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gen_statem Generic State Machine Behaviour

A behaviour module for implementing a state machine. Two callback modes are supported. One for finite state machines (gen_fsm like) that requires the state to be an atom and uses that state as the name of the current callback function, and one without restriction on the state data type that uses one callback function for all states.

This is a new behaviour in OTP-19.0. It has been thoroughly reviewed, is stable enough to be used by at least two heavy OTP applications, and is here to stay. But depending on user feedback, we do not expect but might find it necessary to make minor not backwards compatible changes into OTP-20.0, so its state can be designated as "not quite experimental"...

The gen_statem behaviour is intended to replace gen_fsm for new code. It has the same features and add some really useful:

State code is gathered The state can be any term Events can be postponed Events can be self generated A reply can be sent from a later state There can be multiple sys traceable replies

The callback model(s) for gen_statem differs from the one for gen_fsm, but it is still fairly easy to rewrite from gen_fsm to gen_statem.

A generic state machine process (gen_statem) implemented using this module will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into an OTP supervision tree. Refer to OTP Design Principles for more information.

A gen_statem assumes all specific parts to be located in a callback module exporting a pre-defined set of functions. The relationship between the behaviour functions and the callback functions can be illustrated as follows:

gen_statem module            Callback module
-----------------            ---------------
gen_statem:start
gen_statem:start_link -----> Module:init/1

gen_statem:stop       -----> Module:terminate/3

gen_statem:call
gen_statem:cast
erlang:send
erlang:'!'            -----> Module:StateName/3
                             Module:handle_event/4

-                     -----> Module:terminate/3

-                     -----> Module:code_change/4

Events are of different types so the callback functions can know the origin of an event and how to respond.

If a callback function fails or returns a bad value, the gen_statem will terminate. An exception of class throw, however, is not regarded as an error but as a valid return.

The "state function" for a specific state in a gen_statem is the callback function that is called for all events in this state, and is selected depending on which callback mode that the implementation specifies when the the server starts.

When the callback mode is state_functions, the state has to be an atom and is used as the state function name. See Module:StateName/3 . This gathers all code for a specific state in one function and hence dispatches on state first. Note that in this mode the fact that there is a mandatory callback function Module:terminate/3 makes the state name terminate unusable.

When the callback mode is handle_event_function the state can be any term and the state function name is Module:handle_event/4 . This makes it easy to dispatch on state or on event as you desire. Be careful about which events you handle in which states so you do not accidentally postpone one event forever creating an infinite busy loop.

The gen_statem enqueues incoming events in order of arrival and presents these to the state function in that order. The state function can postpone an event so it is not retried in the current state. After a state change the queue restarts with the postponed events.

The gen_statem event queue model is sufficient to emulate the normal process message queue with selective receive. Postponing an event corresponds to not matching it in a receive statement and changing states corresponds to entering a new receive statement.

The state function can insert events using the action() next_event and such an event is inserted as the next to present to the state function. That is: as if it is the oldest incoming event. There is a dedicated event_type() internal that can be used for such events making them impossible to mistake for external events.

Inserting an event replaces the trick of calling your own state handling functions that you often would have to resort to in for example gen_fsm to force processing an inserted event before others. A warning, though: if you in gen_statem for example postpone an event in one state and then call some other state function of yours, you have not changed states and hence the postponed event will not be retried, which is logical but might be confusing.

See the type transition_option() for the details of a state transition.

A gen_statem handles system messages as documented in sys. The sysmodule can be used for debugging a gen_statem.

Note that a gen_statem does not trap exit signals automatically, this must be explicitly initiated in the callback module (by calling process_flag(trap_exit, true).

Unless otherwise stated, all functions in this module fail if the specified gen_statem does not exist or if bad arguments are given.

The gen_statem process can go into hibernation; see proc_lib:hibernate/3. It is done when a state function or Module:init/1 specifies hibernate in the returned Actions list. This feature might be useful to reclaim process heap memory while the server is expected to be idle for a long time. However, use this feature with care since hibernation can be too costly to use after every event; see erlang:hibernate/3.

