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Node的藝術

Node.js入門

本文件假設讀者已經懂了以下的兩樣東西：

  • 至少懂得一種程式語言。例如：JavaScript，Ruby，Python，Perl或其他程式語言。如果你還不是程式設計師，你也不懂程式語言，你可以閱讀JavaScript for Cats。:cat2:
  • git和github。這是一個開源的協作工具，Node社群的參與者使用git共享模組。你只需要懂得基本操作就夠了。這裏有三篇很好的入門教學：1, 2, 3

This short book is a work in progress + I don't have a job right now (if I did I wouldn't have the time to write this). If you like it then please consider donating via gittip so that I can write more!

譯者: 上面這段我沒有翻譯，因爲我希望保持原文。上面作者提到，目前他還沒找到工作。如果你喜歡這份文件，希望你可以通過gittip樂捐給作者。這樣作者才能夠寫更多內容。

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目錄

瞭解Node

Node.js是一個自由軟體專案，目的是讓你通過編寫JavaScript的程式進行網路、檔案系統或者與其他輸入/輸出裝置溝通的程式。就這樣！它只是一個簡單而穩定的輸入/輸出平臺，你可以在這個平臺上建構模組。

那有沒有一些關於輸入/輸出的實際例子？ 這裏有張我用Node.js製作的應用程式結構圖，你可以看到上面有很多輸入/輸出裝置：

server diagram

如果你無法完全瞭解上圖顯示的所有東西，那沒關係。重點是你看到一個Node的運作（中間的六邊形那個），它就像經紀人，管理全部輸入/輸出的節點（橙色和紫色的線條代表輸入/輸出）。

一般上我們撰寫的程式可以分爲以下兩類：

  • 很難寫，但是效率超高（就像用C從零開始編寫一個網頁伺服器）
  • 很好寫，但是不夠效率/強大（就像有人試圖上傳5GB的檔案去你伺服器，但是伺服器卻當掉了）

Node試圖在這兩者之間做到平衡：容易理解與使用，並且在多數的情況下能夠快速開發。

Node不是以下兩樣東西：

  • 不是Web框架 （不像Rails或Django，儘管它可以被用來做這樣的事情）
  • 不是程式語言（Node使用JavaScript程式語言，它沒有自己的語言）

相反，Node是：

  • 設計上簡單，而且容易明白和使用的平臺
  • 適合那些需要快速和處理很多輸入/輸出連接的程式

在底層，Node可以作爲一種工具，並編寫出以下兩類程式：

  • 需要使用到Web協議（如：HTTP、TCP、UDP、DNS和SSL）的網路程式
  • 需要對檔案系統或者本機執行緒/記憶體進行寫入或讀出操作的程式

什麼是“輸入/輸出程式”？ 這裏有一些常見的輸入/輸出裝置：

  • 資料庫 （如：MySQL、PostgreSQL、MongoDB、Redis、CouchDB）
  • APIs（如：Twitter、Facebook、Apple Push Notifications）
  • HTTP/WebSocket的連線（從使用者的Web應用程式）
  • 文件檔（圖像縮放軟件、影音編輯器、網路收音機）

Node能夠非同步處理多個不同種類的輸入/輸出來源。比如說，假設你來到快餐店，你向店員要了一個起士堡，他們會馬上爲你下單和準備漢堡。然後，他們會要求你在旁邊等漢堡完成。在你等待這段時間，他們可以接受其他訂單和幫其他人準備漢堡。試想一下，如果你站在櫃檯前面，一直等到你的起士堡完成，那麼你就阻礙了後面的人下訂單，廚師也不能幫其他人準備漢堡！我們稱這個爲阻塞式I/O，因爲一次只能處理一個I/O操作（廚師一次只能準備一個漢堡）。Node並不是這樣的，它是非阻塞的，也就是說它能一次準備很多漢堡。

感謝Node的非阻塞特性，讓我們可以實現以下這些有趣事情：

核心模組

首先，你需要安裝Node到你的電腦。Node安裝很簡單，只需瀏覽nodejs.org和按下Install.

