path: root/lib/stdlib/doc/src/gen_server.xml

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  <header>
    <copyright>
      <year>1996</year><year>2014</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
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    </legalnotice>

    <title>gen_server</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>gen_server</module>
  <modulesummary>Generic Server Behaviour</modulesummary>
  <description>
    <p>A behaviour module for implementing the server of a client-server
      relation. A generic server process (gen_server) 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
      <seealso marker="doc/design_principles:gen_server_concepts">OTP Design Principles</seealso> for more information.</p>
    <p>A gen_server 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:</p>
    <pre>
gen_server module            Callback module
-----------------            ---------------
gen_server:start
gen_server:start_link -----> Module:init/1

gen_server:stop       -----> Module:terminate/2

gen_server:call
gen_server:multi_call -----> Module:handle_call/3

gen_server:cast
gen_server:abcast     -----> Module:handle_cast/2

-                     -----> Module:handle_info/2

-                     -----> Module:terminate/2

-                     -----> Module:code_change/3    </pre>
    <p>If a callback function fails or returns a bad value,
      the gen_server will terminate.</p>
    <p>A gen_server handles system messages as documented in
       <seealso marker="sys">sys(3)</seealso>. The <c>sys</c> module
       can be used for debugging a gen_server.</p>
    <p>Note that a gen_server does not trap exit signals automatically,
      this must be explicitly initiated in the callback module.</p>
    <p>Unless otherwise stated, all functions in this module fail if
      the specified gen_server does not exist or if bad arguments are
      given.</p>

    <p>The gen_server process can go into hibernation 
    (see <seealso marker="erts:erlang#erlang:hibernate/3">erlang(3)</seealso>) if a callback
    function specifies <c>'hibernate'</c> instead of a timeout value. This
    might be useful if the server is expected to be idle for a long
    time. However this feature should be used with care as hibernation
    implies at least two garbage collections (when hibernating and
    shortly after waking up) and is not something you'd want to do
    between each call to a busy server.</p>

  </description>
  <funcs>
    <func>
      <name>start_link(Module, Args, Options) -> Result</name>
      <name>start_link(ServerName, Module, Args, Options) -> Result</name>
      <fsummary>Create a gen_server process in a supervision tree.</fsummary>
      <type>
        <v>ServerName = {local,Name} | {global,GlobalName}
	| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Module = atom()</v>
        <v>Args = term()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>&nbsp;&nbsp;SOpts = [term()]</v>
        <v>Result = {ok,Pid} | ignore | {error,Error}</v>
        <v>&nbsp;Pid = pid()</v>
        <v>&nbsp;Error = {already_started,Pid} | term()</v>
      </type>
      <desc>
        <p>Creates a gen_server process as part of a supervision tree.
          The function should be called, directly or indirectly, by
          the supervisor. It will, among other things, ensure that
          the gen_server is linked to the supervisor.</p>
        <p>The gen_server process calls <c>Module:init/1</c> to
          initialize. To ensure a synchronized start-up procedure,
          <c>start_link/3,4</c> does not return until
          <c>Module:init/1</c> has returned.</p>
        <p>If <c>ServerName={local,Name}</c> the gen_server is
          registered locally as <c>Name</c> using <c>register/2</c>.
          If <c>ServerName={global,GlobalName}</c> the gen_server is
          registered globally as <c>GlobalName</c> using
          <c>global:register_name/2</c>. If no name is provided,
          the gen_server is not registered.
	  If <c>ServerName={via,Module,ViaName}</c>, the gen_server will
	  register with the registry represented by <c>Module</c>.
	  The <c>Module</c> callback should export the functions
	  <c>register_name/2</c>, <c>unregister_name/1</c>,
	  <c>whereis_name/1</c> and <c>send/2</c>, which should behave like the
	  corresponding functions in <c>global</c>. Thus,
	<c>{via,global,GlobalName}</c> is a valid reference.</p>
        <p><c>Module</c> is the name of the callback module.</p>
        <p><c>Args</c> is an arbitrary term which is passed as
          the argument to <c>Module:init/1</c>.</p>
        <p>If the option <c>{timeout,Time}</c> is present,
          the gen_server is allowed to spend <c>Time</c> milliseconds
          initializing or it will be terminated and the start function
          will return <c>{error,timeout}</c>.