EXAMPLE

This example shows a simple pushbutton model for a toggling pushbutton implemented with callback mode state_functions. You can push the button and it replies if it went on or off, and you can ask for a count of how many times it has been pushed to on.

This is the complete callback module file pushbutton.erl:

-module(pushbutton). -behaviour(gen_statem). -export([start/0,push/0,get_count/0,stop/0]). -export([terminate/3,code_change/4,init/1]). -export([on/3,off/3]). name() -> pushbutton_statem. % The registered server name callback_mode() -> state_functions. %% API. This example uses a registered name name() %% and does not link to the caller. start() -> gen_statem:start({local,name()}, ?MODULE, [], []). push() -> gen_statem:call(name(), push). get_count() -> gen_statem:call(name(), get_count). stop() -> gen_statem:stop(name()). %% Mandatory callback functions terminate(_Reason, _State, _Data) -> void. code_change(_Vsn, State, Data, _Extra) -> {callback_mode(),State,Data}. init([]) -> %% Set the callback mode and initial state + data. %% Data is used only as a counter. State = off, Data = 0, {callback_mode(),State,Data}. %%% State functions off({call,From}, push, Data) -> %% Go to 'on', increment count and reply %% that the resulting status is 'on' {next_state,on,Data+1,[{reply,From,on}]}; off(EventType, EventContent, Data) -> handle_event(EventType, EventContent, Data). on({call,From}, push, Data) -> %% Go to 'off' and reply that the resulting status is 'off' {next_state,off,Data,[{reply,From,off}]}; on(EventType, EventContent, Data) -> handle_event(EventType, EventContent, Data). %% Handle events common to all states handle_event({call,From}, get_count, Data) -> %% Reply with the current count {keep_state,Data,[{reply,From,Data}]}; handle_event(_, _, Data) -> %% Ignore all other events {keep_state,Data}.

And this is a shell session when running it:

1> pushbutton:start().
{ok,<0.36.0>}
2> pushbutton:get_count().
0
3> pushbutton:push().
on
4> pushbutton:get_count().
1
5> pushbutton:push().
off
6> pushbutton:get_count().
1
7> pushbutton:stop().
ok
8> pushbutton:push().
** exception exit: {noproc,{gen_statem,call,[pushbutton_statem,push,infinity]}}
     in function  gen:do_for_proc/2 (gen.erl, line 261)
     in call from gen_statem:call/3 (gen_statem.erl, line 386)

And just to compare styles here is the same example using callback mode state_functions, or rather here is code to replace from the init/1 function of the pushbutton.erl example file above:

init([]) -> %% Set the callback mode and initial state + data. %% Data is used only as a counter. State = off, Data = 0, {handle_event_function,State,Data}. %%% Event handling handle_event({call,From}, push, off, Data) -> %% Go to 'on', increment count and reply %% that the resulting status is 'on' {next_state,on,Data+1,[{reply,From,on}]}; handle_event({call,From}, push, on, Data) -> %% Go to 'off' and reply that the resulting status is 'off' {next_state,off,Data,[{reply,From,off}]}; %% %% Event handling common to all states handle_event({call,From}, get_count, State, Data) -> %% Reply with the current count {next_state,State,Data,[{reply,From,Data}]}; handle_event(_, _, State, Data) -> %% Ignore all other events {next_state,State,Data}.

Name specification to use when starting a gen_statem server. See start_link/3 and server_ref() below.

Server specification to use when addressing a gen_statem server. See call/2 and server_name() above.

It can be:

pid()
LocalName The gen_statem is locally registered. {Name,Node} The gen_statem is locally registered on another node. {global,GlobalName} The gen_statem is globally registered in global. {via,RegMod,ViaName} The gen_statem is registered through an alternative process registry. The registry callback module RegMod should export the functions register_name/2, unregister_name/1, whereis_name/1 and send/2, which should behave like the corresponding functions in global. Thus, {via,global,GlobalName} is the same as {global,GlobalName}.

Debug option that can be used when starting a gen_statem server through for example enter_loop/5.

For every entry in Dbgs the corresponding function in sys will be called.

Options that can be used when starting a gen_statem server through for example start_link/3.

Return value from the start functions for_example start_link/3.

