Flow control in node.js

Flow Control in Node.js
송형주

Profile
● 삼성전자 소프트웨어센터
Web Convergence Lab.

● 블로그
Inside.JS (http://www.nodejs-kr.
org/insidejs)

var fs = require('fs')
Synchronous Programming
, path = './sync.txt';

var stats = fs.statSync(path);

if (stats == undefined) {
// read the contents of this file
var content = fs.readFileSync(__filename);
}

var code = content.toString();
// write the content to sync.txt
var err = fs.writeFileSync(path, code);
if(err) throw err;
console.log('sync.txt created!');

console.log(‘Another Task’);

Node.js programming is not easy.
var open = false;

setTimeout(function () {
open = true;
}, 1000);

while(!open) {
// wait
}

console.log('opened!');

Nested Callback Problem
async1(function(input, result1) {
async2(function(result2) {
async5(function(output) {
// do something with output
});
});
});
});
})

Flow Control Problem

for(var i = 1; i <= 1000; i++) {
fs.readFile('./'+i+'.txt',
function() {
// do something with the file
}
);
}

do_next_part();

Why Flow Control?
• Asynchronous Programming

• Issues
– Doing a bunch of things in a specific order
– Knowing when task is done.
– Nested Callback
– Collecting result data

Flow Control Pattern

• Serial Execution
• Parallel Execution
• Limited Parallel Execution

Serial Execution
Start

Task1

Task2

Task3

Continue when tasks
complete

Parallel Execution
Start

Task1 Task2 Task3

Continue When tasks
complete

Flow Control Libraries
• Async – providing various control function
• Step – simple and easy to use
• Slide – used in the npm client code
• Seq
• Nue
•…

Node.js Coding Conventions
• Two kinds of functions :
– Action Function : Take action
– Callback Function : Get results

• Action functions
– last argument is always a callback function

• Callback functions
– first argument is always an error or null.

Action Callback
Function Result Function

(Action Function Developer) (Action Function User)

Action Function Example
function actor (some, args, cb) {
// last argument is callback
// optional args:
if (!cb && typeof(args) === "function")
cb = args, args = [];

// do something, and then:

if (failed) cb(new Error("failed!"))
else cb(null, optionalData)
}

Action Callback
Function Result Function

Action Function과 Callback
Function 사이는 결과를 어떻게 넘길 지에 대
한 인터페이스가 명확히 규정되어 한다.

Action Function Example
// return true if a path is either
// a symlink or a directory.

function isLinkOrDir (path, cb) {
fs.lstat(path, function (er, s) {
if (er) return cb(er);
return cb(null,
s.isDirectory() ||
s.isSymbolicLink());
});
} isLinkorDir
Result cb
fs.lstat _cb
er, s er, result

Usecases : Parallel Execution
● I have a list of 10 ﬁles, and need to read all of
them, and then continue when they're all done.

● I have a dozen URLs, and need to fetch them all,
and then continue when they're all done.

●I have 4 connected users, and need to send a
message to all of them, and then continue when
that's done.

asyncMap
list
result

중간 결과 저 cb_
장

fn cb
err, err, result
data

Usecases : AsyncMap
function writeFiles (files, what, cb) {
asyncMap(files,
function (f, cb_) {
fs.writeFile(f,what,cb_);
}, cb_는 asyncMap의 내부 함수임
cb
);
}

writeFiles([my,file,list], "foo", cb);

writeFiles
asyncMap
files cb
list

fn cb_
err

This implementation is ﬁne if order doesn't
matter, but what if it does?

usecase: Serial Execution

• I have to do a bunch of things, in order.
Get db credentials out of a file, read the data
from the db, write that data to another file.

• If anything fails, do not continue.

function chain (things, cb) {
(function LOOP (i, len) {
if (i >= len)
return cb();
things[i](function (er) {
if (er)
return cb(er);
LOOP(i + 1, len)
})
})(0, things.length)
}

chain
things
LOOP,0

things[0] LOOP,1

things[1] LOOP,2

things[2] LOOP, 3 cb

Step Module Example

Step(
function readSelf() {
fs.readFile(__filename, 'utf8', this);
},
function capitalize(err, text) {
if (err) throw err;
return text.toUpperCase();
},
function showIt(err, newText) {
if (err) throw err;
console.log(newText);
}
);

Step Module Example
Step(
// Loads two files in parallel
function loadStuff() {
fs.readFile(__filename, this.parallel());
fs.readFile("/etc/passwd", this.
parallel());
},
// Show the result when done
function showStuff(err, code, users) {
if (err) throw err;
console.log(code);
console.log(users);
}
)

Reference
● slide module guide (Issacs) https://github.
com/isaacs/slide-flow-control/blob/master/nodejs-
controlling-flow.pdf

Flow control in node.js

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