The document discusses a case study of rewriting the architecture of a dance matching app called DuoDisco using the Erlang programming language. Originally the app had issues with race conditions and debugging complex interactions between connected players. By modeling each player as an Erlang process and using processes to represent rooms and matchmaking, the rewritten app in Erlang had no more bugs. The presenter advocates using the right tools like Erlang for concurrency and fault tolerance when needed.

1. Doing the Erlang dance
A case study
Arjan Scherpenisse
@acscherp

2. Who am I?
● Wide range of interests
● Polyglot programmer
● Contributor to Open Source Software
● Doing things differently...

3. Where others use … , I use …
● Mac → Linux
● iOS → Android
● Textmate Sublime Text → Emacs
● … → Erlang!

4. Introducing:

5. Do the DuoDisco!
● Mark Meeuwenoord, Klasien van de Zandschulp,
James Bryan Graves
● It’s fun. Profit comes later
1. Download app
2. Join disco
3. Dance
4. Find partner → score!
5. goto 3

6. DuoDisco Architecture
● Table: player
○ id
○ name
○ status (“waiting”, “playing”, “loading”, etc)
○ connected (true / false)
○ connected_to (player id FK)
○ current_song_id

8. Disco architecture issues
● Race Condition Paradise
○ Connection drops
○ Connection latency
○ Subtle client differences
● These bugs never happen while you’re
developing
● Player A → Player B, Player B → Player C
○ Debugging nightmare..

10. So what about that Erlang thing?
● A language, VM and runtime system
● Fault tolerant
● Concurrent
● Functional
● In use by major players

11. Erlang: Fault Tolerance
● Systems will error
● “Let it crash”
○ Only program the happy path
● You will be restarted
○ by your supervisor
○ with a clean slate

12. Erlang: Concurrence
● Processes (“Actors”) are cheap
● Processes communicate
● Processes contain state

13. Erlang: It’s Functional
● Assign-once
● Pattern matching
● No loops
○ use (tail)recursion
○ list comprehensions, map/reduce
● It has a WEIRD SYNTAX? ?F()(*$#)(*#
○ I had to say it somewhere
○ Look at Elixir, it rocks. http://elixir-lang.org/

14. Erlang + Web = Zotonic
● “The Django of Erlang”
● Web development framework
○ Built-in web server; CMS
○ Opinionated & extensible
● Great performance out-of-the-box
● Easy to use data model
○ Semantic web “everything is a thing”

15. Back in the Disco…
● 3 clients, iOS, Android & HTML
● Backend: Erlang + Zotonic (obviously...)
● JSON over WebSockets
● REST for login / discovery
● Zotonic manages data model
○ (media files, multiple disco’s)

16. DuoDisco refactoring
● Model it the Erlang way
● Every player is a process
● “Room” processes for connected players
● One match-making process
○ Uses process registry for querying processes

19. DuoDisco findings
● First try-out at Extrapool Festival 2 wks ago
● No more weird bugs!
● Lessons learned
○ Think (a bit) before you code
○ Use the right tool for the job
○ Dare to iterate and start over

20. Links
http://erlang.org/ - the Erlang language
http://zotonic.com/ - The web framework
http://duodisco.nl/ - Duo Disco app info
http://elixir-lang.org/ - Erlang with different syntax

21. Thanks! Questions?
Contact:
● http://twitter.com/acscherp
● http://miraclethings.nl/
● http://github.com/arjan
● buy me a beer :p