	</p>
        <p>If the option <c>{debug,Dbgs}</c> is present,
          the corresponding <c>sys</c> function will be called for each
          item in <c>Dbgs</c>. See
          <seealso marker="sys">sys(3)</seealso>.</p>
        <p>If the option <c>{spawn_opt,SOpts}</c> is present,
          <c>SOpts</c> will be passed as option list to
          the <c>spawn_opt</c> BIF which is used to spawn
          the gen_server. See
          <seealso marker="erts:erlang#spawn_opt/2">erlang(3)</seealso>.</p>
        <note>
          <p>Using the spawn option <c>monitor</c> is currently not
            allowed, but will cause the function to fail with reason
            <c>badarg</c>.</p>
        </note>
        <p>If the gen_server is successfully created and initialized
          the function returns <c>{ok,Pid}</c>, where <c>Pid</c> is
          the pid of the gen_server. If there already exists a process
          with the specified <c>ServerName</c> the function returns
          <c>{error,{already_started,Pid}}</c>, where <c>Pid</c> is
          the pid of that process.</p>
        <p>If <c>Module:init/1</c> fails with <c>Reason</c>,
          the function returns <c>{error,Reason}</c>. If
          <c>Module:init/1</c> returns <c>{stop,Reason}</c> or
          <c>ignore</c>, the process is terminated and the function
          returns <c>{error,Reason}</c> or <c>ignore</c>, respectively.</p>
      </desc>
    </func>
    <func>
      <name>start(Module, Args, Options) -> Result</name>
      <name>start(ServerName, Module, Args, Options) -> Result</name>
      <fsummary>Create a stand-alone gen_server process.</fsummary>
      <type>
        <v>ServerName = {local,Name} | {global,GlobalName}
	| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Module = atom()</v>
        <v>Args = term()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {timeout,Time} | {spawn_opt,SOpts}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics | {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>&nbsp;&nbsp;SOpts = [term()]</v>
        <v>Result = {ok,Pid} | ignore | {error,Error}</v>
        <v>&nbsp;Pid = pid()</v>
        <v>&nbsp;Error = {already_started,Pid} | term()</v>
      </type>
      <desc>
        <p>Creates a stand-alone gen_server process, i.e. a gen_server
          which is not part of a supervision tree and thus has no
          supervisor.</p>
        <p>See <seealso marker="#start_link/3">start_link/3,4</seealso>
          for a description of arguments and return values.</p>
      </desc>
    </func>
    <func>
      <name>stop(ServerRef) -> ok</name>
      <name>stop(ServerRef, Reason, Timeout) -> ok</name>
      <fsummary>Synchronously stop a generic server.</fsummary>
      <type>
        <v>ServerRef = Name | {Name,Node} | {global,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Reason = term()</v>
        <v>Timeout = int()>0 | infinity</v>
      </type>
      <desc>
	<p>Orders a generic server to exit with the
	  given <c>Reason</c> and waits for it to terminate. The
	  gen_server will call
          <seealso marker="#Module:terminate/2">Module:terminate/2</seealso>
	  before exiting.</p>
	<p>The function returns <c>ok</c> if the server terminates
	  with the expected reason. Any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c> will cause an
          error report to be issued using
          <seealso marker="kernel:error_logger#format/2">error_logger:format/2</seealso>.
