Supervisors, Links and Monitors

Using OTP tools in your Elixir application for great justice

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()
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