Benoit Chesneau - What Python can learn from Erlang?

What can we learn from Erlang for building reliable high concurrency services? This talk will shows some techniques used in Erlang and how they can be used to solve problems in a more efficient way in Python. It will also discuss how Python could evolve accordingly.

https://us.pycon.org/2015/schedule/presentation/388/

Transcript

1. What Python can learn from Erlang? benoît chesneau @benoitc PYCON US 2015 - Montreal
2. OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late- binding of all things. It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I'm not aware of them. Alan Kay http://userpage.fu-berlin.de/~ram/pub/pub_jf47ht81Ht/doc_kay_oop_en
3. It’s about reliability
4. What is reliable program ? ‣Resistant to failures ‣Recover easily ‣Hot-Upgrade
5. Problem?
 Stay alive
6. Reliability ‣Pattern matching ‣Process isolation ➡ assertion ‣Message passing ‣No Shared memory
7. Process isolation
8. P P P P Each process is isolated
9. P P P P Failure The system continues to operate
10. P P P P Recover Detect the failures and relaunch
11. Program to Systems ‣Supervisor ‣Load-balancer & Proxy ‣Containers
12. • Everything run in 1 thread • GIL • Use OS processes (multiprocessing, gunicorn, …) • PyParallel ? • PyPy STM ? Process isolation is hard in Python
13. • Fail fast • Crash early • Don’t catch too much • Crash in a way it can recover fast Let it crash
14. Immutability
15. • For one input we will always get the same output • An expression can be replaced b its value: F(I) = V, V = F(I) Immutability
16. • Easier to think about it • Easy to test • Easy to share even between different processes • Thread-Safe Immutability
17. Immutability • real world is mutable • but at some time, each mutation is immutable • Need to reference at some point the value of a data (identity) • snapshots can easily be stored
18. Identity class Person(object): def __init__(self, occupation, age): self.occupation = occupation self.age = age
19. Identity dev1 = Person("student", 23) dev1.occupation = "developer" dev1.age = 28 developers = dict( dev1 = dev1, ... devN = devN)
20. Identity developers = { "dev1": Person("dev", 20), ... "devN": .. }
21. dict should also be immutable for devid, dev in developers.items(): developers[devid] = dev.update()
22. immutable data structures • dict should also be immutable • Funktown: https://github.com/zhemao/funktown • Pysistence https://pythonhosted.org/pysistence • fn.py https://github.com/kachayev/fn.py • Need more.
23. immutable data structures • dict should also be immutable • Funktown: https://github.com/zhemao/funktown • Pysistence https://pythonhosted.org/pysistence • fn.py https://github.com/kachayev/fn.py • Need more.
24. Don’t be too catchy
25. try…except but not so much • Try to fail fast, do not catch everything • reraise once logged • Pattern-Matching
26. Pattern matching in Erlan {ok, Socket} = connect() {error, Error} will crash receive {say, Msg} -> say(Msg); quit -> quit(); _ -> % other value will crash throw(Error); end case msg() of {say, Msg} -> say(Msg); quit -> quit() % other value will crash end
27. patterns from patterns import patterns, Mismatch @patterns def factorial(): if 0: 1 if n is int: n * factorial(n-1) if []: [] if [x] + xs: [factorial(x)] + factorial(xs) if {'n': n, 'f': f}: f(factorial(n)) assert factorial(0) == 1 assert factorial(5) == 120 assert factorial([3,4,2]) == [6, 24, 2] assert factorial({'n': [5, 1], 'f': sum}) == 121 factorial('hello') # raises Mismatch https://github.com/Suor/patterns
28. separate actions from decision class Action(object): ... class SendMessage(Action): ... class Log(Action): ... def send_message(to, msg): if is_exists(to): SendMessage(to, msg) else: Log("error %s not found" % to) run(): for action in actions: action.do()
29. ? @benoitc http://enkim.eu