). Tetapi dengan fs.createReadStream, tidak ada masalah sama sekali mengalirkan data 2 GB ke pemohon, dan yang paling penting, penggunaan memori proses akan hampir sama.

Bersedia untuk belajar aliran sekarang?

Artikel ini adalah penulisan sebahagian daripada kursus Pluralsight saya mengenai Node.js. Saya merangkumi kandungan yang serupa dalam format video di sana.

Aliran 101

Terdapat empat jenis aliran asas dalam Node.js: Aliran boleh dibaca, Boleh ditulis, Dupleks, dan Transform.

  • Aliran yang dapat dibaca adalah pengabstrakan untuk sumber dari mana data dapat digunakan. Contohnya ialah fs.createReadStreamkaedah.
  • Aliran yang dapat ditulis adalah pengabstrakan untuk tujuan data boleh ditulis. Contohnya ialah fs.createWriteStreamkaedah.
  • 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:

  • Learn React.js by Building Games
  • Node.js Beyond the Basics