In this document we'll cover:
- The Event Loop
- What is single threaded?
- Thread Pool
- FAQ
- Callback Hell
- The Problem
- The Solution
- Frameworks
- CO Example
A single threaded application can be thought of as a long line at a cash register. Everything executes sequentially in order. The important take away is YOUR application does one thing at a time.
console.log('Reading file...');
var fileBytes = fs.readFileSync(filename, "utf8");
console.log('The file contains ' + fileBytes.length + ' bytes.');
console.log('Done.'); OUTPUT:
Reading file...
The file contains 142 bytes.
Done.
Handling a web request
function handleRequest() {
// Retrieve data from database
// Calculate averages
// Write data to database
// Send JSON object to client
}In the example above, no requests will be processed while data is being retrieve or written to the database. Node is single threaded and can only perform ONE action at a time.
This will reduce the number of request that your apllication can handle per second.
Apart from your application, Node runs an IO and Network thread pool which consists of 4 (by default) threads.
These threads can be used to offload IO and Network tasks.
Example of a non-blocking task.
console.log('Reading file...');
fs.readFile('/etc/hosts', 'utf8', function (err,fileBytes) {
console.log('The file contains ' + fileBytes.length + ' bytes');
});
console.log('Done.'); OUTPUT:
Reading file...
Done.
The file contains 142 bytes.
Example of a blocking task.
console.log('Reading file...');
fs.readFile('/etc/hosts', 'utf8', function (err,fileBytes) {
console.log('The file contains ' + fileBytes.length + ' bytes');
});
doSomeStuffThatTakesVeryLong();
console.log('Done.');What do you think the output will be?
OUTPUT:
Reading file...
Done.
The file contains 142 bytes.
Lets take another look at our snippet for handling a request.
function handleRequest() {
// Retrieve data from database
sql.query('SELECT * FROM ....', function (err1, result1) {
// Calculate averages
calculateAverages();
// Write data to database
sql.query('INSERT INTO ....', function (err2, result2) {
// Send JSON object to client
sendJSONToClient();
}
}
} // Find user by username
findUserByUsername(username, function (err, user) {
// Check if user is active
if (user.isActive) {
// Find user permissions by user id
findUserPermissions(user.id, function (err, permissions) {
// Check for error
if (err) {
console.error(err);
return;
}
if (permission.contains('admin')) {
// Insert data into database
sql.query('INSERT INTO ....', function (err, result) {
// Check for error
if (err) {
console.error(err);
return;
}
// Update data in database
sql.query('UPDATE ....', function (err, result) {
// Check for error
if (err) {
console.error(err);
return;
}
// Call third party api
http.get(url, function (err, response) {
// Check for error
if (err) {
console.error(err);
return;
}
// Send JSON object to client
sendJSONToClient();
});
});
});
}else {
// Insert data into database
sql.query('INSERT INTO ....', function (err, result) {
// Check for error
if (err) {
console.error(err);
return;
}
// Send JSON object to client
sendJSONToClient();
});
}
});
}
}); // Find user by username
findUserByUsername(username).then(function(user) {
// Check if user is active
if (user.isActive) {
// Find user permissions by user id
return findUserPermissions(user.id);
}
}).then(function (permission) {
if (permission.contains('admin')) {
// Insert data into database
return sql.query('INSERT INTO ....');
} else {
// Insert data into database
return sql.query('INSERT INTO ....');
}
}).then(function (result) {
// Call third party api
return http.get(url);
}).then(function (response) {
// Send JSON object to client
sendJSONToClient();
}).catch(function (err) {
console.error(err);
});Frameworks to make async calls read better:
- CO
- BlueBird
- Async.js
co(function* () {
// Find user by username
var user = yield findUserByUsername(username);
// Check if user is active
if (user.isActive) {
// Find user permissions by user id
var permissions = yield findUserPermissions(user.id);
if (permission.contains('admin')) {
// Insert data into database
var result = yield sql.query('INSERT INTO ....');
var response = yield http.get(url);
// Send JSON object to client
sendJSONToClient();
} else {
// Insert data into database
var result = yield sql.query('INSERT INTO ....');
}
}
});Copyright © 2017 Barend Erasmus
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