Facebook chat architecture

Erlang at Facebook
Eugene Letuchy
Apr 30, 2009
2

1 Facebook ... and Erlang
2 Story of Facebook Chat
3 Facebook Chat Architecture
4 Key Erlang Features
5 Then and Now
Agenda
3

The Facebook Environment
▪ The Site
▪ More than 200 million active users
▪ More than 3.5 billion minutes are spent on Facebook each day
▪ Fewer than 900 employees
▪ The Engineering Team
▪ Fast iteration: code gets out to production within a week
▪ Polyglot programming: interoperability is key
▪ Practical: high-leverage tools win
5

Erlang Projects
▪ Chat: the biggest and best known user
▪ AIM Presence: a JSONP validator
▪ Chat Jabber support (ejabberd)
6

2007: Facebook needs Chat
Messages, Wall, Links aren’t enough
8

Enter a Hackathon (Jan 2007)
▪ Chat started in one night of coding
▪ Floating conversation windows
▪ No buddy list
▪ One server (no distribution)
▪ Erlang was there!
9

Enter Eugene (Feb 2007)
▪ I joined Facebook after Chat Hackathon
▪ What is this Erlang?
▪ Spring 2007:
▪ Learning Erlang from Joe Armstrong's thesis
▪ Lots of prototyping
▪ Evaluating infrastructure needs
▪ Summer 2007:
▪ Chris Piro works on Erlang Thrift bindings
10

Let’s do this!
▪ Mid-Fall 2007: Chat becomes a “real” project
▪ 4 engineers, 0.5 designer
▪ Infrastructure components get built and improved
▪ Feb 2008: “Dark launch” testing begins
▪ Simulates load on the Erlang servers ... they hold up
▪ Apr 6, 2008: First real Chat message sent
▪ Apr 23, 2008: 100% rollout (Facebook has 70M users at the time)
11

Launch: April 2008
▪ Apr 6, 2008: gradual live rollout starts
▪ First message: "msn chat?"
▪ Apr 23, 2008: 100% rollout (to Facebook’s 70M users)
▪ Graph of sends in the ﬁrst days of launch
0
3
6
9
12
15
Tue 00:00 12:00 Wed 00:00 12:00
millions of sends per hour
12

Chat ... one year later
▪ Facebook has 200M active users
▪ 800+ million user messages / day
▪ 7+ million active channels at peak
▪ 1GB+ in / sec at peak
▪ 100+ channel machines
▪ ~9-10 times the work at launch;
~2 as many machines
13

System challenges
▪ How does synchronous messaging work on the Web?
▪ “Presence” is hard to scale
▪ Need a system to queue and deliver messages
▪ Millions of connections, mostly idle
▪ Need logging, at least between page loads
▪ Make it work in Facebook’s environment
15

System overview - User Interface
Chat in the browser?
▪ Chat bar afﬁxed to the bottom of each Facebook page
▪ Mix of client-side Javascript and server-side PHP
▪ Works around transport errors, browser differences
▪ Regular AJAX for sending messages, fetching conversation history
▪ Periodic AJAX polling for list of online friends
▪ AJAX long-polling for messages (Comet)
17

System Overview - Back End
How does the back end service requests?
▪ Discrete responsibilities for each service
▪ Communicate via Thrift
▪ Channel (Erlang): message queuing and delivery
▪ Queue messages in each user’s “channel”
▪ Deliver messages as responses to long-polling HTTP requests
▪ Presence (C++): aggregates online info in memory (pull-based presence)
▪ Chatlogger (C++): stores conversations between page loads
▪ Web tier (PHP): serves our vanilla web requests
18

Message send
Me:
Lunch?
Eugene:
Lunch?
1 - ajax
2a - thrift
2b - thrift
3 - long poll
20

Channel servers
Architectural overview
▪ One channel per user
▪ Web tier delivers messages for that user
▪ Channel State: short queue of sequenced messages
▪ Long poll for streaming (Comet)
▪ Clients make an HTTP request
▪ Server replies when a message is ready
▪ One active request per browser tab
22

channel application
messages
authentication
online list messages
23

Channel servers
Architectural details
▪ Distributed design
▪ User id space is partitioned (division of labor)
▪ Each partition is serviced by a cluster (availability)
▪ Presence aggregation
▪ Channel servers are authoritative
▪ Periodically shipped to presence servers
▪ Open source: Erlang, Mochiweb, Thrift, Scribe, fb303,et al.
24