Node擁有一組核心模組（通常被稱爲Node核心）提供公用 API 讓你開發時呼叫。我們可以呼叫fs模組來操作檔案系統。當我們要進行網路操作時候，我們會呼叫網路模組，例如：net（TCP），http，dgram（UDP）。

除了fs和網路模組之外，Node核心還有很多其他的核心模組。如dns模組用來非同步解析DNS查詢。os模組可以用來收集作業系統的資訊，如tempdir的路徑。buffer模組可以處理二進制資料。還有些模組可以處理URL和路徑，如：url，querystring和path等等。大部分的核心模組都支援Node的主要使用目標：快速編寫能夠進行檔案或網路操作的程式。

Node通過回呼機制，事件，串流和模組來控制I/O。如果你知道這四樣東西是如何工作的，那麼你就能夠靈活使用任何核心模組，並且懂得如何與這些模組串聯。

回呼機制

如果你想瞭解如何使用node，這將會是最重要的課題。幾乎在node中的所有事情都會使用到回呼機制。這並不是node發明的，它只是一種呼叫javascript函示的特殊方式。

Callbacks are functions that are executed asynchronously, or at a later time. Instead of the code reading top to bottom procedurally, async programs may execute different functions at different times based on the order and speed that earlier functions like http requests or file system reads happen.

The difference can be confusing since determining if a function is asynchronous or not depends a lot on context. Here is a simple synchronous example:

var myNumber = 1
function addOne() { myNumber++ } // define the function
addOne() // run the function
console.log(myNumber) // logs out 2

The code here defines a function and then on the next line calls that function, without waiting for anything. When the function is called it immediately adds 1 to the number, so we can expect that after we call the function the number should be 2.

Let's suppose that we want to instead store our number in a file called number.txt:

var fs = require('fs') // require is a special function provided by node
var myNumber = undefined // we dont know what the number is yet since it is stored in a file

function addOne() {
  fs.readFile('./number.txt', function doneReading(err, fileContents) {
    myNumber = parseInt(fileContents)
    myNumber++
  })
}

addOne()

console.log(myNumber) // logs out undefined

Why do we get undefined when we log out the number this time? In this code we use the fs.readFile method, which happens to be an asynchronous method. Usually things that have to talk to hard drives or networks will be asynchronous. If they just have to access things in memory or do some work on the CPU they will be synchronous. The reason for this is that I/O is reallyyy reallyyy sloowwww. A ballpark figure would be that talking to a hard drive is about 100,000 times slower than talking to memory (RAM).

When we run this program all of the functions are immediately defined, but they don't all execute immediately. This is a fundamental thing to understand about async programming. When addOne is called it kicks off a readFile and then moves on to the next thing that is ready to execute. If there is nothing to execute node will either wait for pending fs/network operations to finish or it will stop running and exit to the command line.

When readFile is done reading the file (this may take anywhere from milliseconds to seconds to minutes depending on how fast the hard drive is) it will run the doneReading function and give it an error (if there was an error) and the file contents.

The reason we got undefined above is that nowhere in our code exists logic that tells the console.log statement to wait until the readFile statement finishes before it prints out the number.

If you have some code that you want to be able to execute over and over again or at a later time the first step is to put that code inside a function. Then you can call the function whenever you want to run your code. It helps to give your functions descriptive names.

Callbacks are just functions that get executed at some later time. The key to understanding callbacks is to realize that they are used when you don't know when some async operation will complete, but you do know where the operation will complete — the last line of the async function! The top-to-bottom order that you declare callbacks does not necessarily matter, only the logical/hierarchical nesting of them. First you split your code up into functions, and then use callbacks to declare if one function depends on another function finishing.

The fs.readFile method is provided by node, is asynchronous and happens to take a long time to finish. Consider what it does: it has to go to the operating system, which in turn has to go to the file system, which lives on a hard drive that may or may not be spinning at thousands of revolutions per minute. Then it has to use a laser to read data and send it back up through the layers back into your javascript program. You give readFile a function (known as a callback) that it will call after it has retrieved the data from the file system. It puts the data it retrieved into a javascript variable and calls your function (callback) with that variable, in this case the variable is called fileContents because it contains the contents of the file that was read.

Think of the restaurant example at the beginning of this tutorial. At many restaurants you get a number to put on your table while you wait for your food. These are a lot like callbacks. They tell the server what to do after your cheeseburger is done.

Let's put our console.log statement into a function and pass it in as a callback.

var fs = require('fs')
var myNumber = undefined

function addOne(callback) {
  fs.readFile('./number.txt', function doneReading(err, fileContents) {
    myNumber = parseInt(fileContents)
    myNumber++
    callback()
  }
}

function logMyNumber() {
  console.log(myNumber)
}

addOne(logMyNumber)

Now the logMyNumber function can get passed in an argument that will become the callback variable inside the addOne function. After readFile is done the callback variable will be invoked (callback()). Only functions can be invoked, so if you pass in anything other than a function it will cause an error.

When a function get invoked in javascript the code inside that function will immediately get executed. In this case our log statement will execute since callback is actually logMyNumber. Remember, just because you define a function it doesn't mean it will execute. You have to invoke a function for that to happen.

