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

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  <header>
    <copyright>
      <year>1996</year><year>2017</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.
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      distributed under the License is distributed on an "AS IS" BASIS,
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    <title>gen_server</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>gen_server</module>
  <modulesummary>Generic server behavior.</modulesummary>
  <description>
    <p>This behavior module provides the server of a client-server
      relation. A generic server process (<c>gen_server</c>) implemented using
      this module has a standard set of interface functions and
      includes functionality for tracing and error reporting. It also
      fits into an OTP supervision tree. For more information, see section
      <seealso marker="doc/design_principles:gen_server_concepts">
      gen_server Behaviour</seealso> in OTP Design Principles.</p>

    <p>A <c>gen_server</c> process assumes all specific parts to be located in
      a callback module exporting a predefined set of functions.
      The relationship between the behavior functions and the callback
      functions is 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:async_call
gen_server:multi_call -----> Module:handle_call/3

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

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

-                     -----> Module:handle_continue/2

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

-                     -----> Module:code_change/3</pre>

    <p>If a callback function fails or returns a bad value, the
      <c>gen_server</c> process terminates.</p>

    <p>A <c>gen_server</c> process handles system messages as described in
      <seealso marker="sys"><c>sys(3)</c></seealso>. The <c>sys</c> module
      can be used for debugging a <c>gen_server</c> process.</p>

    <p>Notice that a <c>gen_server</c> process 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 <c>gen_server</c> process does not exist or if bad
      arguments are specified.</p>

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

    <p>If the <c>gen_server</c> process needs to perform an action
      immediately after initialization or to break the execution of a
      callback into multiple steps, it can return <c>{continue,Continue}</c>
      in place of the time-out or hibernation value, which will immediately
      invoke the <c>handle_continue/2</c> callback.</p>

  </description>

  <funcs>
    <func>
      <name>abcast(Name, Request) -> abcast</name>
      <name>abcast(Nodes, Name, Request) -> abcast</name>
      <fsummary>Send an asynchronous request to many 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 <c>gen_server</c> processes
          locally registered as <c>Name</c> at the specified nodes. The function
          returns immediately and ignores nodes that do not exist, or
          where the <c>gen_server</c> <c>Name</c> does not exist.
          The <c>gen_server</c> processes call
          <seealso marker="#Module:handle_cast/2">
          <c>Module:handle_cast/2</c></seealso> to handle the request.</p>
        <p>For a description of the arguments, see
          <seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>.</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}</v>
        <v>&nbsp;&nbsp;| {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 <c>ServerRef</c> of the
          <c>gen_server</c> process
          by sending a request and waiting until a reply arrives or a
          time-out occurs. The <c>gen_server</c> process calls
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso> to handle the request.</p>
        <p><c>ServerRef</c> can be any of the following:</p>
        <list type="bulleted">
          <item>The pid</item>
          <item><c>Name</c>, if the <c>gen_server</c> process is locally
            registered</item>
          <item><c>{Name,Node}</c>, if the <c>gen_server</c> process is locally
            registered at another node</item>
          <item><c>{global,GlobalName}</c>, if the <c>gen_server</c> process is
            globally registered</item>
          <item><c>{via,Module,ViaName}</c>, if the <c>gen_server</c> process is
            registered through an alternative process registry</item>
        </list>
        <p><c>Request</c> is any term that 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 that
          specifies how many milliseconds to wait for a reply, or
          the atom <c>infinity</c> to wait indefinitely. Defaults to
          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 can arrive at any time later into the message queue of the caller.
          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 can fail for many reasons, including time-out and the
          called <c>gen_server</c> process dying before or during the call.</p>
      </desc>
    </func>