Key Erlang Features we love
25

Concurrency
▪ Cheap parallelism at massive scale
▪ Simpliﬁes modeling concurrent interactions
▪ Chat users are independent and concurrent
▪ Mapping onto traditional OS threads is unnatural
▪ Locality of reference
▪ Bonus: carries over to non-Erlang concurrent programming
26

Distribution
▪ Connected network of nodes
▪ Remote processes look like local processes
▪ Any node in a channel server cluster can route requests
▪ Naive load balancing
▪ Distributed Erlang works out-of-the-box (all nodes are trusted)
27

Fault Isolation
▪ Bugs in the initial versions of Chat:
▪ Process leaks in the Thrift bindings
▪ Unintended multicasting of messages
▪ Bad return state for presence aggregators
▪ (Horrible) bugs don’t kill a mostly functional system:
▪ C/C++ segfault takes down the OS process and your server state
▪ Erlang badmatch takes down an Erlang process
▪ ... and notiﬁes linked processes
28

Error logging (Crash Reports)
▪ Any proc_lib-compliant process generates crash reports
▪ Error reports can be handled out of band (not where generated)
▪ Stacktraces point the way to bugs (functional languages win big here)
▪ ... but they could be improved with source line numbers
▪ Writing error_log handlers is simple:
▪ gen_event behavior
▪ Allows for massaging of the crash and error messages (binaries!)
▪ Thrift client in the error log
▪ WARNING: error logging can OOM the Erlang node
29

Hot code swapping
▪ Restart-free upgrades are awesome (!)
▪ Pushing new functional code for Chat takes ~20 seconds
▪ No state is lost
▪ Test on a running system
▪ Provides a safety net ... rolling back bad code is easy
▪ NOTE: we don’t use the OTP release/upgrade strategies
30

Monitoring and Error Recovery
▪ Supervision hierarchies
▪ Organize (and control) processes
▪ Organize thoughts
▪ Systematize restarts and error recovery
▪ simple_one_for_one for dynamic child processes
▪ net_kernel (Distributed Erlang)
▪ sends nodedown, nodeup messages
▪ any process can subscribe
▪ heart: monitors and restarts the OS process
31

Remote Shell
▪ To invoke:
> erl -name hidden -hidden -remsh <node_name> -setcookie <cookie>
Eshell V5.7.1 (abort with ^G)
(<node_name>)1>
▪ Ad-hoc inspection of a running node
▪ Command-and-control from a console
▪ Combines with hot code loading
32

Erlang top (etop)
▪ Shows Erlang processes, sorted by
reductions, memory and message
queue
▪ OS functionality ... for free
33

Hibernation
▪ Drastically shrink memory usage with erlang:hibernate/3
▪ Throws away the call stack
▪ Minimizes the heap
▪ Enters a wait state for new messages
▪ “Jumps” into a passed-in function for a received message
▪ Perfect for a long-running, idling HTTP request handler
▪ But ... not compatible with gen_server:call (and gen_server:reply)
▪ gen_server:call has its own receive() loop
▪ hibernate() doesn’t support have an explicit timeout
▪ Fixed with a few hours and a look at gen.erl
34

Symmetric MultiProcessing (SMP)
▪ Take advantage of multi-core servers
▪ erl -smp runs multiple scheduler threads inside the node
▪ SMP is emphasized in recent Erlang development
▪ Added to Erlang R11B
▪ Erlang R12B-0 through R13B include ﬁxes and perf boosts
▪ Smart people have been optimizing our code for a year (!)
▪ Upgraded to R13B last night with about 1/3 less load
35

hipe_bifs
Cheating single assignment
▪ Erlang is opinionated:
▪ Destructive assignment is hard because it should be
▪ hipe_bifs:bytearray_update() allows for destructive array assignment
▪ Necessary for aggregating Chat users’ presence
▪ Don’t tell anyone!
36

Then and now Erlang in Progress
37

Then ... a steep learning curve
▪ Start of 2007:
▪ Few industry-focused English-language resources
▪ Few blogs (outside of Yariv’s and Joel Reymont’s)
▪ Code examples spread out and disorganized
▪ U.S. Erlang community limited in number and visibility
38

Now ...
▪ Programming Erlang (Jun 2007)
▪ Erlang Programming (upcoming...)
▪ More blogs and blog aggregators:
▪ Planet Erlang, Planet TrapExit
▪ Erlang Factory aggregates Erlang developments
▪ More code available:
▪ GitHub, CEAN
▪ More general-purpose Open Source Libraries
▪ U.S. -located conference and ErlLounges
39

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