Destination to use when replying through for example the action() {reply,From,Reply} to a process that has called the gen_statem server using call/2.

After a state change (NextState =/= State) all postponed events are retried.

If the callback mode is state_functions, the state has to be of this type.

A term in which the state machine implementation should store any server data it needs. The difference between this and the state() itself is that a change in this data does not cause postponed events to be retried. Hence if a change in this data would change the set of events that are handled than that data item should be made a part of the state.

External events are of 3 different type: {call,From}, cast or info. Calls (synchronous) and casts originate from the corresponding API functions. For calls the event contain whom to reply to. Type info originates from regular process messages sent to the gen_statem. It is also possible for the state machine implementation to generate events of types timeout and internal to itself.

The callback mode is selected when starting the gen_statem using the return value from Module:init/1 or when calling enter_loop/5-7, and with the return value from Module:code_change/4.

state_functions The state has to be of type state_name() and one callback function per state that is Module:StateName/3 is used. handle_event_function The state can be any term and the callback function Module:handle_event/4 is used for all states.

Transition options may be set by actions and they modify some details below in how the state transition is done:

All actions are processed in order of appearance. If postpone() is true the current event is postponed. If the state changes the queue of incoming events is reset to start with the oldest postponed. All events stored with action() next_event are inserted in the queue to be processed before all other events. If an event_timeout() is set through action() timeout an event timer may be started or a timeout zero event may be enqueued. The (possibly new) state function is called with the oldest enqueued event if there is any, otherwise the gen_statem goes into receive or hibernation (if hibernate() is true) to wait for the next message. In hibernation the next non-system event awakens the gen_statem, or rather the next incoming message awakens the gen_statem but if it is a system event it goes right back into hibernation.

If true postpone the current event and retry it when the state changes (NextState =/= State).

If true hibernate the gen_statem by calling proc_lib:hibernate/3 before going into receive to wait for a new external event. If there are enqueued events, to prevent receiving any new event; a garbage_collect/0 is done instead to simulate that the gen_statem entered hibernation and immediately got awakened by the oldest enqueued event.

Generate an event of event_type() timeout after this time (in milliseconds) unless some other event arrives in which case this timeout is cancelled. Note that a retried or inserted event counts just like a new in this respect.

If the value is infinity no timer is started since it will never trigger anyway.

If the value is 0 the timeout event is immediately enqueued unless there already are enqueued events since then the timeout is immediately cancelled. This is a feature ensuring that a timeout 0 event will be processed before any not yet received external event.

Note that it is not possible nor needed to cancel this timeout since it is cancelled automatically by any other event.

These state transition actions may be invoked by returning them from the state function, from Module:init/1 or by giving them to enter_loop/6,7.

Actions are executed in the containing list order.

Actions that set transition options overrides any previous of the same type, so the last in the containing list wins. For example the last event_timeout() overrides any other event_timeout() in the list.

postpone Set the transition_option() postpone() for this state transition. This action is ignored when returned from Module:init/1 or given to enter_loop/5,6 since there is no event to postpone in those cases. hibernate Set the transition_option() hibernate() for this state transition. Timeout Short for {timeout,Timeout,Timeout} that is the timeout message is the timeout time. This form exists to make the state function return value {next_state,NextState,NewData,Timeout} allowed like for gen_fsm Module:StateName/2. timeout Set the transition_option() event_timeout() to Time with EventContent. reply_action() Reply to a caller. next_event Store the given EventType and EventContent for insertion after all actions have been executed.

The stored events are inserted in the queue as the next to process before any already queued events. The order of these stored events is preserved so the first next_event in the containing list will become the first to process.

An event of type internal should be used when you want to reliably distinguish an event inserted this way from any external event.

