OTP Behavior: gen_fsm

gen_fsm is a generate state machine pattern.  With gen_fsm, each state is represented with a function that has the form:

<state name>(Event, LoopData) -> { next_state, NewState, NewLoopData }


The FSM can also tell the framework to terminate via the following:


<state name>(Event, LoopData) -> { stop, Reason, NewLoopData }


Client API

• start(Module, Args, Options)
• start(Name, Module, Args, Options)
• start_link(Module, Args, Options)
• start_link(Name, Module, Args, Options)
  Start up the FSM.  These functions will call init/1 in the specified Module.  start_link links the new FSM to the caller, start does not.  The name will register the newly created process locally or globally, as appropriate.
• send_event (Reference, Event)
  This will cause the corresponding <state name> function to be called.  The call returns immediately.
• send_all_state_event (Reference, Event)
  Will cause handle_event to be called on the server instead of the function for the current state.
• sync_send_event (Reference, Event)
  This will cause the corresponding <state name> function to be called.  The call waits for the FSM to complete before continuing
• sync_send_all_state_event (Reference, Event)
  This will cause the handle_event function to be called on the server.  The caller will wait for the event to be processed before continuing.
• reply(Caller, Reply)
  This is actually an internal call that is used to set the reply that will be returned in the case of sync_send_event and sync_send_all_state_event.

Server API

• init(Args) -> { ok, StateName, StateData }
  Start up the state machine.
• <State Name>(Event, Data) -> {next_state, NextState, NewData}
  Handle an event sent to the state machine via send_event or sync_send_event  Can also return:
• { stop, Reason, NewData} in order to terminate the FSM.
• handle_event(Event, State, Data) ->
      {next_state, NextState, NewData}
  Handle an event sent to the FSM via send_all_state_event.
• handle_sync_event(Event, State, Data) ->
      {next_state, NextState, NewData}
  Handle an event sent to the FSM via sync_send_all_state_event.
• terminate(Reason, StateName, Data)
  Clean up hook.

A slight change of plan: I am not going to cover the application behavior.