path: root/system/doc/design_principles/fsm.xml

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Gen_Fsm Behaviour</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>fsm.xml</file>
  </header>
  <p>This chapter should be read in conjunction with <c>gen_fsm(3)</c>,
    where all interface functions and callback functions are described
    in detail.</p>

  <section>
    <title>Finite State Machines</title>
    <p>A finite state machine, FSM, can be described as a set of
      relations of the form:</p>
    <pre>
State(S) x Event(E) -> Actions(A), State(S')</pre>
    <p>These relations are interpreted as meaning:</p>
    <quote>
      <p>If we are in state <c>S</c> and the event <c>E</c> occurs, we
        should perform the actions <c>A</c> and make a transition to
        the state <c>S'</c>.</p>
    </quote>
    <p>For an FSM implemented using the <c>gen_fsm</c> behaviour,
      the state transition rules are written as a number of Erlang
      functions which conform to the following convention:</p>
    <pre>
StateName(Event, StateData) ->
    .. code for actions here ...
    {next_state, StateName', StateData'}</pre>
  </section>

  <section>
    <title>Example</title>
    <p>A door with a code lock could be viewed as an FSM. Initially,
      the door is locked. Anytime someone presses a button, this
      generates an event. Depending on what buttons have been pressed
      before, the sequence so far may be correct, incomplete or wrong.</p>
    <p>If it is correct, the door is unlocked for 30 seconds (30000 ms).
      If it is incomplete, we wait for another button to be pressed. If
      it is is wrong, we start all over, waiting for a new button
      sequence.</p>
    <p>Implementing the code lock FSM using <c>gen_fsm</c> results in
      this callback module:</p>
    <marker id="ex"></marker>
    <code type="none"><![CDATA[
-module(code_lock).
-behaviour(gen_fsm).

-export([start_link/1]).
-export([button/1]).
-export([init/1, locked/2, open/2]).

start_link(Code) ->
    gen_fsm:start_link({local, code_lock}, code_lock, Code, []).

button(Digit) ->
    gen_fsm:send_event(code_lock, {button, Digit}).

init(Code) ->
    {ok, locked, {[], Code}}.

locked({button, Digit}, {SoFar, Code}) ->
    case [Digit|SoFar] of
        Code ->
            do_unlock(),
            {next_state, open, {[], Code}, 30000};
        Incomplete when length(Incomplete)<length(Code) ->
            {next_state, locked, {Incomplete, Code}};
        _Wrong ->
            {next_state, locked, {[], Code}}
    end.

open(timeout, State) ->
    do_lock(),
    {next_state, locked, State}.]]></code>
    <p>The code is explained in the next sections.</p>
  </section>

  <section>
    <title>Starting a Gen_Fsm</title>
    <p>In the example in the previous section, the gen_fsm is started by
      calling <c>code_lock:start_link(Code)</c>:</p>
    <code type="none">
start_link(Code) ->
    gen_fsm:start_link({local, code_lock}, code_lock, Code, []).</code>
    <p><c>start_link</c> calls the function <c>gen_fsm:start_link/4</c>.
      This function spawns and links to a new process, a gen_fsm.</p>
    <list type="bulleted">
      <item>
        <p>The first argument <c>{local, code_lock}</c> specifies
          the name. In this case, the gen_fsm will be locally registered
          as <c>code_lock</c>.</p>
        <p>If the name is omitted, the gen_fsm is not registered.
          Instead its pid must be used. The name could also be given as
          <c>{global, Name}</c>, in which case the gen_fsm is registered
          using <c>global:register_name/2</c>.</p>
      </item>
      <item>
        <p>The second argument, <c>code_lock</c>, is the name of
          the callback module, that is the module where the callback
          functions are located.</p>
        <p>In this case, the interface functions (<c>start_link</c> and
          <c>button</c>) are located in the same module as the callback
          functions (<c>init</c>, <c>locked</c> and <c>open</c>). This
          is normally good programming practice, to have the code
          corresponding to one process contained in one module.</p>
      </item>
      <item>
        <p>The third argument, <c>Code</c>, is a term which is passed
          as-is to the callback function <c>init</c>. Here, <c>init</c>
          gets the correct code for the lock as indata.</p>
      </item>
      <item>
        <p>The fourth argument, [], is a list of options. See
          <c>gen_fsm(3)</c> for available options.</p>
      </item>
    </list>
    <p>If name registration succeeds, the new gen_fsm process calls
      the callback function <c>code_lock:init(Code)</c>. This function
      is expected to return <c>{ok, StateName, StateData}</c>, where
      <c>StateName</c> is the name of the initial state of the gen_fsm.
      In this case <c>locked</c>, assuming the door is locked to begin
      with. <c>StateData</c> is the internal state of the gen_fsm. (For
      gen_fsms, the internal state is often referred to 'state data' to
      distinguish it from the state as in states of a state machine.)
      In this case, the state data is the button sequence so far (empty
      to begin with) and the correct code of the lock.</p>
    <code type="none">
init(Code) ->
    {ok, locked, {[], Code}}.</code>
    <p>Note that <c>gen_fsm:start_link</c> is synchronous. It does not
      return until the gen_fsm has been initialized and is ready to
      receive notifications.</p>
    <p><c>gen_fsm:start_link</c> must be used if the gen_fsm is part of
      a supervision tree, i.e. is started by a supervisor. There is
      another function <c>gen_fsm:start</c> to start a stand-alone
      gen_fsm, i.e. a gen_fsm which is not part of a supervision tree.</p>
  </section>