To break down this example even more, here is a timeline of events that happen when we run this program:

  • 1: the code is parsed, which means if there are any syntax errors they would make the program break.
  • 2: addOne gets invoked, getting passed in the logMyNumber function as callback, which is what we want to be called when addOne is complete. This immediately causes the asynchronous fs.readFile function to kick off. This part of the program takes a while to finish.
  • 3: with nothing to do, node idles for a bit as it waits for readFile to finish
  • 4: readFile finishes and calls its callback, doneReading, which then in turn increments the number and then immediately invokes the function that addOne passed in (its callback), logMyNumber.

Perhaps the most confusing part of programming with callbacks is how functions are just objects that be stored in variables and passed around with different names. Giving simple and descriptive names to your variables is important in making your code readable by others. Generally speaking in node programs when you see a variable like callback or cb you can assume it is a function.

You may have heard the terms 'evented programming' or 'event loop'. They refer to the way that readFile is implemented. Node first dispatches the readFile operation and then waits for readFile to send it an event that it has completed. While it is waiting node can go check on other things. Inside node there is a list of things that are dispatched but haven't reported back yet, so node loops over the list again and again checking to see if they are finished. After they finished they get 'processed', e.g. any callbacks that depended on them finishing will get invoked.

Here is a pseudocode version of the above example:

function addOne(thenRunThisFunction) {
  waitAMinute(function waitedAMinute() {
    thenRunThisFunction()
  })
}

addOne(function thisGetsRunAfterAddOneFinishes() {})

Imagine you had 3 async functions a, b and c. Each one takes 1 minute to run and after it finishes it calls a callback (that gets passed in the first argument). If you wanted to tell node 'start running a, then run b after a finishes, and then run c after b finishes' it would look like this:

a(function() {
  b(function() {
    c()
  })
})

When this code gets executed, a will immediately start running, then a minute later it will finish and call b, then a minute later it will finish and call c and finally 3 minutes later node will stop running since there would be nothing more to do. There are definitely more elegant ways to write the above example, but the point is that if you have code that has to wait for some other async code to finish then you express that dependency by putting your code in functions that get passed around as callbacks.

The design of node requires you to think non-linearly. Consider this list of operations:

read a file
process that file

If you were to naively turn this into pseudocode you would end up with this:

var file = readFile()
processFile(file)

This kind of linear (step-by-step, in order) code is isn't the way that node works. If this code were to get executed then readFile and processFile would both get executed at the same exact time. This doesn't make sense since readFile will take a while to complete. Instead you need to express that processFile depends on readFile finishing. This is exactly what callbacks are for! And because of the way that JavaScript works you can write this dependency many different ways:

var fs = require('fs')
fs.readFile('movie.mp4', finishedReading)

function finishedReading(error, movieData) {
  if (error) return console.error(error)
  // do something with the movieData
}

But you could also structure your code like this and it would still work:

var fs = require('fs')

function finishedReading(error, movieData) {
  if (error) return console.error(error)
  // do something with the movieData
}

fs.readFile('movie.mp4', finishedReading)

Or even like this:

var fs = require('fs')

fs.readFile('movie.mp4', function finishedReading(error, movieData) {
  if (error) return console.error(error)
  // do something with the movieData
})

Events

In node if you require the events module you can use the so-called 'event emitter' that node itself uses for all of its APIs that emit things.

Events are a common pattern in programming, known more widely as the 'observer pattern' or 'pub/sub' (publish/subscribe). Whereas callbacks are a one-to-one relationship between the thing waiting for the callback and the thing calling the callback, events are the same exact pattern except with a many-to-many API.

Here are few common use cases for using events instead of plain callbacks:

  • Chat room where you want to broadcast messages to many listeners
  • Game server that needs to know when new players connect, disconnect, move, shoot and jump
  • Database connector that might need to know when the database connection opens, closes or sends an error

If we were trying to write a module that connects to a chat server using only callbacks it would look like this:

var chatClient = require('my-chat-client')

function onConnect() {
  // have the UI show we are connected
}

function onConnectionError(error) {
  // show error to the user
}

function onDisconnect() {
 // tell user that they have been disconnected
}

function onMessage(message) {
 // show the chat room message in the UI
}

chatClient.connect(
  'http://mychatserver.com',
  onConnect,
  onConnectionError,
  onDisconnect,
  onMessage
)

As you can see this is really cumbersome because of all of the functions that you have to pass in a specific order to the .connect function. Writing this with events would look like this:

var chatClient = require('my-chat-client').connect()

chatClient.on('connect', function() {
  // have the UI show we are connected
}) 

chatClient.on('connectionError', function() {
  // show error to the user
})

chatClient.on('disconnect', function() {
  // tell user that they have been disconnected
})

chatClient.on('message', function() {
  // show the chat room message in the UI
})

