gen_statem

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gen_statem Generic State Machine Behaviour

A behaviour module for implementing a state machine, primarily a finite state machine, but an infinite state space is possible. 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/2

gen_statem:call
gen_statem:cast
erlang:send
erlang:'!'            -----> Module:State/5
                             Module:handle_event/5

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

-                     -----> Module:code_change/3

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. An event can can be postponed causing it to be retried after the state has changed, or more precisely; after a state change all postponed events are retried in the new state.

The state machine State is normally an atom in which case the state function that will be called is Module:State/5. For a State that is not an atom the state function Module:handle_event/5 will be called. If you use handle_event as a state and later decide to use non-atom states you will then have to rewrite your code to stop using that state.

When using an atom-only State it becomes fairly obvious in the implementation code which events are handled in which state since there are different callback functions for different states.

When using a non-atom State all events are handled in the callback function Module:handle_event/5 so it may require more coding discipline to ensure what events are handled in which state. Therefore it might be a wee bit easier to accidentally postpone an event in two or more states and not handling it in any of them causing a tight infinite loop when the event bounces to be retried between the states.

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

Note that a gen_statem does not trap exit signals automatically, this must be explicitly initiated by the callback module.

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 erlang:hibernate/3 ) if a state function or Module:init/1 specifies 'hibernate' in the returned StateOps list. This might be useful if the server is expected to be idle for a long time. However use this feature with care since hibernation implies at least two garbage collections (when hibernating and shortly after waking up) and that is not something you'd want to do between each event on a busy server.

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:

the pid(), Name, if the gen_statem is locally registered, {Name,Node}, if the gen_statem is locally registered at another node, or {global,GlobalName}, if the gen_statem is globally registered. {via,RegMod,ViaName}, if 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/4.

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.

Client address to use when replying through for example the state_op() {reply,Client,Reply} to a client that has called the gen_statem server using call/2.

If the gen_statem State is an atom(), the state function is Module:State/5. If it is any other term() the state function is Module:handle_event/5 . After a state change (NewState =/= State) all postponed events are retried.

A term() in which the state machine implementation should store any state data it needs. The difference between this data and the state() itself is that a change in this data does not cause postponed events to be retried.

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

A fun() of arity 2 that takes an event and returns a boolean. When used in {remove_event,RemoveEventPredicate} from state_op(). The event for which the predicate returns true will be removed.

The predicate may not use a throw exception to return its result.

Either a state_option() of which the first occurence in the containing list takes precedence, or a state_operation() that are performed in order of the containing list.

These may be returned from the state function or from Module:init/1.

The gen_statem enqueues postponed events and not yet processed events in order of arrival, except for an event that a callback function inserts with {insert_event,EventType,EventContent} that is inserted as the next event to process.

When the state machine changes states all enqueued events becomes not yet processed to be processed before the old not yet processed events. In other words; the order of arrival is retained.

The processing order is:

If the option retry is true the current event is enqueued as postponed to be retried. If the state changes all postponed events are transferred to not yet processed to be processed before other not yet processed events. All operations are processed in order of appearance. The timeout option is processed if present. So a state timer may be started or a timeout zero event may be inserted as if just received. The (possibly new) state function is called with the next not yet processed event if there is any, otherwise the gen_statem goes into receive or hibernation (if the option hibernate is true) to wait for the next message. In hibernation the next non-system event awakens the gen_statem. retry {retry,Retry} If Retry =:= true or plain retry postpone the current event to be retried after a state change. hibernate {hibernate,Hibernate} If Hibernate =:= true or plain hibernate hibernate the gen_statem by calling proc_lib:hibernate/3 before receive to wait for a new event. If there are not yet processed events the hibernate operation is ignored as if an event just arrived and awakened the gen_statem. {timeout,Time,Msg} Generate an event of type timeout after Time milliseconds unless some other event is received before that time. Note that a retried event counts just like a new in this respect. If Time =:= infinity or Time =:= 0 no timer is started but for zero time the timeout event is enqued as just received after all other already received events. Also note that it is not possible to cancel this timeout using the state_operation() cancel_timer. This timeout is cancelled automatically by any event. {reply,Client,Reply} Reply to a client that called call/2. Client must be the term from the {call,Client} argument to the state function. {stop,Reason} The gen_statem will call Module:terminate/3 with Reason and terminate. {insert_event,EventType,EventContent} Insert the given event as the next to process before any other not yet processed events. An event of type internal should be used when you want to reliably distinguish an event inserted this way from any external event. {remove_event,EventType,EventContent} Remove the oldest queued event that matches equal to the given event. {remove_event,EventPredicate} Remove the oldest queued event for which the EventPredicate returns true. {cancel_timer,TimerRef} Uses TimerRef when calling erlang:cancel_timer/2 to cancel a timer, cleans the gen_statem's message queue from any late timeout message from the timer, and removes any late timeout message from the queued events using {remove_event,EventPredicate} above. This is a convenience function that saves quite some lines of code and testing time over doing it from the primitives mentioned above. {demonitor,MonitorRef} Like {cancel_timer,_} above but for demonitor/2 . {unlink,Id} Like {cancel_timer,_} above but for unlink/1 . 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 Options, 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 Options, debugging through sys is activated.