	  The default <c>Reason</c> is <c>normal</c>.</p>
	<p><c>Timeout</c> is an integer greater than zero which
          specifies how many milliseconds to wait for the server to
          terminate, or the atom <c>infinity</c> to wait
          indefinitely. The default value is <c>infinity</c>. If the
          server has not terminated within the specified time, a
          <c>timeout</c> exception is raised.</p>
	<p>If the process does not exist, a <c>noproc</c> exception
	  is raised.</p>
      </desc>
    </func>
    <func>
      <name>call(ServerRef, Request) -> Reply</name>
      <name>call(ServerRef, Request, Timeout) -> Reply</name>
      <fsummary>Make a synchronous call to a generic server.</fsummary>
      <type>
        <v>ServerRef = Name | {Name,Node} | {global,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Request = term()</v>
        <v>Timeout = int()>0 | infinity</v>
        <v>Reply = term()</v>
      </type>
      <desc>
        <p>Makes a synchronous call to the gen_server <c>ServerRef</c>
          by sending a request and waiting until a reply arrives or a
          timeout occurs. The gen_server will call
          <c>Module:handle_call/3</c> to handle the request.</p>
        <p><c>ServerRef</c> can be:</p>
        <list type="bulleted">
          <item>the pid,</item>
          <item><c>Name</c>, if the gen_server is locally registered,</item>
          <item><c>{Name,Node}</c>, if the gen_server is locally
           registered at another node, or</item>
          <item><c>{global,GlobalName}</c>, if the gen_server is
           globally registered.</item>
          <item><c>{via,Module,ViaName}</c>, if the gen_server is
           registered through an alternative process registry.</item>
        </list>
        <p><c>Request</c> is an arbitrary term which is passed as one of
          the arguments to <c>Module:handle_call/3</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero which
          specifies how many milliseconds to wait for a reply, or
          the atom <c>infinity</c> to wait indefinitely. Default value
          is 5000. If no reply is received within the specified time,
          the function call fails. If the caller catches the failure
          and continues running, and the server is just late with the reply,
          it may arrive at any time later into the caller's message queue.
          The caller must in this case be prepared for this
          and discard any such garbage messages that are two element
          tuples with a reference as the first element.</p>
        <p>The return value <c>Reply</c> is defined in the return value
          of <c>Module:handle_call/3</c>.</p>
        <p>The call may fail for several reasons, including timeout and
          the called gen_server dying before or during the call.</p>
        <p>The ancient behaviour of sometimes consuming the server
          exit message if the server died during the call while
          linked to the client has been removed in OTP R12B/Erlang 5.6.</p>
      </desc>
    </func>
    <func>
      <name>multi_call(Name, Request) -> Result</name>
      <name>multi_call(Nodes, Name, Request) -> Result</name>
      <name>multi_call(Nodes, Name, Request, Timeout) -> Result</name>
      <fsummary>Make a synchronous call to several generic servers.</fsummary>
      <type>
        <v>Nodes = [Node]</v>
        <v>&nbsp;Node = atom()</v>
        <v>Name = atom()</v>
        <v>Request = term()</v>
        <v>Timeout = int()>=0 | infinity</v>
        <v>Result = {Replies,BadNodes}</v>
        <v>&nbsp;Replies = [{Node,Reply}]</v>
        <v>&nbsp;&nbsp;Reply = term()</v>
        <v>BadNodes = [Node]</v>
      </type>
      <desc>
        <p>Makes a synchronous call to all gen_servers locally
          registered as <c>Name</c> at the specified nodes by first
          sending a request to every node and then waiting for
          the replies. The gen_servers will call
          <c>Module:handle_call/3</c> to handle the request.</p>
        <p>The function returns a tuple <c>{Replies,BadNodes}</c> where
          <c>Replies</c> is a list of <c>{Node,Reply}</c> and
          <c>BadNodes</c> is a list of node that either did not exist,
          or where the gen_server <c>Name</c> did not exist or did not
          reply.</p>
        <p><c>Nodes</c> is a list of node names to which the request
          should be sent. Default value is the list of all known nodes
          <c>[node()|nodes()]</c>.</p>
        <p><c>Name</c> is the locally registered name of each
          gen_server.</p>
        <p><c>Request</c> is an arbitrary term which is passed as one of
          the arguments to <c>Module:handle_call/3</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero which
          specifies how many milliseconds to wait for each reply, or
          the atom <c>infinity</c> to wait indefinitely. Default value
          is <c>infinity</c>. If no reply is received from a node within
          the specified time, the node is added to <c>BadNodes</c>.</p>
        <p>When a reply <c>Reply</c> is received from the gen_server at
          a node <c>Node</c>, <c>{Node,Reply}</c> is added to
          <c>Replies</c>. <c>Reply</c> is defined in the return value of
          <c>Module:handle_call/3</c>.</p>
        <warning>
          <p>If one of the nodes is not capable of process monitors,
            for example C or Java nodes, and the gen_server is not started
             when the requests are sent, but starts within 2 seconds,
             this function waits the whole <c>Timeout</c>,
             which may be infinity.</p>
          <p>This problem does not exist if all nodes are Erlang nodes.</p>
        </warning>
        <p>To prevent late answers (after the timeout) from polluting
          the caller's message queue, a middleman process is used to
          do the actual calls. Late answers will then be discarded
          when they arrive to a terminated process.</p>
      </desc>
    </func>
    <func>
      <name>cast(ServerRef, Request) -> ok</name>
      <fsummary>Send an asynchronous request to a generic server.</fsummary>
      <type>
        <v>ServerRef = Name | {Name,Node} | {global,GlobalName}
	| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Request = term()</v>
      </type>
      <desc>
        <p>Sends an asynchronous request to the gen_server
          <c>ServerRef</c> and returns <c>ok</c> immediately, ignoring
          if the destination node or gen_server does not exist.