  <section>
    <title>Notifying About Events</title>
    <p>The function notifying the code lock about a button event is
      implemented using <c>gen_fsm:send_event/2</c>:</p>
    <code type="none">
button(Digit) ->
    gen_fsm:send_event(code_lock, {button, Digit}).</code>
    <p><c>code_lock</c> is the name of the gen_fsm and must agree with
      the name used to start it. <c>{button, Digit}</c> is the actual
      event.</p>
    <p>The event is made into a message and sent to the gen_fsm. When
      the event is received, the gen_fsm calls
      <c>StateName(Event, StateData)</c> which is expected to return a
      tuple <c>{next_state, StateName1, StateData1}</c>.
      <c>StateName</c> is the name of the current state and
      <c>StateName1</c> is the name of the next state to go to.
      <c>StateData1</c> is a new value for the state data of
      the gen_fsm.</p>
    <code type="none"><![CDATA[
locked({button, Digit}, {SoFar, Code}) ->
    case [Digit|SoFar] of
        Code ->
            do_unlock(),
            {next_state, open, {[], Code}, 30000};
        Incomplete when length(Incomplete)<length(Code) ->
            {next_state, locked, {Incomplete, Code}};
        _Wrong ->
            {next_state, locked, {[], Code}};
    end.

open(timeout, State) ->
    do_lock(),
    {next_state, locked, State}.]]></code>
    <p>If the door is locked and a button is pressed, the complete
      button sequence so far is compared with the correct code for
      the lock and, depending on the result, the door is either unlocked
      and the gen_fsm goes to state <c>open</c>, or the door remains in
      state <c>locked</c>.</p>
  </section>

  <section>
    <title>Timeouts</title>
    <p>When a correct code has been given, the door is unlocked and
      the following tuple is returned from <c>locked/2</c>:</p>
    <code type="none">
{next_state, open, {[], Code}, 30000};</code>
    <p>30000 is a timeout value in milliseconds. After 30000 ms, i.e.
      30 seconds, a timeout occurs. Then <c>StateName(timeout, StateData)</c> is called. In this case, the timeout occurs when
      the door has been in state <c>open</c> for 30 seconds. After that
      the door is locked again:</p>
    <code type="none">
open(timeout, State) ->
    do_lock(),
    {next_state, locked, State}.</code>
  </section>

  <section>
    <title>All State Events</title>
    <p>Sometimes an event can arrive at any state of the gen_fsm.
      Instead of sending the message with <c>gen_fsm:send_event/2</c>
      and writing one clause handling the event for each state function,
      the message can be sent with <c>gen_fsm:send_all_state_event/2</c>
      and handled with <c>Module:handle_event/3</c>:</p>
    <code type="none">
-module(code_lock).
...
-export([stop/0]).
...

stop() ->
    gen_fsm:send_all_state_event(code_lock, stop).

...

handle_event(stop, _StateName, StateData) ->
    {stop, normal, StateData}.</code>
  </section>

  <section>
    <title>Stopping</title>

    <section>
      <title>In a Supervision Tree</title>
      <p>If the gen_fsm is part of a supervision tree, no stop function
        is needed. The gen_fsm will automatically be terminated by its
        supervisor. Exactly how this is done is defined by a
        <seealso marker="sup_princ#shutdown">shutdown strategy</seealso>
        set in the supervisor.</p>
      <p>If it is necessary to clean up before termination, the shutdown
        strategy must be a timeout value and the gen_fsm must be set to
        trap exit signals in the <c>init</c> function. When ordered
        to shutdown, the gen_fsm will then call the callback function
        <c>terminate(shutdown, StateName, StateData)</c>:</p>
      <code type="none">
init(Args) ->
    ...,
    process_flag(trap_exit, true),
    ...,
    {ok, StateName, StateData}.

...

terminate(shutdown, StateName, StateData) ->
    ..code for cleaning up here..
    ok.</code>
    </section>

    <section>
      <title>Stand-Alone Gen_Fsms</title>
      <p>If the gen_fsm is not part of a supervision tree, a stop
        function may be useful, for example:</p>
      <code type="none">
...
-export([stop/0]).
...

stop() ->
    gen_fsm:send_all_state_event(code_lock, stop).
...

handle_event(stop, _StateName, StateData) ->
    {stop, normal, StateData}.

...

terminate(normal, _StateName, _StateData) ->
    ok.</code>
      <p>The callback function handling the <c>stop</c> event returns a
        tuple <c>{stop,normal,StateData1}</c>, where <c>normal</c>
        specifies that it is a normal termination and <c>StateData1</c>
        is a new value for the state data of the gen_fsm. This will
        cause the gen_fsm to call
        <c>terminate(normal,StateName,StateData1)</c> and then
        terminate gracefully:</p>
    </section>
  </section>

  <section>
    <title>Handling Other Messages</title>
    <p>If the gen_fsm should be able to receive other messages than
      events, the callback function <c>handle_info(Info, StateName, StateData)</c> must be implemented to handle them. Examples of
      other messages are exit messages, if the gen_fsm is linked to
      other processes (than the supervisor) and trapping exit signals.</p>
    <code type="none">
handle_info({'EXIT', Pid, Reason}, StateName, StateData) ->
    ..code to handle exits here..
    {next_state, StateName1, StateData1}.</code>
    <p>The code_change method also has to be implemented.</p>
    <code type="none">
code_change(OldVsn, StateName, StateData, Extra) ->
    ..code to convert state (and more) during code change
    {ok, NextStateName, NewStateData}</code>
  </section>
</chapter>