Supervisors, Links and Monitors

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SUPERVISORS, LINKS & MONITORS Sean Cribbs

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FLASHBACK: 2008 a more innocent time

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PLAYSKOOL: MY FIRST ERLANG twitter client mochiweb handler mnesia

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PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH twitter client mochiweb handler mnesia

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PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤ Buggy request handler? CRASH twitter client mochiweb handler mnesia

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PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤ Buggy request handler? CRASH ➤ Mnesia acting weird? CRASH twitter client mochiweb handler mnesia

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PLAYSKOOL: MY FIRST ERLANG ➤ Twitter API changes? CRASH ➤ Buggy request handler? CRASH ➤ Mnesia acting weird? CRASH twitter client mochiweb handler mnesia RESULT: Unpredictable behavior

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SUPERVISORS are not for restarting processes

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SUPERVISORS are for structuring your application

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

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HINDSIGHT IS 20/20 ➤ mnesia as application dependency twitter client mochiweb handler mnesia my_app (supervisor)

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HINDSIGHT IS 20/20 ➤ mnesia as application dependency ➤ Client and mochiweb handler as children twitter client mochiweb handler mnesia my_app (supervisor)

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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)

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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)

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:observer.start()

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

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SUPERVISOR STRATEGIES ➤ :one_for_one ➤ :one_for_all ➤ :rest_for_one ➤ :simple_one_for_one 㱺 Task.Supervisor

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SUPERVISORS use links and trap exits

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LINKS intertwine fates of two processes

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A B

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A B Process.link(b)

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file_server_2 Port (driver)

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file_server_2 Port (driver) shutdown!

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file_server_2 Port (driver) shutdown! EXIT close()

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No content

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TRAP EXITS if you need to handle exit signals

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

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TRAP EXITS if you need to handle exit signals Process.flag(:trap_exit, true) {:EXIT, pid, reason}

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MONITORS notify of process exit without linking

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A B

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A B Process.monitor(b)

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

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A B GENSERVER.CALL/3

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A B GENSERVER.CALL/3 ➤ Monitor the target {:ok, ref} = Process.monitor(b)

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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}

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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}

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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}

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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}

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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}

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registry conn  sup app sup CUSTOM PROCESS REGISTRY

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registry conn  sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL: Lookup or start a connection to another node

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registry conn  sup app sup CUSTOM PROCESS REGISTRY ➤ GOAL: Lookup or start a connection to another node ➤ Client (GenServer.call) ⇒ Registry client

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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

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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

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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

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TIPS AND TRICKS ➤ Don’t put too much on your Supervisor ➤ Child order matters ➤ Use links and monitors ➤ Read the Elixir and Erlang source

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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