gen_statem - OTP's Unsung Hero

Given at AlchemyConf 2025 in Braga, Portugal.

Transcript

2. Andrea Leopardi @whatyouhide
6. gen_statem
7. but why?!
8. FSM
9. FSM initial state states state transitions
11. chilling_and_looking_sad_for_no_reason
14. Finite?
15. Age of AI
16. gen_statem
18. @behaviour :gen_statem
20. init/1 terminate/3 code_change/4 format_status/2
21. def callback_mode do :state_functions end
22. handle_call/3 handle_cast/2 handle_info/2
23. <state_name>/3
24. <state_name>( event_type, event_content, data )
25. <state_name>( event_type, event_content, data )
26. <state_name>( event_type, event_content, data )
27. def handle_cast( {:put_coin, coin}, state ) do {:noreply, update_in(state.credit, &add_coin/1)} end
28. def accepting_coins( :cast, {:put_coin, coin}, data ) do {:keep_state, update_in(data.credit, &add_coin/1)} end
31. def accepting_coins( {:call, from}, {:press_button, beverage}, data ) do if enough_credit?(data, beverage) do dispense_beverage(data, beverage) actions = [{:reply, from, :ok}] {:next_state, :dispensing, data, actions} else # ... end end
32. actions = [{:reply, from, :ok}] {:next_state, :dispensing, data, actions}
33. that’s kind of it?
34. Feature tour
35. Postponing events State enter callbacks Timeouts (state, event, named) Internal events Non-finite state machines
36. Postponing events
38. def dispensing(:cast, {:add_coin, _}, data) do actions = [:postpone] {:keep_state_and_data, actions} end def accepting_coins(:cast, {:add_coin, _}, data) do # Normal implementation end
39. “State enter”
40. def callback_mode do [ :state_functions, :state_enter ] end
41. def accepting_coins(:enter, _old_state, data) do update_display(data.credit) :keep_state_and_data end
42. Timeouts
43. Event timeouts
44. def accepting_coins(:cast, {:put_coin, coin}, data) do {:keep_state, update_in(data.credit, &add_coin/1)} end
45. def accepting_coins(:cast, {:put_coin, coin}, data) do actions = [ {:timeout, to_timeout(second: 30), :give_back_coins} ] {:keep_state, update_in(data.credit, &add_coin/1), actions} end def accepting_coins(:timeout, :give_back_coins, data)
46. {:timeout, to_timeout(second: 30), :give_back_coins} def accepting_coins(:timeout, :give_back_coins, data)
47. State timeouts
48. def dispensing(:enter, _old_state, _data) do actions = [ {:state_timeout, to_timeout(minute: 1), :stuck} ] {:keep_state_and_data, actions} end def dispensing(:state_timeout, :stuck, data) do # Call for help, the machine got stuck. end
49. Named timeouts
50. def init(options) do actions = [ {{:timeout, :self_health_check}, to_timeout(hour: 1), :no_content} ] {:ok, :idle, data, actions} end def idle({:timeout, :self_health_check}, _content, data) do # ... end
51. def accepting_coins({:timeout, :self_health_check}, _, _) do {:keep_state_and_data, [:postpone]} end def dispensing({:timeout, :self_health_check}, _, _) do {:keep_state_and_data, [:postpone]} end
52. {:timeout, ms, content} {:state_timeout, ms, content} {{:timeout, :some_name}, ms, content}
53. “Internal” events
54. def disconnected({:timeout, :reconnect}, _, _data) do {:keep_state_and_data, {:next_event, :internal, :connect}} end
55. Non-finite state machines
56. def callback_mode do :handle_event_function end
57. def accepting_coins( :cast, {:put_coin, coin}, data ) do # ... end
58. def handle_event( :cast, {:put_coin, coin}, :accepting_coins, data ) do # ... end
59. def handle_event( event_type, event_content, current_state, data )
60. def handle_event( :cast, {:put_coin, coin}, {:accepting_coins, credit}, data ) do new_state = {:accepting_coins, credit + coin} {:next_state, new_state, data} end
61. Use cases
63. GenServer :gen_statem or
64. def handle_call(call, from, %{user: nil}) def handle_call(call, from, %{user: %User{}}) def handle_call(call, from, %{user: %Admin{}})
65. def no_user({:call, from}, _, _) def user({:call, from}, _, _) def admin({:call, from}, _, _)
66. Connections!
67. DBConnection Redix HTTP/2 (?)
69. state/named timeouts -> reconnection w/ backoff event timeouts -> keepalive postponed events -> retrying requests
70. Workflow engines IoT sensors …
71. Wrapping up
72. Timeouts
73. No Elixir wrapper it’s fine
74. Data structures Processes vs
75. HTTP/2 stream state machine diagram
76. Composability
78. Resources Wikipedia Docs
79. Thanks! @whatyouhide AlchemyConf 2025