Quick introduction to Node.js
- It is a runtime environment that allows you to run Javascript code in the backend (eg on your computer), outside a browser
- The JS is executed by the V8 javascript engine (the thing that makes Google Chrome so fast)
- Node.js uses an event-driven, non-blocking I/O model
- Node ships with a lot of useful modules, so you don't have to write everything from scratch
Netflix, New York Times, PayPal, LinkedIn, Uber and many more..
If you don’t have the latest version of Node.js installed in your local machine you should do that now: Head out to Node.js and click on the “Install” button.
I/O stands for input/output
Here are some common sources of I/O based programs:
- Databases (e.g. MySQL, PostgreSQL, MongoDB, Redis, CouchDB)
- APIs (e.g. Twitter, Facebook, Apple Push Notifications)
- HTTP/Web socket connections (from users of a web app)
- Files (e.g image resizer, video editor, internet radio)
Node is very good when you need to do several things at the same time.
Excellent explanation by art-of-node:
For example, if you go down to a fast food joint and order a cheeseburger they will immediately take your order and then make you wait around until the cheeseburger is ready. In the meantime they can take other orders and start cooking cheeseburgers for other people. Imagine if you had to wait at the register for your cheeseburger, blocking all other people in line from ordering while they cooked your burger! This is called blocking I/O because all I/O (cooking cheeseburgers) happens one at a time. Node, on the other hand, is non-blocking, which means it can cook many cheeseburgers at once.
setTimeout(function() {
console.log('World');
}, 2000);
console.log('2000');
Run this code in repl.it (or something similar).
You will see that first output is 2000, and that the second is World.
2000 is not 'blocked' by the previous operation, which needs 2 seconds to be completed. Node just gets on executing the next function whilst the first one is still going on. This is asynchronous.
If it were a blocking model, there would be a 2-second pause, followed by World, and then 2000. That would be synchronous.
When code runs synchronously, things happen strictly in order - you wait for it to finish before moving on to another task. When you execute something asynchronously, you can move on to another task before it finishes.
To fully understand how Node.js works you need to know what a callback is.
A 'callback' is just a fancy term for a function that is passed as an argument to another function.
The point of callbacks is that you pass them into some function that runs asynchronously - it stores your callback away. When that function is done with whatever it needs to do, it will call your callback with the necessary parameters.
Example:
Imagine we have some operation that runs for a long time, such as fetching some data from the database. And let's say once we have that data, we want to run another function on it.
function getStuffFromDatabase() {
// this takes a long time
}
function doSomethingWithData (dataFromDatabase) {
// some data manipulation happens here
}
* other code *
...
...
We don't want our code to run synchronously and block other code from happening, which would waste precious time.
But then our doSomethingWithData function mustn't run before the dataFromDatabase is ready.
So how do we make sure our doSomethingWithData function runs at the right time, whilst everything is running asynchronously?
We can re-write dataFromDatabase to take a function as an argument. Then when we run dataFromDatabase, we can pass in doSomethingWithData as its argument.
In other words, we'll use doSomethingWithData as a callback function.
function getStuffFromDatabase (callback) {
// this takes a long time
callback(dataFromDatabase);
}
function doSomethingWithData (dataFromDatabase) {
// some data manipulation happens here
}
// Let's run the code now.
getStuffFromDatabase(doSomethingWithData);
* other code *
Note that the doSomethingWithData callback doesn't need invocation brackets - it will still work.
Let's test this out in real life.
- We'll simulate taking a long time with a call to
setTimeout. - We'll also pretend we got some data from the database, but we'll just hardcode a string value.
- Then we will console.log the data after the getting-from-database step is finished.
function getStuffFromDatabase (callback) {
setTimeout(function() {
var results = "database data";
}, 5000);
};
Not quite finished: let's make getStuffFromDatabase call the callback function, passing to it our database data.
function getStuffFromDatabase (callback) {
setTimeout(function() {
var results = "database data";
callback(results);
}, 5000);
}
Now lets run the function with our callback function.
function displayData(data) {
console.log(data);
}
getStuffFromDatabase(displayData);
You don't have to declare your callback function separately, by the way. You can also pass it straight in as an anonymous function. The following is exactly the same as above:
getStuffFromDatabase(function(data) {
console.log("The database data is " + data);
});
So, as you can see data came from the function that you pass your callback into;
it gives that data to you when it knows what that data should be.
Look at this example. This won't work how we want it to:
// executed immediately executed sometime in the future by getStuffFromDatabase
// | |
// v v
getStuffFromDatabase(function(data) {
var results = data; // <- this isn't available until sometime in the future!
});
console.log(results); // <- executed immediately
When the console.log runs we will get undefined. Because of Node's non-blocking model, it will not wait for getStuffFromDatabase to complete - it will just move straight on to the next task.
When Node reaches the console.log, the assignment of var results hasn't happened yet!
So the reason you can't set a value in your callback and use it outside the callback is because the callback itself doesn't happen until later in time.
That's exactly how Node.js works, it requires you to think non-linearly. This means that you can fire off several async calls to retrieve files, fetch data (or any event which takes time to complete) and not lock the main thread while it renders the content.
The power of async is being able to do multiple things at the same time without locking the main thread.
Node.js platform uses a “Single Threaded Model with Event Loop” architecture to handle multiple concurrent clients.
-
Client sends request to web server
-
Node.js web server receives those requests and places them into a queue – known as a “event queue”.
-
Node.js has a component internally, known as a the “Event Loop” - which picks up those requests one by one.
-
Event Loop uses a single thread only – which is a heart of Node.js model. Single Thread means that the program can run one piece of code at a time.
The event loop's job is:
- to check whether a client request needs blocking I/O operations (like interacting with a database, file system or external API )
or non-blocking I/O task
- if a request is non-blocking task, it prepare response and send it back to the client
- if a request is a blocking I/O operation, the event loop does not process this request. Instead it picks up one thread from the internal thread pool and assigns this client request to it. In this way the event loop can serve other clients without locking the main thread.
- This thread is responsible for taking that request, processing it, preparing the response and sending it back to the event loop
- The event loop, in turn, sends this response back to the client
- to check whether a client request needs blocking I/O operations (like interacting with a database, file system or external API )
or non-blocking I/O task
To understand more about the single-threaded versus multi-threaded model please refer here
Ryan Dahl (creator of Node.js), wants a completely asynchronous net server
- Ruby VM was slow...
- C/C++ is too complicated
- Lua is perfect except its I/O libraries were only synchronous
- Enter JavaScript:
- A naturally single-threaded/event-driven model, asynchronisation
- almost blank area on the server side, so no issues of breaking old habits
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