          The gen_server will call <c>Module:handle_cast/2</c> to
          handle the request.</p>
        <p>See <seealso marker="#call/2">call/2,3</seealso> for a
          description of <c>ServerRef</c>.</p>
        <p><c>Request</c> is an arbitrary term which is passed as one
          of the arguments to <c>Module:handle_cast/2</c>.</p>
      </desc>
    </func>
    <func>
      <name>abcast(Name, Request) -> abcast</name>
      <name>abcast(Nodes, Name, Request) -> abcast</name>
      <fsummary>Send an asynchronous request to several generic servers.</fsummary>
      <type>
        <v>Nodes = [Node]</v>
        <v>&nbsp;Node = atom()</v>
        <v>Name = atom()</v>
        <v>Request = term()</v>
      </type>
      <desc>
        <p>Sends an asynchronous request to the gen_servers locally
          registered as <c>Name</c> at the specified nodes. The function
          returns immediately and ignores nodes that do not exist, or
          where the gen_server <c>Name</c> does not exist.
          The gen_servers will call <c>Module:handle_cast/2</c> to
          handle the request.</p>
        <p>See
          <seealso marker="#multi_call/2">multi_call/2,3,4</seealso>
          for a description of the arguments.</p>
      </desc>
    </func>
    <func>
      <name>reply(Client, Reply) -> Result</name>
      <fsummary>Send a reply to a client.</fsummary>
      <type>
        <v>Client - see below</v>
        <v>Reply = term()</v>
	<v>Result = term()</v>
      </type>
      <desc>
        <p>This function can be used by a gen_server to explicitly send
          a reply to a client that called <c>call/2,3</c> or
          <c>multi_call/2,3,4</c>, when the reply cannot be defined in
          the return value of <c>Module:handle_call/3</c>.</p>
        <p><c>Client</c> must be the <c>From</c> argument provided to
          the callback function. <c>Reply</c> is an arbitrary term,
          which will be given back to the client as the return value of
          <c>call/2,3</c> or <c>multi_call/2,3,4</c>.</p>
        <p>The return value <c>Result</c> is not further defined, and
	   should always be ignored.</p>
      </desc>
    </func>
    <func>
      <name>enter_loop(Module, Options, State)</name>
      <name>enter_loop(Module, Options, State, ServerName)</name>
      <name>enter_loop(Module, Options, State, Timeout)</name>
      <name>enter_loop(Module, Options, State, ServerName, Timeout)</name>
      <fsummary>Enter the gen_server receive loop</fsummary>
      <type>
        <v>Module = atom()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs}</v>
        <v>&nbsp;&nbsp;Dbgs = [Dbg]</v>
        <v>&nbsp;&nbsp;&nbsp;Dbg = trace | log | statistics</v>
        <v>&nbsp;&nbsp;&nbsp;&nbsp;| {log_to_file,FileName} | {install,{Func,FuncState}}</v>
        <v>State = term()</v>
        <v>ServerName = {local,Name} | {global,GlobalName}
	| {via,Module,ViaName}</v>
        <v>&nbsp;Name = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Timeout = int() | infinity</v>
      </type>
      <desc>
        <p>Makes an existing process into a gen_server. Does not return,
          instead the calling process will enter the gen_server receive
          loop and become a gen_server process. The process
          <em>must</em> have been started using one of the start
          functions in <c>proc_lib</c>, see
          <seealso marker="proc_lib">proc_lib(3)</seealso>. The user is
          responsible for any initialization of the process, including
          registering a name for it.</p>
        <p>This function is useful when a more complex initialization
          procedure is needed than the gen_server behaviour provides.</p>
        <p><c>Module</c>, <c>Options</c> and <c>ServerName</c> have
          the same meanings as when calling
          <seealso marker="#start_link/3">gen_server:start[_link]/3,4</seealso>.
