Node.js Streams: Alt hvad du behøver at vide

Opdatering: Denne artikel er nu en del af min bog "Node.js Beyond The Basics".

Læs den opdaterede version af dette indhold og mere om Node på jscomplete.com/node-beyond-basics .

Node.js-streams har ry for at være svære at arbejde med og endnu sværere at forstå. Nå, jeg har gode nyheder til dig - det er ikke længere tilfældet.

I årenes løb skabte udviklere mange pakker derude med det ene formål at gøre det lettere at arbejde med streams. Men i denne artikel vil jeg fokusere på den oprindelige Node.js stream API.

"Streams er Nodes bedste og mest misforståede idé."

- Dominic Tarr

Hvad er streams?

Streams er samlinger af data - ligesom arrays eller strenge. Forskellen er, at streams muligvis ikke er tilgængelige på én gang, og at de ikke behøver at passe i hukommelsen. Dette gør streams virkelig effektive, når du arbejder med store mængder data eller data, der kommer fra en ekstern kilde, en del ad gangen.

Imidlertid handler streams ikke kun om at arbejde med big data. De giver os også kraften i komposibilitet i vores kode. Ligesom vi kan komponere kraftige linux-kommandoer ved at pipere andre mindre Linux-kommandoer, kan vi gøre nøjagtigt det samme i Node med streams.

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

Mange af de indbyggede moduler i Node implementerer streaming-interface:

Ovenstående liste har nogle eksempler på native Node.js-objekter, der også er læsbare og skrivbare streams. Nogle af disse objekter er både læsbare og skrivbare streams, som TCP-sockets, zlib og crypto streams.

Bemærk, at genstandene også er tæt beslægtede. Mens et HTTP-svar er en læsbar strøm på klienten, er det en skrivbar strøm på serveren. Dette skyldes, at vi i HTTP-sagen grundlæggende læser fra det ene objekt ( http.IncomingMessage) og skriver til det andet ( http.ServerResponse).

Bemærk også, hvordan stdiovandløb ( stdin, stdout, stderr) har de inverse stream typer, når det kommer til børns processer. Dette giver en rigtig nem måde at pibe til og fra disse streams fra de vigtigste processtrømme stdio.

Et streams praktisk eksempel

Teorien er stor, men ofte ikke 100% overbevisende. Lad os se et eksempel, der viser forskellen, som streams kan skabe i kode, når det kommer til hukommelsesforbrug.

Lad os først oprette en stor fil:

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();

Se hvad jeg plejede at oprette den store fil. En skrivbar strøm!

Den fsModulet kan bruges til at læse fra og skrive til filer ved hjælp af en strøm-interface. I eksemplet ovenfor skriver vi til det big.filegennem en skrivbar strøm 1 million linjer med en løkke.

At køre ovenstående script genererer en fil, der handler om ~ 400 MB.

Her er en simpel Node-webserver designet til udelukkende at betjene 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);

Når serveren modtager en anmodning, serverer den den store fil ved hjælp af den asynkrone metode fs.readFile. Men hej, det er ikke som om vi blokerer begivenhedssløjfen eller noget. Hver ting er fantastisk, ikke? Ret?

Lad os se, hvad der sker, når vi kører serveren, opretter forbindelse til den og overvåger hukommelsen, mens vi gør det.

Da jeg kørte serveren, startede det med en normal mængde hukommelse, 8,7 MB:

Så oprettede jeg forbindelse til serveren. Bemærk hvad der skete med den forbrugte hukommelse:

Wow - hukommelsesforbruget sprang til 434,8 MB.

Vi lægger stort set hele big.fileindholdet i hukommelsen, før vi skrev det ud til responsobjektet. Dette er meget ineffektivt.

HTTP-responsobjektet ( resi koden ovenfor) er også en skrivbar strøm. Dette betyder, at hvis vi har en læsbar strøm, der repræsenterer indholdet af big.file, kan vi bare pibe de to på hinanden og for det meste opnå det samme resultat uden at forbruge ~ 400 MB hukommelse.

Nodes fsmodul kan give os en læsbar strøm til enhver fil ved hjælp af createReadStreammetoden. Vi kan røre det til responsobjektet:

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);

Nu når du opretter forbindelse til denne server, sker der en magisk ting (se på hukommelsesforbruget):

Hvad sker der?

Når en klient beder om den store fil, streamer vi den en klump ad gangen, hvilket betyder, at vi slet ikke buffer den i hukommelsen. Hukommelsesforbruget voksede med ca. 25 MB, og det er det.

Du kan skubbe dette eksempel til dets grænser. Genopret big.filemed fem millioner linjer i stedet for kun en million, hvilket ville føre filen til langt over 2 GB, og det er faktisk større end standardbuffergrænsen i Node.

Hvis du forsøger at servere den fil ved hjælp af fs.readFile, kan du simpelthen ikke som standard (du kan ændre grænserne). Men med fs.createReadStreamer der slet ingen problemer med at streame 2 GB data til rekvirenten, og bedst af alt er proceshukommelsesforbruget nogenlunde det samme.

Klar til at lære streams nu?

Denne artikel er en opskrivning af en del af mit Pluralsight-kursus om Node.js. Jeg dækker lignende indhold i videoformat der.

Strømme 101

Der er fire grundlæggende streamtyper i Node.js: læsbare, skrivbare, duplex- og transform-streams.

  • En læsbar strøm er en abstraktion for en kilde, hvorfra data kan forbruges. Et eksempel på det er fs.createReadStreammetoden.
  • En skrivbar strøm er en abstraktion for en destination, hvortil data kan skrives. Et eksempel på det er fs.createWriteStreammetoden.
  • 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'));

Forudsat at den beståede fil er den komprimerede version, opretter koden ovenfor en læsestrøm ud fra den, rør den i kryptostrømmen createDecipher()(ved hjælp af den samme hemmelighed), rør output af den i zlib- createGunzip()strømmen og skriv derefter tingene tilbage til en fil uden udvidelsesdelen.

Det er alt, hvad jeg har til dette emne. Tak for læsningen! Indtil næste gang!

Learning React eller Node? Tjek mine bøger:

  • Lær React.js ved at bygge spil
  • Node.js ud over det grundlæggende