Reply to a caller waiting for a reply in call/2. From must be the term from the {call,From} argument to the state function.

next_state The gen_statem will do a state transition to NextStateName (which may be the same as the current state), set NewData and execute all Actions

All these terms are tuples or atoms and this property will hold in any future version of gen_statem, just in case you need such a promise.

next_state The gen_statem will do a state transition to NextState (which may be the same as the current state), set NewData and execute all Actions

All these terms are tuples or atoms and this property will hold in any future version of gen_statem, just in case you need such a promise.

stop Terminate the gen_statem by calling Module:terminate/3 with Reason and NewData, if given. stop_and_reply Send all Replies then terminate the gen_statem by calling Module:terminate/3 with Reason and NewData, if given. keep_state The gen_statem will keep the current state, or do a state transition to the current state if you like, set NewData and execute all Actions. This is the same as {next_state,CurrentState,NewData,Actions}. keep_state_and_data The gen_statem will keep the current state or do a state transition to the current state if you like, keep the current server data, and execute all Actions. This is the same as {next_state,CurrentState,CurrentData,Actions}.

All these terms are tuples or atoms and this property will hold in any future version of gen_statem, just in case you need such a promise.

Create a linked gen_statem process

Creates a gen_statem process according to OTP design principles (using proc_lib primitives) that is linked to the calling process. This is essential when the gen_statem shall be part of a supervision tree so it gets linked to its supervisor.

The gen_statem process calls Module:init/1 to initialize the server. To ensure a synchronized start-up procedure, start_link/3,4 does not return until Module:init/1 has returned.

ServerName specifies the server_name() to register for the gen_statem. If the gen_statem is started with start_link/3 no ServerName is provided and the gen_statem is not registered.

Module is the name of the callback module.

Args is an arbitrary term which is passed as the argument to Module:init/1.

If the option {timeout,Time} is present in Opts, the gen_statem is allowed to spend Time milliseconds initializing or it will be terminated and the start function will return {error,timeout}.

If the option {debug,Dbgs} is present in Opts, debugging through sys is activated.

If the option {spawn_opt,SpawnOpts} is present in Opts, SpawnOpts will be passed as option list to spawn_opt/2 which is used to spawn the gen_statem process.

Using the spawn option monitor is currently not allowed, but will cause this function to fail with reason badarg.

If the gen_statem is successfully created and initialized this function returns {ok,Pid}, where Pid is the pid() of the gen_statem. If there already exists a process with the specified ServerName this function returns {error,{already_started,Pid}}, where Pid is the pid() of that process.

If Module:init/1 fails with Reason, this function returns {error,Reason}. If Module:init/1 returns {stop,Reason} or ignore, the process is terminated and this function returns {error,Reason} or ignore, respectively.

Create a stand-alone gen_statem process

Creates a stand-alone gen_statem process according to OTP design principles (using proc_lib primitives). Since it does not get linked to the calling process this start function can not be used by a supervisor to start a child.

See start_link/3,4 for a description of arguments and return values.

Synchronously stop a generic server

The same as stop(ServerRef, normal, infinity).

Synchronously stop a generic server

Orders the gen_statem ServerRef to exit with the given Reason and waits for it to terminate. The gen_statem will call Module:terminate/3 before exiting.

This function returns ok if the server terminates with the expected reason. Any other reason than normal, shutdown, or {shutdown,Term} will cause an error report to be issued through error_logger:format/2. The default Reason is normal.

Timeout is an integer greater than zero which specifies how many milliseconds to wait for the server to terminate, or the atom infinity to wait indefinitely. The default value is infinity. If the server has not terminated within the specified time, a timeout exception is raised.

If the process does not exist, a noproc exception is raised.

Make a synchronous call to a gen_statem

Makes a synchronous call to the gen_statem ServerRef by sending a request and waiting until its reply arrives. The gen_statem will call the state function with event_type() {call,From} and event content Request.

A Reply is generated when a state function returns with {reply,From,Reply} as one action(), and that Reply becomes the return value of this function.

Timeout is an integer greater than zero which specifies how many milliseconds to wait for a reply, or the atom infinity to wait indefinitely, which is the default. If no reply is received within the specified time, the function call fails.

To avoid getting a late reply in the caller's inbox this function spawns a proxy process that does the call. A late reply gets delivered to the dead proxy process hence gets discarded. This is less efficient than using Timeout =:= infinity.

The call may fail for example if the gen_statem dies before or during this function call.

