Asynchronous programming with java script and node.js

Asynchronous Programming
with JavaScript and Node.js
Timur Shemsedinov
Software Architect at Metarhia, Lecturer at KPI

Asynchronous programming in JavaScript as of today
● callbacks
● async.js
● promises
● async/await
● ?

● callbacks
● async.js
● promises
● async/await
● generators/yield
● events
● functor + chaining + composition

● callbacks > async.js
● promises > async/await
● events
● functor + chaining + composition

Callbacks
(callback) => callback(data)
(...args, callback) => callback(err, data)
Use contracts: callback-last, error-first
You can implement hell easely

Callbacks
readConfig('myConfig', (e, data) => {
query('select * from cities', (e, data) => {
httpGet('http://kpi.ua', (e, data) => {
readFile('README.md', (e, data) => {
});
});
});
});

Callbacks
readConfig('myConfig',
query.bind(null, 'select * from cities',
httpGet.bind(null, 'http://kpi.ua',
readFile.bind('README.md', () => {
});
});
});
});

Callbacks
readConfig('myConfig');
function readConfig(fileName) {
...; query('select * from cities');
}
function query(statement) {
...; httpGet('http://kpi.ua');
}
...

Library async.js or analogues
async.method(
[... (data, cb) => cb(err, result) ...],
(err, result) => {}
);
Use callback-last, error-first
Define functions separately, descriptive names
Hell remains

Events
const ee = new EventEmitter();
const f1 = () => ee.emit('step2');
const f2 = () => ee.emit('step3');
const f3 = () => ee.emit('done');
ee.on('step1', f1.bind(null, par));
ee.on('done', () => console.log('done'));
ee.emit('step1');

Promise
new Promise((resolve, reject) => {
resolve(data);
reject(new Error(...));
})
.then(result => {}, reason => {})
.catch(err => {});
Separated control flow for success and fail
Hell remains for complex parallel/sequential code

Promise Sequential
Promise.resolve()
.then(readConfig.bind(null, 'myConfig'))
.then(query.bind(null, 'select * from cities'))
.then(httpGet.bind(null, 'http://kpi.ua'))
.catch((err) => console.log(err.message))
.then(readFile.bind(null, 'README.md'))
.catch((err) => console.log(err.message))
.then((data) => {
console.dir({ data });
});

Promise Parallel
Promise.all([
readConfig('myConfig'),
doQuery('select * from cities'),
httpGet('http://kpi.ua'),
readFile('README.md')
]).then((data) => {
console.log('Done');
});

Promise Mixed: parallel / sequential
Promise.resolve()
.then(readConfig.bind(null, 'myConfig'))
.then(() => Promise.all([
query('select * from cities'),
gttpGet('http://kpi.ua')
]))
.then(readFile.bind(null, 'README.md'))
.then((data) => {
console.log('Done');
});

async/await
async function f() {
return await new Promise(...);
}
f().then(console.log).catch(console.error);
Promises under the hood, Control-flow separated
Hell remains, Performance reduced

Functor + Chaining + composition
const c1 = chain()
.do(readConfig, 'myConfig')
.do(doQuery, 'select * from cities')
.do(httpGet, 'http://kpi.ua')
.do(readFile, 'README.md');
c1();

Functor + chaining + composition
function chain(prev = null) {
const cur = () => {
if (cur.prev) {
cur.prev.next = cur;
cur.prev();
} else {
cur.forward();
}
};
cur.prev = prev;
cur.fn = null;
cur.args = null;
...
...
cur.do = (fn, ...args) => {
cur.fn = fn;
cur.args = args;
return chain(cur);
};
cur.forward = () => {
if (cur.fn) cur.fn(cur.args, () => {
if (cur.next) cur.next.forward();
});
};
return cur;
}

Problems
of callbacks, async.js, Promise, async/await
● Nesting and syntax
● Different contracts
● Not cancellable, no timeouts
● Complexity and Performance

Add timeout to any function
const fn = (par) => {
console.log('Function called, par: ' + par);
};
const fn100 = timeout(100, fn);
const fn200 = timeout(200, fn);
setTimeout(() => {
fn100('first'); fn200('second');
}, 150);

Add timeout to any function
function timeout(msec, fn) {
let timer = setTimeout(() => {
if (timer) console.log('Function timedout');
timer = null;
}, msec);
return (...args) => {
if (timer) {
timer = null;
fn(...args);
}
};
}

Make function cancelable
const fn = (par) => {
console.log('Function called, par: ' + par);
};
const f = cancelable(fn);
f('first');
f.cancel();
f('second');

Make function cancelable
const cancelable = (fn) => {
const wrapper = (...args) => {
if (fn) return fn(...args);
};
wrapper.cancel = () => {
fn = null;
};
return wrapper;
};

More wrappers
const f1 = timeout(1000, fn);
const f2 = cancelable(fn);
const f3 = once(fn);
const f4 = limit(10, fn);
const f5 = throttle(10, 1000, fn);
const f6 = debounce(1000, fn);
const f7 = utils(fn)
.limit(10)
.throttle(10, 100)
.timeout(1000);

