Error and exception handling examined - looking at how erlang works

1. @michaelneale
www.CloudBees.com

2. @michaelneale
github.com/michaelneale
currently: trying to make less reliable things more
reliable.

3. Some things we use to get it done:
- scala (back end apps)
- ruby (chef, rails, proxymachine, lots)
- java (jenkins and plugins)
- erlang (“agents” and lots more..)

4. Exceptions. Errors.
What The Hell Is That Process Doing?
and how it relates to..

5. On Call PTSD

6. Exception handling is wrong...

7. But it seems harmless...

10. Erlang says no no no...

11. Question: a process on server not responding
Answer:
1) panic
2) take time thinking about underlying cause
3) bounce the bastard and pray **

12. FACT: Answer#3 will often work so well, you
often don’t really spend time to think about
doing #2**
** FibreError?

14. Imagine a programming environment:
- unlimited process spawning
- concurrency someone else’s problem
- don’t try to handle errors - let it fail

15. Erlang
Built for stability, and not bothering me.
Anything else is gravy
I wasn’t an erlang programmer

16. Process isolation
- it works so well
- nothing shared
- has worked so well for so long with OSes
- in erlang - each ‘process’ is like a process !

17. nginx Example: My Host OS
haproxy Erlang process
supervisor
...
p1 p999
Each: own heap, garbage collector

18. Erlang is:
- A vm based runtime (light !)
- language with prolog heritage (!)
- compiler
- interpreter
- debug tools - batteries included
- OTP - an application (server?) platform

19. Erlang is:
- Not OO
- no shared state **
- state kept in “term stores” - mini DBs
- processes easy to hot swap
- Quasi functional programming
(immutable variables, higher order)
- distributed (if you want)

23. OTP
- Open Telecommunications Platform
- not about telco !
- network stacks/apis, supervisor trees
- toolkit for reliable servers (middleware)
- behaviours

27. State in erlang
files
Host + real kernel
Erlang process
DETS supervisor
...
ETS p1 p999
Send Receive

28. Erlang processes, like OS processes
- Can only send messages to each other
- Can fail independently
- Can be restarted, safely, independently
- Easy to write apps for - Old School !

29. Better
- Tiny overhead (can have 100K+ of them !)
- perfect for supervisory agents
- distribute: run on every box if you like !
- lots of processes == lots of concurrency
- if you need to block, block !

30. Have used erlang for
- Supervisory agents - controllers
- Autoscaling services (realtime data analytics)
- misc backend systems
- fun command line tools !
- is “lite” VM (startup, footprint)

31. Supervisor configuration
- Declare rules for restarting, failure
- So you don’t have to intervene !
- can hot reload changes
- can have dependent processes

32. Restart just busted process

33. Program 1 (module with behaviour)

34. Program 2 (module with behaviour)

35. Anyway, that is why we use Erlang, also...
- RabbitMQ
- Riak (nosql DB)
- CouchDB (nosql DB)
- Probably your telephone exchange

36. Alternative approaches:
- Use “native” with daemontools/monit etc
- Akka (Scala) has similar supervisor
- library for your favourite language
- ephemeral “state less” apps
- Write software without bugs
- Or just restart everything 400 times a day

37. What is not awesome:
- OTP boiler plate
- VM not as fast as some
- No one seems to like the language
- tools? IDEs?

38. Thank you