Node js lecture

Node.js
Threads – Modules - Callbacks

2
Threads versus Events
request = readRequest(socket);
reply = processRequest(request);
sendReply(socket, reply);
Implementation:
- Thread switching and scheduler
readRequest(socket, function(request) {
processRequest(request,
function (reply) {
sendReply(socket, reply);
});
});
Implementation:
- Event queue processing

3
Event queue
while (true) {
if (!eventQueue.notEmpty()) {
eventQueue.pop().call();
}
}
Inner loop
1. launchReadRequest(socket); // Returns immediately
2. eventQueue.push(readDoneEventHandler); // when readRequest is complete
Never wait/block in event handler [ex: readRequest(socket);]

4
Node.js
● Takes JavaScript engine from a browser (Chrome's V8 JavaScript Engine)
○ Get same JavaScript on both browser and server
○ Don't need the DOM on the server
● Adds events and an event queue
○ Everything runs as a call from the event loop
● Makes event interface to all OS operations
○ Wrap all the OS blocking calls (file and socket/network io)
○ Add some data handle support
● Adds a proper module system
○ Each module gets its own scope (not everything in document.window)

5
Example: Node.js callback
var fs = require("fs"); // require is a Node module call
// fs object wraps OS sync file system calls
// OS read() is synchronous but Node's fs.readFile is asynchronous
fs.readFile("smallFile", readDoneCallback); // Start read
function readDoneCallback(error, dataBuffer) {
// By convention: First argument is JavaScript Error object
// dataBuffer is a special Node Buffer object
if (!error) {
console.log("smallFile contents", dataBuffer.toString());
}
}

6
Node Modules
var notGlobal;
function func1() {}
function func2() {}
module.exports = {func1: func1, func2: func2};
● Import using require()
○ System module: require("fs"); // Looks in node_module directories
○ From a file: require("./XXX.js"); // Reads specified file
○ From a directory: require("./myModule"); // Reads myModule/index.js
● Module files have a private scope
○ Can declare variables that would be global in the browser
○ Require returns what is assigned to module.exports

7
Node modules
● Many standard Node modules
○ File system, process access, networking, timers, devices, crypto, etc.
● Huge library of modules (npm)
○ Do pretty much anything you want
● We use:
○ Express - Fast, unopinionated, minimalist web framework (speak HTTP)
○ Mongoose - Elegant mongodb object modeling (speak to the database)

8
Node Buffer class
● Manipulating lots of binary data wasn't a strength of the JavaScript engine
○ Of course, that is what web servers do:
DBMS ⇔ Web Server ⇔ Browser
● Node added a Buffer class - Optimized for storing and operating on
binary data
○ Interface looks an array of bytes (like the OS system calls use)
○ Memory is allocated outside of the V8 heap
● Used by the “wrapped” OS I/O calls (fs, net, …)
● Thus optimizing sharing with pointers rather than always copying
○ buffer.copy()
○ For example: socket.write instead of fs.readFile

9
Buffer operations
● Supports operations for selecting or updating certain values
○ Can peek at some values and send the bulk of it on its way
● Can convert a buffer or parts of a buffer to a JavaScript string
○ buf.toString("utf8"); // Convert to UTF8 - commonly used on the web
○ buf.toString("hex"); // Convert to hex encoding (2 digits per byte)
○ buf.toString("base64"); // Convert to base64 encoding

10
Example: Node.js callbacks (redux)
var fs = require("fs");
// fs has 81 properties: readFile, writeFile, most OS calls, etc.
fs.readFile("smallFile", readDoneCallback);
// Read has been launched - JavaScript execution continues
// Node.js exits when no callbacks are outstanding
function readDoneCallback(error, dataBuffer) {
// console.log(dataBuffer) prints <Buffer 66 73 20 3d 20 72 65 71 ...
if (!error) {
console.log("smallFile contents", dataBuffer.toString());
}
}

11
Programming with Events/Callbacks
r1 = step1();
console.log('step1 done', r1);
r2 = step2(r1);
r3 = step3(r2);
console.log('All Done!');
● Threads: Blocking/waiting is transparent
● Events: Blocking/waiting requires callback
step1(function(r1) {
step2(r1, function (r2) {
console.log('step3 done',r3);
});
});
}):
THREADS EVENTS

12
Programming with Events/Callbacks
● Key difference
○ Threads: Blocking/waiting is transparent
○ Events: Blocking/waiting requires callback
● Mental model
○ If code doesn't block: Same as thread programming
○ If code does block (or needs to block): Need to setup callback
○ Often what was a return statement becomes a function call

13
Example: Three step process(broken)
});
});
}):
// Won't work! Call outside scope!
THREADS (or non-blocking) CALLBACKS
r1 = step1();
r2 = step2(r1);
r3 = step3(r2);

14
Example: Three-step process(repaired)
});
});
}):
EVENTS
r1 = step1();
r2 = step2(r1);
r3 = step3(r2);
THREADS

15
Listener/emitter pattern
● When programing with events (rather than threads) a
listener/emitter pattern is used.
● Listener - Function to be called when the event is signaled
○ Should be familiar from DOM programming
(addEventListerner)
● Emitter - Signal that an event has occurred
○ Emit an event cause all the listener functions to be called

16
Node: EventEmitter = require('events');
myEmitter.on('myEvent', function(param1, param2) {
console.log('myEvent occurred with ' + param1 + ' and ' + param2 + '!');
});
myEmitter.emit('myEvent', 'arg1', 'arg2');
● On emit call listeners are called synchronously and in the
order the listeners were registered
● If no listener, then emit() is non-operative
● Listen with on() and signal with emit()

