Lessons learned from Node.js - Callbacks / Promises

Lessons Learned
from Node.js
Jason Yau
Director of Engineering
Samsung 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.

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

Fast
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 / waiting

Scale with Threads
Scale with Processes
Thread 1
Thread 2
Thread 3
Thread 4
Process 1
Process 2
Process 3
Process 4

Scale with Event Loop
Handles many concurrent requests in a single
process / thread
Process 1
Thread Pool

Analogy

Event Queue Thread Pool
Event Loop
Filesystem
Network
Process
Other
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 value
2. rejected - The promise is broken

Instead of calling a
passed callback,
return a promise.

Callback example

Return a promise

Passed callback

Parallel I/O

Promises are cleaner

Instead of calling a
passed callback,
return a promise.
Promise me you’ll
remember promises.

Thank You
Jason Yau
Director of Engineering, Samsung
work: jason@samsungaccelerator.com

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