If the option {spawn_opt,SOpts} is present in Options, SOpts will be passed as option list to the spawn_opt BIF which is used to spawn the gen_statem.

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,Client} and event content Request.

A Reply is generated when a state function returns with {reply,Client,Reply} as one state_op(), 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.

Send a reply to a client

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

Client must be the term from the {call,Client} argument to the state function.

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/6 except that no server_name() must have been registered.

Enter the gen_statem receive loop

If Server_or_StateOps is a list() the same as enter_loop/6 except that no server_name() must have been registered and StateOps = Server_or_StateOps.

Otherwise the same as enter_loop/6 with Server = Server_or_StateOps and StateOps = [].

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, Options 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.

State and StateData 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 = {ok,State,StateData} | {ok,State,StateData,StateOps} | {stop,Reason} | ignore State = state() StateData = state_data() StateOps = [state_op()] Reason = term()

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

Args is the Args argument provided to the start function.

If the initialization is successful, the function should return {ok,State,StateData} or {ok,State,StateData,StateOps}. State is the state of the gen_statem.

The StateOps are executed before 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 gen_statem:start_link/3,4.

Module:handle_event(EventType, EventContent, PrevState, State, StateData) -> Result Module:State(EventType, EventContent, PrevState, State, StateData) -> Result Handle an event EventType = event_type() EventContent = term() Result = {NewState,NewStateData,StateOps} | {NewState,NewStateData} The same as {NewState,NewStateData,[]} | {NewStateData} The same as {State,NewStateData,[retry]} | {} The same as {State,StateData,[]} | StateOps The same as {State,StateData,StateOps} PrevState = State = NewState = state() StateData = NewStateData = state_data() StateOps = [state_op()]

Whenever a gen_statem receives an event from gen_statem:call/2, gen_statem:cast/2 or as a normal process message this function is called. If the EventType is {call,Client} the client is waiting for a reply. The reply can be sent from this or from any other state function by returning with {reply,Client,Reply} in StateOps or by calling gen_statem:reply(Client, Reply) .

State is the internal state of the gen_statem which when State is an atom() is the same as this function's name, so it is seldom useful, except for example when comparing with PrevState that is the gen_statem's previous state, or in Module:handle_event/5 since that function is common for all states that are not an atom().

If this function returns with NewState =/= State all postponed events will be retried in the new state.

See state_op() for the operations that can be done by gen_statem after returning from this function.

Module:terminate(Reason, State, StateData) Clean up before termination Reason = normal | shutdown | {shutdown,term()} | term() State = state() StateData = state_data()

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 StateOps, 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 .

Module:code_change(OldVsn, OldState, OldStateData, Extra) -> Result Update the internal state during upgrade/downgrade OldVsn = Vsn | {down,Vsn} Vsn = term() OldState = NewState = term() Extra = term() Result = {ok,{NewState,NewStateData}} | Reason OldState = NewState = state() OldStateData = NewStateData = state_data() Reason = term()

This function is called by a gen_statem when it should update its internal state during a release upgrade/downgrade, i.e. 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.

OldState and OldStateData 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 {ok,{NewState,NewStateData}} tuple.

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

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

This callback is optional, so callback modules need not export it. The gen_statem module provides a default implementation of this function that returns the callback module state.

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.

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.