Supervisors, Links and Monitors

Transcript

1. SUPERVISORS, LINKS & MONITORS Sean Cribbs

2. FLASHBACK: 2008 a more innocent time

3. PLAYSKOOL: MY FIRST ERLANG twitter client mochiweb handler mnesia

4. PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH twitter

   client mochiweb handler mnesia

5. PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤

   Buggy request handler? CRASH twitter client mochiweb handler mnesia

6. PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤

   Buggy request handler? CRASH ➤ Mnesia acting weird? CRASH twitter client mochiweb handler mnesia

7. PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤

   Buggy request handler? CRASH ➤ Mnesia acting weird? CRASH twitter client mochiweb handler mnesia RESULT: Unpredictable behavior

8. SUPERVISORS are not for restarting processes

9. SUPERVISORS are for structuring your application

10. HINDSIGHT IS 20/20 twitter client mochiweb handler mnesia my_app (supervisor)

11. HINDSIGHT IS 20/20 ➤ mnesia as application dependency twitter client

    mochiweb handler mnesia my_app (supervisor)

12. HINDSIGHT IS 20/20 ➤ mnesia as application dependency ➤ Client

    and mochiweb handler as children twitter client mochiweb handler mnesia my_app (supervisor)

13. HINDSIGHT IS 20/20 ➤ mnesia as application dependency ➤ Client

    and mochiweb handler as children ➤ Supervisor initializes mnesia tables twitter client mochiweb handler mnesia my_app (supervisor)

14. HINDSIGHT IS 20/20 ➤ mnesia as application dependency ➤ Client

    and mochiweb handler as children ➤ Supervisor initializes mnesia tables twitter client mochiweb handler mnesia RESULT: Reliability my_app (supervisor)

15. :observer.start()

16. SUPERVISOR STRATEGIES ➤ :one_for_one ➤ :one_for_all ➤ :rest_for_one ➤ :simple_one_for_one

17. SUPERVISOR STRATEGIES ➤ :one_for_one ➤ :one_for_all ➤ :rest_for_one ➤ :simple_one_for_one

    㱺 Task.Supervisor

18. SUPERVISORS use links and trap exits

19. LINKS intertwine fates of two processes

20. A B

21. A B Process.link(b)

22. file_server_2 Port (driver)

23. file_server_2 Port (driver) shutdown!

24. file_server_2 Port (driver) shutdown! EXIT close()

25. None

26. TRAP EXITS if you need to handle exit signals

27. TRAP EXITS if you need to handle exit signals Process.flag(:trap_exit,

    true)

28. TRAP EXITS if you need to handle exit signals Process.flag(:trap_exit,

    true) {:EXIT, pid, reason}

29. MONITORS notify of process exit without linking

30. A B

31. A B Process.monitor(b)

32. A B Process.monitor(b) {:DOWN, ref, :process, pid, reason}

33. A B GENSERVER.CALL/3

34. A B GENSERVER.CALL/3 ➤ Monitor the target {:ok, ref} =

    Process.monitor(b)

35. A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the

    message tagged with the monitor reference {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}

36. A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the

    message tagged with the monitor reference ➤ Receive {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}

37. A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the

    message tagged with the monitor reference ➤ Receive ➤ {^ref, reply} ⇒ 
 {:ok, reply} {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}

38. A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the

    message tagged with the monitor reference ➤ Receive ➤ {^ref, reply} ⇒ 
 {:ok, reply} ➤ :DOWN ⇒ exit(reason) {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}

39. A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the

    message tagged with the monitor reference ➤ Receive ➤ {^ref, reply} ⇒ 
 {:ok, reply} ➤ :DOWN ⇒ exit(reason) ➤ timeout ⇒ exit(:timeout) {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}

40. registry conn
 sup app sup CUSTOM PROCESS REGISTRY

41. registry conn
 sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:

    Lookup or start a connection to another node

42. registry conn
 sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:

    Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry client

43. registry conn
 sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:

    Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry ➤ Registry ⇒ Supervisor.start_child client

44. registry conn
 sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:

    Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry ➤ Registry ⇒ Supervisor.start_child conn client

45. registry conn
 sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:

    Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry ➤ Registry ⇒ Supervisor.start_child ➤ Registry ⇒ monitor connection conn client

46. registry conn
 sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:

    Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry ➤ Registry ⇒ Supervisor.start_child ➤ Registry ⇒ monitor connection ➤ Client ⇐ connection pid 
 (maybe monitor?) conn client

47. registry conn
 sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:

    Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry ➤ Registry ⇒ Supervisor.start_child ➤ Registry ⇒ monitor connection ➤ Client ⇐ connection pid 
 (maybe monitor?) ➤ DOWN ⇒ Registry cleans up client

48. TIPS AND TRICKS ➤ Don’t put too much on your

    Supervisor ➤ Child order matters ➤ Use links and monitors ➤ Read the Elixir and Erlang source

49. REFERENCES ➤ Erlang Reference Manual: Processes
 http://erlang.org/doc/reference_manual/processes.html ➤ Erlang Reference

    Manual: Errors
 http://erlang.org/doc/reference_manual/errors.html ➤ OTP Design Principles: Overview
 http://erlang.org/doc/design_principles/des_princ.html ➤ OTP Design Principles: Supervisors
 http://erlang.org/doc/design_principles/sup_princ.html ➤ OTP Design Principles: Applications
 http://erlang.org/doc/design_principles/applications.html