          However, if <c>ServerName</c> is specified, the process must
          have been registered accordingly <em>before</em> this function
          is called.</p>
        <p><c>State</c> and <c>Timeout</c> have the same meanings as in
          the return value of
          <seealso marker="#Moduleinit">Module:init/1</seealso>.
          Also, the callback module <c>Module</c> does not need to
          export an <c>init/1</c> function. </p>
        <p>Failure: If the calling process was not started by a
          <c>proc_lib</c> start function, or if it is not registered
          according to <c>ServerName</c>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>CALLBACK FUNCTIONS</title>
    <p>The following functions
      should be exported from a <c>gen_server</c> callback module.</p>
  </section>
  <funcs>
    <func>
      <name>Module:init(Args) -> Result</name>
      <fsummary>Initialize process and internal state.</fsummary>
      <type>
        <v>Args = term()</v>
        <v>Result =  {ok,State} | {ok,State,Timeout} | {ok,State,hibernate}</v>
        <v>&nbsp;| {stop,Reason} | ignore</v>
        <v>&nbsp;State = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <marker id="Moduleinit"></marker>
        <p>Whenever a gen_server is started using
          <seealso marker="#start/3">gen_server:start/3,4</seealso> or
          <seealso marker="#start_link/3">gen_server:start_link/3,4</seealso>,
          this function is called by the new process to initialize.</p>
        <p><c>Args</c> is the <c>Args</c> argument provided to the start
          function.</p>
        <p>If the initialization is successful, the function should
          return <c>{ok,State}</c>, <c>{ok,State,Timeout}</c> or <c>{ok,State,hibernate}</c>, where
          <c>State</c> is the internal state of the gen_server.</p>
        <p>If an integer timeout value is provided, a timeout will occur
          unless a request or a message is received within
          <c>Timeout</c> milliseconds. A timeout is represented by
          the atom <c>timeout</c> which should be handled by
          the <c>handle_info/2</c> callback function. The atom
          <c>infinity</c> can be used to wait indefinitely, this is
          the default value.</p>
	<p>If <c>hibernate</c> is specified instead of a timeout value, the process will go 
	   into hibernation when waiting for the next message to arrive (by calling 
	   <seealso marker="proc_lib#hibernate/3">proc_lib:hibernate/3</seealso>).</p>
        <p>If something goes wrong during the initialization
          the function should return <c>{stop,Reason}</c> where
          <c>Reason</c> is any term, or <c>ignore</c>.</p>
      </desc>
    </func>
    <func>
      <name>Module:handle_call(Request, From, State) -> Result</name>
      <fsummary>Handle a synchronous request.</fsummary>
      <type>
        <v>Request = term()</v>
        <v>From = {pid(),Tag}</v>
        <v>State = term()</v>
        <v>Result = {reply,Reply,NewState} | {reply,Reply,NewState,Timeout}</v> 
	<v>&nbsp;&nbsp;| {reply,Reply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState} | {noreply,NewState,Timeout}</v> 
	<v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,Reply,NewState} | {stop,Reason,NewState}</v>
        <v>&nbsp;Reply = term()</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a gen_server receives a request sent using
          <seealso marker="#call/2">gen_server:call/2,3</seealso> or
          <seealso marker="#multi_call/2">gen_server:multi_call/2,3,4</seealso>,
          this function is called to handle the request.</p>
        <p><c>Request</c> is the <c>Request</c> argument provided
          to <c>call</c> or <c>multi_call</c>.</p>