Send an asynchronous event to a gen_statem

Sends an asynchronous event to the gen_statem ServerRef and returns ok immediately, ignoring if the destination node or gen_statem does not exist. The gen_statem will call the state function with event_type() cast and event content Msg.

Reply to a caller

This function can be used by a gen_statem to explicitly send a reply to a process that waits in call/2 when the reply cannot be defined in the return value of a state function.

From must be the term from the {call,From} argument to the state function. From and Reply can also be specified using a reply_action() and multiple replies with a list of them.

A reply sent with this function will not be visible in sys debug output.

Enter the gen_statem receive loop

The same as enter_loop/7 except that no server_name() must have been registered.

Enter the gen_statem receive loop

If Server_or_Actions is a list() the same as enter_loop/7 except that no server_name() must have been registered and Actions = Server_or_Actions.

Otherwise the same as enter_loop/7 with Server = Server_or_Actions and Actions = [].

Enter the gen_statem receive loop

Makes an the calling process become a gen_statem. Does not return, instead the calling process will enter the gen_statem receive loop and become a gen_statem server. The process must have been started using one of the start functions in proc_lib. The user is responsible for any initialization of the process, including registering a name for it.

This function is useful when a more complex initialization procedure is needed than the gen_statem behaviour provides.

Module, Opts and Server have the same meanings as when calling gen_statem:start[_link]/3,4. However, the server_name() name must have been registered accordingly before this function is called.

CallbackMode, State, Data and Actions have the same meanings as in the return value of Module:init/1. Also, the callback module Module does not need to export an init/1 function.

Failure: If the calling process was not started by a proc_lib start function, or if it is not registered according to server_name().

CALLBACK FUNCTIONS

The following functions should be exported from a gen_statem callback module.

Module:init(Args) -> Result Initialize process and internal state Args = term() Result = {CallbackMode,State,Data}  | {CallbackMode,State,Data,Actions}  | {stop,Reason} | ignore CallbackMode = callback_mode() State = state() Data = data() Actions = [action()] | action() Reason = term()

Whenever a gen_statem is started using start_link/3,4 or start/3,4, this function is called by the new process to initialize the implementation state and server data.

Args is the Args argument provided to the start function.

If the initialization is successful, the function should return {CallbackMode,State,Data} or {CallbackMode,State,Data,Actions}. CallbackMode selects the callback mode. of the gen_statem. State is the initial state() and Data the initial server data().

The Actions are executed when entering the first state just as for a state function.

If something goes wrong during the initialization the function should return {stop,Reason} or ignore. See start_link/3,4.

This function may use throw/1 to return Result.

Module:StateName(EventType, EventContent, Data) -> StateFunctionResult Module:handle_event(EventType, EventContent, State, Data) -> HandleEventResult Handle an event EventType = event_type() EventContent = term() State = state() Data = NewData = data() StateFunctionResult = state_function_result() HandleEventResult = handle_event_result()

Whenever a gen_statem receives an event from call/2, cast/2 or as a normal process message one of these functions is called. If callback mode is state_functions then Module:StateName/3 is called, and if it is handle_event_function then Module:handle_event/4 is called.

If EventType is {call,From} the caller is waiting for a reply. The reply can be sent from this or from any other state function by returning with {reply,From,Reply} in Actions, in Replies or by calling reply(From, Reply).

If this function returns with a next state that does not match equal (=/=) to the current state all postponed events will be retried in the next state.

The only difference between StateFunctionResult and HandleEventResult is that for StateFunctionResult the next state has to be an atom but for HandleEventResult there is no restriction on the next state.

See action() for options that can be set and actions that can be done by gen_statem after returning from this function.

These functions may use throw/1, to return the result.

Module:terminate(Reason, State, Data) -> Ignored Clean up before termination Reason = normal | shutdown | {shutdown,term()} | term() State = state() Data = data() Ignored = term()

This function is called by a gen_statem when it is about to terminate. It should be the opposite of Module:init/1 and do any necessary cleaning up. When it returns, the gen_statem terminates with Reason. The return value is ignored.

Reason is a term denoting the stop reason and State is the internal state of the gen_statem.

Reason depends on why the gen_statem is terminating. If it is because another callback function has returned a stop tuple {stop,Reason} in Actions, Reason will have the value specified in that tuple. If it is due to a failure, Reason is the error reason.

