gen_statem - OTP's Unsung Hero

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Andrea Leopardi @whatyouhide

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gen_statem

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but why?!

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FSM

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FSM initial state states state transitions

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chilling_and_looking_sad_for_no_reason

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Finite?

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Age of AI

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gen_statem

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@behaviour :gen_statem

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init/1 terminate/3 code_change/4 format_status/2

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def callback_mode do :state_functions end

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handle_call/3 handle_cast/2 handle_info/2

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( event_type, event_content, data )

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( event_type, event_content, data )

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( event_type, event_content, data )

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def handle_cast( {:put_coin, coin}, state ) do {:noreply, update_in(state.credit, &add_coin/1)} end

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def accepting_coins( :cast, {:put_coin, coin}, data ) do {:keep_state, update_in(data.credit, &add_coin/1)} end

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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

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actions = [{:reply, from, :ok}] {:next_state, :dispensing, data, actions}

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that’s kind of it?

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Feature tour

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Postponing events State enter callbacks Timeouts (state, event, named) Internal events Non-finite state machines

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Postponing events

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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

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“State enter”

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def callback_mode do [ :state_functions, :state_enter ] end

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def accepting_coins(:enter, _old_state, data) do update_display(data.credit) :keep_state_and_data end

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Timeouts

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Event timeouts

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def accepting_coins(:cast, {:put_coin, coin}, data) do {:keep_state, update_in(data.credit, &add_coin/1)} end

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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)

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{:timeout, to_timeout(second: 30), :give_back_coins} def accepting_coins(:timeout, :give_back_coins, data)

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State timeouts

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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

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Named timeouts

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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

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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

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{:timeout, ms, content} {:state_timeout, ms, content} {{:timeout, :some_name}, ms, content}

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“Internal” events

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def disconnected({:timeout, :reconnect}, _, _data) do {:keep_state_and_data, {:next_event, :internal, :connect}} end

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Non-finite state machines

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def callback_mode do :handle_event_function end

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def accepting_coins( :cast, {:put_coin, coin}, data ) do # ... end

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def handle_event( :cast, {:put_coin, coin}, :accepting_coins, data ) do # ... end

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def handle_event( event_type, event_content, current_state, data )

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def handle_event( :cast, {:put_coin, coin}, {:accepting_coins, credit}, data ) do new_state = {:accepting_coins, credit + coin} {:next_state, new_state, data} end

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Use cases

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GenServer :gen_statem or

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def handle_call(call, from, %{user: nil}) def handle_call(call, from, %{user: %User{}}) def handle_call(call, from, %{user: %Admin{}})

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def no_user({:call, from}, _, _) def user({:call, from}, _, _) def admin({:call, from}, _, _)

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Connections!

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DBConnection Redix HTTP/2 (?)

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state/named timeouts -> reconnection w/ backoff event timeouts -> keepalive postponed events -> retrying requests