        <p><c>From</c> is a tuple <c>{Pid,Tag}</c> where <c>Pid</c> is
          the pid of the client and <c>Tag</c> is a unique tag.</p>
        <p><c>State</c> is the internal state of the gen_server.</p>
        <p>If the function returns <c>{reply,Reply,NewState}</c>, 
          <c>{reply,Reply,NewState,Timeout}</c> or 
	  <c>{reply,Reply,NewState,hibernate}</c>, <c>Reply</c> will be
          given back to <c>From</c> as the return value of
          <c>call/2,3</c> or included in the return value of
          <c>multi_call/2,3,4</c>. The gen_server then continues
          executing with the possibly updated internal state
          <c>NewState</c>. See <c>Module:init/1</c> for a description
          of <c>Timeout</c> and <c>hibernate</c>.</p>
        <p>If the functions returns <c>{noreply,NewState}</c>,
          <c>{noreply,NewState,Timeout}</c> or <c>{noreply,NewState,hibernate}</c>, 
	  the gen_server will
          continue executing with <c>NewState</c>. Any reply to
          <c>From</c> must be given explicitly using
          <seealso marker="#reply/2">gen_server:reply/2</seealso>.</p>
        <p>If the function returns <c>{stop,Reason,Reply,NewState}</c>,
          <c>Reply</c> will be given back to <c>From</c>. If
          the function returns <c>{stop,Reason,NewState}</c>, any reply
          to <c>From</c> must be given explicitly using
          <c>gen_server:reply/2</c>. The gen_server will then call
          <c>Module:terminate(Reason,NewState)</c> and terminate.</p>
      </desc>
    </func>
    <func>
      <name>Module:handle_cast(Request, State) -> Result</name>
      <fsummary>Handle an asynchronous request.</fsummary>
      <type>
        <v>Request = term()</v>
        <v>State = term()</v>
        <v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a gen_server receives a request sent using
          <seealso marker="#cast/2">gen_server:cast/2</seealso> or
          <seealso marker="#abcast/2">gen_server:abcast/2,3</seealso>,
          this function is called to handle the request.</p>
        <p>See <c>Module:handle_call/3</c> for a description of
          the arguments and possible return values.</p>
      </desc>
    </func>
    <func>
      <name>Module:handle_info(Info, State) -> Result</name>
      <fsummary>Handle an incoming message.</fsummary>
      <type>
        <v>Info = timeout | term()</v>
        <v>State = term()</v>
        <v>Result = {noreply,NewState} | {noreply,NewState,Timeout} </v>
	<v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Reason = normal | term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_server when a timeout
          occurs or when it receives any other message than a
          synchronous or asynchronous request (or a system message).</p>
        <p><c>Info</c> is either the atom <c>timeout</c>, if a timeout
          has occurred, or the received message.</p>
        <p>See <c>Module:handle_call/3</c> for a description of
          the other arguments and possible return values.</p>
      </desc>
    </func>
    <func>
      <name>Module:terminate(Reason, State)</name>
      <fsummary>Clean up before termination.</fsummary>
      <type>
        <v>Reason = normal | shutdown | {shutdown,term()} | term()</v>
        <v>State = term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_server when it is about to
          terminate. It should be the opposite of <c>Module:init/1</c>
          and do any necessary cleaning up. When it returns,
          the gen_server terminates with <c>Reason</c>. The return
          value is ignored.</p>
        <p><c>Reason</c> is a term denoting the stop reason and
          <c>State</c> is the internal state of the gen_server.</p>
        <p><c>Reason</c> depends on why the gen_server is terminating.