17
Typical EventEmitter patterns
myEmitter.on('conditionA', doConditionA);
myEmitter.on('conditionB', doConditionB);
myEmitter.on('conditionC', doConditionC);
myEmitter.on('error', handleErrorCondition);
● Handling 'error' exceptions is important - Node exits if
they are not caught!
● Have multiple different events for different state or actions
myEmitter.emit('error', new Error('Ouch!'));

18
Streams
● Build modules that produce and/or consume streams of data
● A popular way of structuring servers
○ Network socket ⇔ TCP/IP protocol processing ⇔ HTTP protocol processing ⇔ your code
● Can build connected streams dynamically
○ Add modules on stream: E.g. stream.push(Encryption)
Network socket ⇔ TCP/IP protocol processing ⇔ Encryption ⇔ HTTP processing
● Node's APIs heavily uses streams
○ Readable streams (e.g. fs.createReadStream)
○ Writable stream (e.g. fs.createWriteStream)
○ Duplex stream (e.g. net.createConnection)
○ Transform stream (e.g. zlib, crypto)

19
Readable streams - File reading using streams
var readableStreamEvent = fs.createReadStream("smallFile");
readableStreamEvent.on('data', function (chunkBuffer) {
// Could be called multiple times
console.log('got chunk of', chunkBuffer.length, 'bytes');
});
readableStreamEvent.on('end', function() {
// Called after all chunks read
console.log('got all the data');
});
readableStreamEvent.on('error', function (err) {
console.error('got error', err);
});

20
Writable streams - File writing using streams
var writableStreamEvent = fs.createWriteStream('outputFile');
writableStreamEvent.on('finish', function () {
console.log('file has been written!');
});
writableStreamEvent.write('Hello world!n');
writableStreamEvent.end();

21
Socket setup for TCP connections
sock = socket(AF_INET, SOCK_STREAM,
0);
connect(sock, &serverAddr,
sizeof(serverAddr));
write(sock, "Hi!", 3);
read(sock, buf, 3)
Server (Web Server)
lfd = socket(AF_INET, SOCK_STREAM, 0);
bind(lfd, &serverSddr, sizeof(serveraddr));
listen(lfd, 5);
sock = accept(lgf, &clientaddr, &clientlen);
read(sock, buf, 3);
write(sock, buf, 3)
Client (Browser)
● TCP/IP socket connection is a reliable, in-order byte stream
○ Node: reads can return data in different chunks that sent

22
TCP Networking on Node.js
var net = require('net');
net.createServer(processTCPconnection).listen(4000);
// Creates a socket, binds port 4000, and listens for connections
// Calls function processTCPconnection on each TCP connection
● Node net module wraps OS's network routines
● Includes higher level functionality like:

23
Example: A chat server
var clients = []; // List of connected clients
function processTCPconnection(socket) {
clients.push(socket); // Add this client to our connected list
socket.on('data', function (data) {
broadcast( "> " + data, socket); // Send received data to all
});
socket.on('end', function () {
clients.splice(clients.indexOf(socket), 1); // remove socket
});
}

24
Chat Server: broadcast
// Send message to all clients
function broadcast(message, sender) {
clients.forEach(function (client) {
if (client === sender) return;
// Don't send it to sender
client.write(message);
});
}
● Our chat server implementation is done!

25
Putting it together: A real simple file server
net.createServer(function (socket) {
socket.on('data', function (fileName) {
fs.readFile(fileName.toString(), function (error, fileData) {
if (!error) {
socket.write(fileData); // Writing a Buffer
} else {
socket.write(error.message); // Writing a String
}
socket.end();
});
});
}).listen(4000);

26
Example: Read three files
read(open("f1"), buf1, f1Size);
LINUX NODE.JS
// known by some as "Callback
Hell"
fs.readFile("f1", function
(error, data1) {
(error, data2) {
(error, data3) {
});
});
});

27
Example: Read N files(broken)
var fileContents = {};
['f1','f2','f3'].forEach(function (fileName) {
fs.readFile(fileName, function (error, dataBuffer) {
assert(!error);
fileContents[fileName] = dataBuffer;
});
});
If we use fileContents, how do we
know when ALL reads are
completed?
Recall: Can't wait in NodeJS

28
Example: Read N files(fix)
['f1','f2','f3'].forEach(function (fileName) {
assert(!error);
if (gotLastCallBack()) allDoneCallback(fileContents);
});
});
An 'inelegant' solution...

29
Example: Read N files(fix)
async.each(['f1','f2','f3'], readIt, function (err) {
if (!err) console.log('Done'); // fileContents filled in
if (err) console.error('Got an error:', err.message);
});
function readIt(fileName, callback) {
callback(error);
});
}
● Solution: Write a function that turns waiting into a callback
● 'async' to the rescue!

30
Node.JS - many useful built-in modules
● Buffer
● C/C++ Addons
● Child Processes
● Cluster
● Console
● Crypto
● Debugger
● DNS
● Errors
● Events
● File System
● Globals
● Timers
● TLS/SSL
● TTY
● UDP/Datagram
● URL
● Utilities
● V8
● VM
● ZLIB
● HTTP
● HTTPS
● Modules
● Net
● OS
● Path
● Process
● Punycode
● Query Strings
● Readline
● REPL
● Stream
● String Decoder

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Node js lecture