Lessons learned from Node.js - Callbacks / Promises

Lessons Learned from Node.js

Jason YauDirector of EngineeringSamsung Accelerator

Tuesday, October 8, 13

1. Node.js

2. Methodology

3. Callbacks

4. Promises

Node.js is javascript on the server side.

It’s event-driven, non-blocking I/O model makes it FAST and

scalable. Tuesday, October 8, 13

In most languages, the code we write is

synchronous.

aka Blocking

Blocking

Waiting

In javascript, we do things differently; asynchronously.

aka Non-blocking

Non-Blocking

Parallel

http://benchmarksgame.alioth.debian.org/u64/benchmark.php?test=all&lang=v8&lang2=yarv&data=u64

Javascript

Popular

Methodology

I/O Latency• L1: 3 cycles

• L2: 14 cycles

• RAM: 250 cycles

• DISK: 41,000,000 cycles

• NETWORK: 240,000,000 cycles

A theoretical request.Request Process Results

Query DB or Web Service Write to log file

Format Response

= blocking / waitingTuesday, October 8, 13

Scale with Threads

Scale with Processes

Thread 1Thread 2Thread 3Thread 4

Process 1Process 2Process 3Process 4

Scale with Event LoopHandles many concurrent requests in a single

process / thread

Process 1

Thread Pool

Analogy

Event Queue Thread Pool

Event Loop

FilesystemNetworkProcessOther

Scale with Event Loop

Challenges with Callbacks

Recurring Question

Callbacks Example

Pyramid of Doom

Debugging

Parallel Example

Promises

What are promises?

A promise is an object that represents a one-time event, typically the outcome of an async task like an AJAX call. When a promise is returned, at first it is in a pending state.

Eventually it’s either:1. resolved - The promise is fulfilled with a value2. rejected - The promise is broken

Instead of calling a passed callback, return a promise.

Callback example

Return a promise

Passed callback

Return a promise

Parallel I/O

Promises are cleaner

Instead of calling a passed callback, return a promise.

Promise me you’ll remember promises.

Thank YouJason Yau

Director of Engineering, Samsung work: jason@samsungaccelerator.com

Lessons learned from Node.js - Callbacks / Promises

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