If the gen_statem is part of a supervision tree and is ordered by its supervisor to terminate, this function will be called with Reason = shutdown if the following conditions apply:

the gen_statem has been set to trap exit signals, and the shutdown strategy as defined in the supervisor's child specification is an integer timeout value, not brutal_kill.

Even if the gen_statem is not part of a supervision tree, this function will be called if it receives an 'EXIT' message from its parent. Reason will be the same as in the 'EXIT' message.

Otherwise, the gen_statem will be immediately terminated.

Note that for any other reason than normal, shutdown, or {shutdown,Term} the gen_statem is assumed to terminate due to an error and an error report is issued using error_logger:format/2.

This function may use throw/1 to return Ignored, which is ignored anyway.

Module:code_change(OldVsn, OldState, OldData, Extra) -> Result Update the internal state during upgrade/downgrade OldVsn = Vsn | {down,Vsn}   Vsn = term() OldState = NewState = term() Extra = term() Result = {NewCallbackMode,NewState,NewData} | Reason NewCallbackMode = callback_mode() OldState = NewState = state() OldData = NewData = data() Reason = term()

This function is called by a gen_statem when it should update its internal state during a release upgrade/downgrade, that is when the instruction {update,Module,Change,...} where Change={advanced,Extra} is given in the appup file. See OTP Design Principles for more information.

In the case of an upgrade, OldVsn is Vsn, and in the case of a downgrade, OldVsn is {down,Vsn}. Vsn is defined by the vsn attribute(s) of the old version of the callback module Module. If no such attribute is defined, the version is the checksum of the BEAM file.

If you would dare to change callback mode during release upgrade/downgrade, the upgrade is no problem since the new code surely knows what callback mode it needs, but for a downgrade this function will have to know from the Extra argument that comes from the appup file what callback mode the old code did use. It may also be possible to figure this out from the {down,Vsn} argument since Vsn in effect defines the old callback module version.

OldState and OldData is the internal state of the gen_statem.

Extra is passed as-is from the {advanced,Extra} part of the update instruction.

If successful, the function shall return the updated internal state in an {NewCallbackMode,NewState,NewData} tuple.

If the function returns Reason, the ongoing upgrade will fail and roll back to the old release.

This function may use throw/1 to return Result or Reason.

Module:format_status(Opt, [PDict,State,Data]) -> Status Optional function for providing a term describing the current gen_statem status Opt = normal | terminate PDict = [{Key, Value}] State = state() Data = data() Key = term() Value = term() Status = term()

This callback is optional, so a callback module need not export it. The gen_statem module provides a default implementation of this function that returns {State,Data}. If this callback fails the default function will return {State,Info} where Info informs of the crash but no details, to hide possibly sensitive data.

This function is called by a gen_statem process when:

One of sys:get_status/1,2 is invoked to get the gen_statem status. Opt is set to the atom normal for this case. The gen_statem terminates abnormally and logs an error. Opt is set to the atom terminate for this case.

This function is useful for customising the form and appearance of the gen_statem status for these cases. A callback module wishing to customise the sys:get_status/1,2 return value as well as how its status appears in termination error logs exports an instance of format_status/2 that returns a term describing the current status of the gen_statem.

PDict is the current value of the gen_statem's process dictionary.

State is the internal state of the gen_statem.

Data is the internal server data of the gen_statem.

The function should return Status, a term that customises the details of the current state and status of the gen_statem. There are no restrictions on the form Status can take, but for the sys:get_status/1,2 case (when Opt is normal), the recommended form for the Status value is [{data, [{"State", Term}]}] where Term provides relevant details of the gen_statem state. Following this recommendation isn't required, but doing so will make the callback module status consistent with the rest of the sys:get_status/1,2 return value.

One use for this function is to return compact alternative state representations to avoid having large state terms printed in logfiles. Another is to hide sensitive data from being written to the error log.

This function may use throw/1 to return Status.

SEE ALSO

gen_event(3), gen_fsm(3), gen_server(3), supervisor(3), proc_lib(3), sys(3)