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Supervisors, Links and Monitors
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Sean Cribbs
September 20, 2016
Technology
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75
Supervisors, Links and Monitors
Using OTP tools in your Elixir application for great justice
Sean Cribbs
September 20, 2016
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Transcript
SUPERVISORS, LINKS & MONITORS Sean Cribbs
FLASHBACK: 2008 a more innocent time
PLAYSKOOL: MY FIRST ERLANG twitter client mochiweb handler mnesia
PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH twitter
client mochiweb handler mnesia
PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤
Buggy request handler? CRASH twitter client mochiweb handler mnesia
PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤
Buggy request handler? CRASH ➤ Mnesia acting weird? CRASH twitter client mochiweb handler mnesia
PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤
Buggy request handler? CRASH ➤ Mnesia acting weird? CRASH twitter client mochiweb handler mnesia RESULT: Unpredictable behavior
SUPERVISORS are not for restarting processes
SUPERVISORS are for structuring your application
HINDSIGHT IS 20/20 twitter client mochiweb handler mnesia my_app (supervisor)
HINDSIGHT IS 20/20 ➤ mnesia as application dependency twitter client
mochiweb handler mnesia my_app (supervisor)
HINDSIGHT IS 20/20 ➤ mnesia as application dependency ➤ Client
and mochiweb handler as children twitter client mochiweb handler mnesia my_app (supervisor)
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)
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)
:observer.start()
SUPERVISOR STRATEGIES ➤ :one_for_one ➤ :one_for_all ➤ :rest_for_one ➤ :simple_one_for_one
SUPERVISOR STRATEGIES ➤ :one_for_one ➤ :one_for_all ➤ :rest_for_one ➤ :simple_one_for_one
㱺 Task.Supervisor
SUPERVISORS use links and trap exits
LINKS intertwine fates of two processes
A B
A B Process.link(b)
file_server_2 Port (driver)
file_server_2 Port (driver) shutdown!
file_server_2 Port (driver) shutdown! EXIT close()
None
TRAP EXITS if you need to handle exit signals
TRAP EXITS if you need to handle exit signals Process.flag(:trap_exit,
true)
TRAP EXITS if you need to handle exit signals Process.flag(:trap_exit,
true) {:EXIT, pid, reason}
MONITORS notify of process exit without linking
A B
A B Process.monitor(b)
A B Process.monitor(b) {:DOWN, ref, :process, pid, reason}
A B GENSERVER.CALL/3
A B GENSERVER.CALL/3 ➤ Monitor the target {:ok, ref} =
Process.monitor(b)
A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the
message tagged with the monitor reference {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}
A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the
message tagged with the monitor reference ➤ Receive {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}
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) {:'$gen_call', {a, ref}, :hi}
A B GENSERVER.CALL/3 ➤ Monitor the target ➤ Send the
message tagged with the monitor reference ➤ Receive ➤ {^ref, reply} ⇒ {:ok, reply} ➤ :DOWN ⇒ exit(reason) {:ok, ref} = Process.monitor(b) {:'$gen_call', {a, ref}, :hi}
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}
registry conn sup app sup CUSTOM PROCESS REGISTRY
registry conn sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:
Lookup or start a connection to another node
registry conn sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL:
Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry client
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
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
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
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
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
TIPS AND TRICKS ➤ Don’t put too much on your
Supervisor ➤ Child order matters ➤ Use links and monitors ➤ Read the Elixir and Erlang source
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