    <func>
      <name>async_call(ServerRef, Request) -> Promise</name>
      <fsummary>Make a asynchronous call to a generic server.</fsummary>
      <type>
        <v>ServerRef = Name | {Name,Node} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {via,Module,ViaName} | pid()</v>
        <v>&nbsp;Node = atom()</v>
        <v>&nbsp;GlobalName = ViaName = term()</v>
        <v>Promise = reference()</v>
        <v>Timeout = int()>0 | infinity</v>
        <v>Request = term()</v>
      </type>
      <desc>
        <p>Makes a asynchronous call to the <c>ServerRef</c> of the
        <c>gen_server</c> process and returns a handle <c>Promise</c>.
	The <c>gen_server</c> process calls
        <seealso marker="#Module:handle_call/3">
        <c>Module:handle_call/3</c></seealso> to handle the request.</p>
        <p><c>ServerRef</c> can be any of the following:</p>
        <list type="bulleted">
          <item>The pid</item>
          <item><c>Name</c>, if the <c>gen_server</c> process is locally
            registered</item>
          <item><c>{Name,Node}</c>, if the <c>gen_server</c> process is locally
            registered at another node</item>
          <item><c>{global,GlobalName}</c>, if the <c>gen_server</c> process is
            globally registered</item>
          <item><c>{via,Module,ViaName}</c>, if the <c>gen_server</c> process is
            registered through an alternative process registry</item>
        </list>
        <p><c>Request</c> is any term that is passed as one of
        the arguments to <c>Module:handle_call/3</c>.</p>
        <p>The return value <c>Promise</c> is intended to be used with
        <seealso marker="#yield/2"> <c>yield/1,2</c></seealso>.  Note,
        if the reply message is received and kept without invoking
        <seealso marker="#yield/2"> <c>yield/1,2</c></seealso>, the
        monitor reference needs to be cleaned up with
        <c>erlang:demonitor(Promise, [flush])</c>.
	</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}</v>
        <v>&nbsp;&nbsp;| {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 <c>ServerRef</c> of the
          <c>gen_server</c> process
          and returns <c>ok</c> immediately, ignoring
          if the destination node or <c>gen_server</c> process does not exist.
          The <c>gen_server</c> process calls
          <seealso marker="#Module:handle_cast/2">
          <c>Module:handle_cast/2</c></seealso> to handle the request.</p>
        <p>For a description of <c>ServerRef</c>, see
          <seealso marker="#call/2"><c>call/2,3</c></seealso>.</p>
        <p><c>Request</c> is any term that is passed as one
          of the arguments to <c>Module:handle_cast/2</c>.</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 <c>gen_server</c> receive loop.</fsummary>
      <type>
        <v>Module = atom()</v>
        <v>Options = [Option]</v>
        <v>&nbsp;Option = {debug,Dbgs} | {hibernate_after,HibernateAfterTimeout}</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}</v>
        <v>&nbsp;&nbsp;| {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 <c>gen_server</c> process. Does not
          return, instead the calling process enters the <c>gen_server</c>
          process receive
          loop and becomes a <c>gen_server</c> process. The process
          <em>must</em> have been started using one of the start functions in
          <seealso marker="proc_lib"><c>proc_lib(3)</c></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 <c>gen_server</c> process behavior 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"><c>start[_link]/3,4</c></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="#Module:init/1"><c>Module:init/1</c></seealso>.
          The callback module <c>Module</c> does not need to
          export an <c>init/1</c> function.</p>
        <p>The function fails 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>

    <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 many 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 <c>gen_server</c> processes locally
          registered as <c>Name</c> at the specified nodes by first
          sending a request to every node and then waits for
          the replies. The <c>gen_server</c> process calls
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso> 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 <c>gen_server</c> <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
          is to 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
          <c>gen_server</c> process.</p>
        <p><c>Request</c> is any term that 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 that
          specifies how many milliseconds to wait for each reply, or
          the atom <c>infinity</c> to wait indefinitely. Defaults
          to <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 <c>gen_server</c>
          process 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 cannot process monitors, for example,
            C or Java nodes, and the <c>gen_server</c> process 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 time-out) from polluting
          the message queue of the caller, a middleman process is used to
          do the calls. Late answers are then discarded
          when they arrive to a terminated process.</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 <c>gen_server</c> process to
          explicitly send a reply to a client that called
          <seealso marker="#call/2"><c>call/2,3</c></seealso> or
          <seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></seealso>,
          when the reply cannot be defined in the return value of
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</p>
        <p><c>Client</c> must be the <c>From</c> argument provided to
          the callback function. <c>Reply</c> is any term
          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
          is always to be ignored.</p>
      </desc>
    </func>

