Functional Programming at Facebook. (30 min video, CUFP talk.)
vimeo.com
vimeo.com
[1]: http://cufp.galois.com/2009/abstracts.html#ChristopherPiroEu...
It was an excellent presentation by "the Erlang guys" at Facebook. Makes me wish I had a scaling problem to throw Erlang at. (I wonder if Google App Engine will ever support Erlang!)
"We like to think of Facebook as a communications platform, enabling communications between users". Facebook chat allows users to talk with each other in real-time (vs asynchronous, slow Inbox, Wall, Comments)
"Erlang is the right tool for the job" (they would not change it if they had to do it all over again) "Engineers are uncomfortable with functional programming. Facebook does not hire specifically for FP" Internal tech talks have gotten people excited, even if other FB engineers are not comfortable diving in - advertises the Chat guys as the go-to people for FP
Divisive in company: Majority of code/coders are PHP/C++ - minority in company (they are not outcasts but isolated, adoption outside chat is non-existant). However, the projects at FB tend to be self-contained - as long as it works. They are the people who maintain the chat - so not enough time to move until cool stuff with FP (but there is new stuff coming out with Jabber in Erlang).
Erlang and Haskell in use (~2:00)
Polyglot programming: Use the right language for the job
They use Thrift to write interoperable servers and clients (IDL like Corba but simpler). The Thrift bindings for Erlang were written in-house, as a pre-requisite for that FB chat servers.
Code gets out to production within a week, if not within the same day
Haskell doesn't really run on FB but in a sense it generates the code that runs FB (PHP transforms) (via audience question from Haskell language user - 'now I can impress my kids!')
Chat Development Timeline (~5:30)
2007: There weren't many if any English Erlang resources (blogs, message boards etc.)
Jan 2007: Chat prototyped
Fall 2007: 4 engineers, 0.5 designers
Winter 2007-08: Code, code, code (Learn Erlang - wasn't easy to learn)
Feb 2008: Dark Launch - Simulate load on the Erlang servers
April 6, 2008: First user message
April 23, 2008: 100% roll-out (at the time, 70M users)
Today: 1M messages/day
Peak: 10M active users (Gigabyte of traffic @ peak)
"Workload has increased dramatically. Refined the servers such that we've be able to outpace the growth of the users with the growth of the efficiency of the servers"
Chat Architecture (~7:00 on)
Channel servers deliver messages to browsers. 1 active request/browser tab
The greatest sweet spot of Erlang is the ability to harness concurrency and parallelism on a massive scale.
Very easy to model concurrent interactions: each chat user is independent and concurrent entity (~9:17)
Create an Erlang process for every user
Distribution (~10:00)
Connected network of nodes. Remote processes look just like local processes. Naive load balancing - Rest of nodes pick up work - all nodes are trusted and local and behind firewall
Fault Isolation (~11:00)
Had horrible bugs (process leaks, unintended multicasting, bad return states) in initial version of chat but they didn't kill the functional system - the bad Erlang process didn't crash the system, notifies the link processes and they are able to rebuild the state right away
Error logging
Stack traces from crash reports are very nice.
Since it's a functional language, have all of the arguments that go into execution (can find bug and simulate if necessary)
Too much error logging runs the Erlang node OutOfMemory
Hot-code swapping. (~13:00)
Can push new functional code in about 20 seconds without anyone losing state/kicking anyone off-line. Can push old code over new code quickly if something doesn't work
Also, if code crashes one server (they have 10 servers) - the work can be pushed to the other servers so they have more confidence in the hot code swapping features.
Monitoring and Error Recovery (~13:41)
Supervision hierarchy lets you control your processes
Extended Supervisor to quickly discover what each child name's process id is
Hibernation (~14:30)
Long running, idling HTTP request handlers don't use any memory when they are idling
Erlang hack: Cheating single assignment to allow direct sharing of arrays/mem structs with C++? (~15:33)
I suspect that erlang is successful despite of the language itself, mostly because it's also a mature product: a well tested production quality actor system with decades of tweaking.
I liked the Q&A especially whether a better type system would catch the "horrible bugs" they had :)
They do use C++ for the chat system, it's mainly the channels/presence (If memory serves me right) that are implemented in Erlang.