          If it is because another callback function has returned a
          stop tuple <c>{stop,..}</c>, <c>Reason</c> will have
          the value specified in that tuple. If it is due to a failure,
          <c>Reason</c> is the error reason.</p>
        <p>If the gen_server is part of a supervision tree and is
          ordered by its supervisor to terminate, this function will be
          called with <c>Reason=shutdown</c> if the following
          conditions apply:</p>
        <list type="bulleted">
          <item>the gen_server has been set to trap exit signals, and</item>
          <item>the shutdown strategy as defined in the supervisor's
           child specification is an integer timeout value, not
          <c>brutal_kill</c>.</item>
        </list>
        <p>Even if the gen_server is <em>not</em> part of a supervision tree,
	   this function will be called if it receives an <c>'EXIT'</c>
	   message from its parent. <c>Reason</c> will be the same as in
	   the <c>'EXIT'</c> message.</p>
        <p>Otherwise, the gen_server will be immediately terminated.</p>
        <p>Note that for any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c> the gen_server is
	  assumed to terminate due to an error and
	  an error report is issued using
          <seealso marker="kernel:error_logger#format/2">error_logger:format/2</seealso>.</p>
      </desc>
    </func>
    <func>
      <name>Module:code_change(OldVsn, State, Extra) -> {ok, NewState} | {error, Reason}</name>
      <fsummary>Update the internal state during upgrade/downgrade.</fsummary>
      <type>
        <v>OldVsn = Vsn | {down, Vsn}</v>
        <v>&nbsp;&nbsp;Vsn = term()</v>
        <v>State = NewState = term()</v>
        <v>Extra = term()</v>
	<v>Reason = term()</v>
      </type>
      <desc>
        <p>This function is called by a gen_server when it should
          update its internal state during a release upgrade/downgrade,
          i.e. when the instruction <c>{update,Module,Change,...}</c>
          where <c>Change={advanced,Extra}</c> is given in
          the <c>appup</c> file. See
          <seealso marker="doc/design_principles:release_handling#instr">OTP Design Principles</seealso>
          for more information.</p>
        <p>In the case of an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
          in the case of a downgrade, <c>OldVsn</c> is
          <c>{down,Vsn}</c>. <c>Vsn</c> is defined by the <c>vsn</c>
          attribute(s) of the old version of the callback module
          <c>Module</c>. If no such attribute is defined, the version
          is the checksum of the BEAM file.</p>
        <p><c>State</c> is the internal state of the gen_server.</p>
        <p><c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
          part of the update instruction.</p>
        <p>If successful, the function shall return the updated
          internal state.</p>
	<p>If the function returns <c>{error,Reason}</c>, the ongoing
	  upgrade will fail and roll back to the old release.</p>
      </desc>
    </func>
    <func>
      <name>Module:format_status(Opt, [PDict, State]) -> Status</name>
      <fsummary>Optional function for providing a term describing the
        current gen_server status.</fsummary>
      <type>
        <v>Opt = normal | terminate</v>
        <v>PDict = [{Key, Value}]</v>
        <v>State = term()</v>
        <v>Status = term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The gen_server module provides a default
            implementation of this function that returns the callback
            module state.</p>
        </note>
        <p>This function is called by a gen_server process when:</p>
        <list typed="bulleted">
          <item>One
          of <seealso marker="sys#get_status/1">sys:get_status/1,2</seealso>
          is invoked to get the gen_server status. <c>Opt</c> is set
          to the atom <c>normal</c> for this case.</item>
          <item>The gen_server terminates abnormally and logs an
          error. <c>Opt</c> is set to the atom <c>terminate</c> for this
          case.</item>
        </list>
        <p>This function is useful for customising the form and
          appearance of the gen_server status for these cases. A
          callback module wishing to customise
          the <c>sys:get_status/1,2</c> return value as well as how
          its status appears in termination error logs exports an
          instance of <c>format_status/2</c> that returns a term
          describing the current status of the gen_server.</p>
        <p><c>PDict</c> is the current value of the gen_server's
          process dictionary.</p>
        <p><c>State</c> is the internal state of the gen_server.</p>
        <p>The function should return <c>Status</c>, a term that
          customises the details of the current state and status of
          the gen_server. There are no restrictions on the
          form <c>Status</c> can take, but for
          the <c>sys:get_status/1,2</c> case (when <c>Opt</c>
          is <c>normal</c>), the recommended form for
          the <c>Status</c> value is <c>[{data, [{"State",
          Term}]}]</c> where <c>Term</c> provides relevant details of
          the gen_server state. Following this recommendation isn't
          required, but doing so will make the callback module status
          consistent with the rest of the <c>sys:get_status/1,2</c>
          return value.</p>
        <p>One use for this function is to return compact alternative
          state representations to avoid having large state terms
          printed in logfiles.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="gen_event">gen_event(3)</seealso>,
      <seealso marker="gen_fsm">gen_fsm(3)</seealso>,
      <seealso marker="supervisor">supervisor(3)</seealso>,
      <seealso marker="proc_lib">proc_lib(3)</seealso>,
      <seealso marker="sys">sys(3)</seealso></p>
  </section>
</erlref>