This approach is similar to the pure-callback approach but introduces the .on method, which subscribes a callback to an event. This means you can choose which events you want to subscribe to from the chatClient. You can also subscribe to the same event multiple times with different callbacks:

var chatClient = require('my-chat-client').connect()
chatClient.on('message', logMessage)
chatClient.on('message', storeMessage)

function logMessage(message) {
  console.log(message)
}

function storeMessage(message) {
  myDatabase.save(message)
}

MORE EVENTS CONTENT TODO

Streams

Early on in the project the file system and network APIs had their own separate patterns for dealing with streaming I/O. For example, files in a file system have things called 'file descriptors' so the fs module had to have extra logic to keep track of these things whereas the network modules didn't have such a concept. Despite minor differences in semantics like these, at a fundamental level both groups of code were duplicating a lot of functionality when it came to reading data in and out. The team working on node realized that it would be confusing to have to learn two sets of semantics to essentially do the same thing so they made a new API called the Stream and made all the network and file system code use it.

The whole point of node is to make it easy to deal with file systems and networks so it made sense to have one pattern that was used everywhere. The good news is that most of the patterns like these (there are only a few anyway) have been figured out at this point and it is very unlikely that node will change that much in the future.

THE REST IS TODO, in the meantime read the streams handbook

Modules

TODO

Going with the grain

Like any good tool, node is best suited for a certain set of use cases. For example: Rails, the popular web framework, is great for modeling complex business logic, e.g. using code to represent real life business objects like accounts, loan, itineraries, and inventories. While it is technically possible to do the same type of thing using node, there would be definite drawbacks since node is designed for solving I/O problems and it doesn't know much about 'business logic'. Each tool focuses on different problems. Hopefully this guide will help you gain an intuitive understanding of the strengths of node so that you know when it can be useful to you.

What is outside of node's scope?

Fundamentally node is just a tool used for managing I/O across file systems and networks, and it leaves other more fancy functionality up to third party modules. Here are some things that are outside the scope of node:

Web frameworks

There are a number of web frameworks built on top of node (framework meaning a bundle of solutions that attempts to address some high level problem like modeling business logic), but node is not a web framework. Web frameworks that are written using node don't always make the same kind of decisions about adding complexity, abstractions and tradeoffs that node does and may have other priorities.

Language syntax

Node uses JavaScript and doesn't change anything about it. Felix Geisendörfer has a pretty good write-up of the 'node style' here.

Language abstraction

When possible node will use the simplest possible way of accomplishing something. The 'fancier' you make your JavaScript the more complexity and tradeoffs you introduce. Programming is hard, especially in JS where there are 1000 solutions to every problem! It is for this reason that node tries to always pick the simplest, most universal option. If you are solving a problem that calls for a complex solution and you are unsatisfied with the 'vanilla JS solutions' that node implements, you are free to solve it inside your app or module using whichever abstractions you prefer.

A great example of this is node's use of callbacks. Early on node experimented with a feature called 'promises' that added a number of features to make async code appear more linear. It was taken out of node core for a few reasons:

  • they are more complex than callbacks
  • they can be implemented in userland (distributed on npm as third party modules)

Consider one of the most universal and basic things that node does: reading a file. When you read a file you want to know when errors happen, like when your hard drive dies in the middle of your read. If node had promises everyone would have to branch their code like this:

fs.readFile('movie.mp4')
  .then(function(data) {
    // do stuff with data
  })
  .error(function(error) {
    // handle error
  })

This adds complexity, and not everyone wants that. Instead of two separate functions node just uses a single callback function. Here are the rules:

  • When there is no error pass null as the first argument
  • When there is an error, pass it as the first argument
  • The rest of the arguments can be used for anything (usually data or responses since most stuff in node is reading or writing things)

Hence, the node callback style:

fs.readFile('movie.mp4', function(err, data) {
  // handle error, do stuff with data
})

Threads/fibers/non-event-based concurrency solutions

Note: If you don't know what these things mean then you will likely have an easier time learning node, since unlearning things is just as much work as learning things.

Node uses threads internally to make things fast but doesn't expose them to the user. If you are a technical user wondering why node is designed this way then you should 100% read about the design of libuv, the C++ I/O layer that node is built on top of.

Real-time apps

TODO - this section will have a non-contrived, functioning application with a web UI whose architecture will be dissected and discussed.

License

CCBY

Creative Commons Attribution License (do whatever, just attribute me) http://creativecommons.org/licenses/by/2.0/

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