Promisify and Callbackify
const promise = promisify(asyncFunction);
promise.then(...).catch(...);
const callback = callbackify(promise);
callback((err, value) => { ... });

Sync function to async
const f1 = par => par; const f2 = par => par;
console.log(f4(f3(f2(f1('value')))));
const af1 = toAsync(f1); const af2 = toAsync(f2);
const af3 = toAsync(f3); const af4 = toAsync(f4);
af1('value', (e, data) => {
af2(data, (e, data) => {
console.log(data);
});
});
});
});

Sync function to async
const last = arr => arr[arr.length - 1];
const toAsync = fn => (...args) => {
const callback = last(args);
args.pop();
callback(null, fn(...args));
};

Sync function to Promise
console.log(f4(f3(f2(f1('value')))));
const pf1 = toPromise(f1); const pf2 = toPromise(f2);
const pf3 = toPromise(f3); const pf4 = toPromise(f4);
Promise.resolve()
.then(pf1.bind(null, 'value'))
.then(pf2())
.then(pf3())
.then(pf4())
.then((data) => {
console.log(data);
});

Sync function to Promise
const toPromise = fn => (...args) =>
new Promise(resolve => resolve(fn(...args)));

Convertors
● err-back to Promise
● Promise to err-back
● sync function to Promise
● sync function to err-back
● Events to Promise
● Promise to Events
● Events to err-back
● err-back to Events

Metasync
● Function composition for asynchronous I/O
● Specific asynchronous abstractions
● Short and expressive syntax
● We use errback compatible contract
● IH

Function composition
inc = a => ++a;
square = a => a * a;
lg = x => log(10, x);
f = compose(inc, square, lg);
...but it’s synchronous

Function composition
Function composition is a great idea for asynchronous I/O
But there are questions:
● What about contracts?
○ for calls and callbacks, arguments and errors
○ timeouts, queueing, throttling
● How to add asynchronicity?
○ parallel and sequential

Asynchronous function composition
const readCfg = (name, cb) => fs.readFile(name, cb);
const netReq = (data, cb) => http.get(data.url, cb);
const dbReq = (query, cb) => db.select(query, cb);
const f1 = sequential(readCfg, netReq, dbReq);
const f2 = parallel(dbReq1, dbReq2, dbReq3);
// f1 & f2 contracts (...args, cb) => cb(err, data)

Flow commutation like in electronics
const fx = metasync.flow(
[f1, f2, f3, [[f4, f5, [f6, f7], f8]], f9]
);

Data collector
const dc1 = new metasync.DataCollector(4);
const dc2 = new metasync.DataCollector(4, 5000);
dc1.on('error', (err, key) => {});
dc2.on('timeout', (err, data) => {});
dc2.on('done', (errs, data) => {});
dc1.collect(data);

Key collector
const keyCollector = new KeyCollector(
['k1', 'k2'], (data) => console.dir(data)
);
keyCollector.collect('k1', {});
fs.readFile('HISTORY.md', (err, data) => {
keyCollector.collect('history', data);
});

Key collector
const kc = new metasync.KeyCollector(
['user', 'config', 'readme', 'timer'], (data) => console.dir(data)
);
kc.collect('user', { name: 'Marcus Aurelius' });
fs.readFile('HISTORY.md', (err,data) => kc.collect('history', data));
fs.readFile('README.md', (err,data) => kc.collect('readme', data));
setTimeout(
() => keyCollector.collect('timer', { date: new Date() }),
ASYNC_TIMEOUT
);

Collector
const dc1 = metasync
.collect(3)
.timeout(5000)
.done((err, data) => {});
dc1(item);
const dc2 = metasync
.collect(['key1', 'key2', 'key3'])
.timeout(5000)
dc2(key, value);

Collector features
const dc = metasync
.collect(count)
.distinct()
dc(key, error, value);
dc.pick(key, value);
dc.fail(key, error);
fs.readFile(filename, dc.bind(null, key));
dc.take(key, fs.readFile, filename);

Asynchronous programming with java script and node.js

Throttle
const t1 = metasync.throttle(5000, f);
t1();
t1();
t1(); // single call
setTimeout(t1, 7000); // another call
setTimeout(t1, 7100);
// will be fired at about 7000+5000

Queue
const cq = metasync.queue(3)
.wait(2000)
.timeout(5000)
.throttle(100, 1000)
.process((item, cb) => cb(err, result))
.success((item) => {})
.failure((item) => {})
.done(() => {})
.drain(() => {});

Timur Shemsedinov
tshemsedinov@github, timur.shemsedinov@gmail.com
tshemsedinov@facebook, marcusaurelius@habrahabr
Github repo: github.com/metarhia/metasync
http://how.programming.works
Telegram: t.me/metarhia & t.me/nodeua
Metarhia meetups: meetup.com/NodeUA,
meetup.com/HowProgrammingWorks

Asynchronous programming with java script and node.js

More Related Content

What's hot (20)

Similar to Asynchronous programming with java script and node.js (20)

More from Timur Shemsedinov (15)

Recently uploaded (20)

Asynchronous programming with java script and node.js