    <func>
      <name>start(Module, Args, Options) -> Result</name>
      <name>start(ServerName, Module, Args, Options) -> Result</name>
      <fsummary>Create a standalone <c>gen_server</c> process.</fsummary>
      <type>
        <v>ServerName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {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} | {hibernate_after,HibernateAfterTimeout} | {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 standalone <c>gen_server</c> process, that is, a
          <c>gen_server</c> process that is not part of a supervision tree
          and thus has no supervisor.</p>
        <p>For a description of arguments and return values, see
          <seealso marker="#start_link/3"><c>start_link/3,4</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>start_link(Module, Args, Options) -> Result</name>
      <name>start_link(ServerName, Module, Args, Options) -> Result</name>
      <fsummary>Create a <c>gen_server</c> process in a supervision tree.
      </fsummary>
      <type>
        <v>ServerName = {local,Name} | {global,GlobalName}</v>
        <v>&nbsp;&nbsp;| {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} | {hibernate_after,HibernateAfterTimeout} | {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 <c>gen_server</c> process as part of a supervision tree.
          This function is to be called, directly or indirectly, by
          the supervisor. For example, it ensures that
          the <c>gen_server</c> process is linked to the supervisor.</p>
        <p>The <c>gen_server</c> process calls
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso> to
          initialize. To ensure a synchronized startup procedure,
          <c>start_link/3,4</c> does not return until
          <c>Module:init/1</c> has returned.</p>
        <list type="bulleted">
          <item>
            <p>If <c>ServerName={local,Name}</c>, the <c>gen_server</c> process
              is registered locally as <c>Name</c> using <c>register/2</c>.</p>
          </item>
          <item>
            <p>If <c>ServerName={global,GlobalName}</c>, the <c>gen_server</c>
              process id registered globally as <c>GlobalName</c> using
              <seealso marker="kernel:global#register_name/2">
              <c>global:register_name/2</c></seealso> If no name is
              provided, the <c>gen_server</c> process is not registered.</p>
          </item>
          <item>
            <p>If <c>ServerName={via,Module,ViaName}</c>, the <c>gen_server</c>
              process registers with the registry represented by <c>Module</c>.
              The <c>Module</c> callback is to export the functions
              <c>register_name/2</c>, <c>unregister_name/1</c>,
              <c>whereis_name/1</c>, and <c>send/2</c>, which are to behave
              like the corresponding functions in
              <seealso marker="kernel:global"><c>global</c></seealso>.
              Thus, <c>{via,global,GlobalName}</c> is a valid reference.</p>
          </item>
        </list>
        <p><c>Module</c> is the name of the callback module.</p>
        <p><c>Args</c> is any term that is passed as
          the argument to
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.</p>
        <list type="bulleted">
          <item>
            <p>If option <c>{timeout,Time}</c> is present, the <c>gen_server</c>
              process is allowed to spend <c>Time</c> milliseconds
              initializing or it is terminated and the start function
              returns <c>{error,timeout}</c>.</p>
          </item>
          <item>
            <p>If option <c>{hibernate_after,HibernateAfterTimeout}</c> is present, the <c>gen_server</c>
              process awaits any message for <c>HibernateAfterTimeout</c> milliseconds and
              if no message is received, the process goes into hibernation automatically
              (by calling <seealso marker="proc_lib#hibernate/3"><c>proc_lib:hibernate/3</c></seealso>).</p>
          </item>
          <item>
            <p>If option <c>{debug,Dbgs}</c> is present,
              the corresponding <c>sys</c> function is called for each
              item in <c>Dbgs</c>; see
              <seealso marker="sys"><c>sys(3)</c></seealso>.</p>
          </item>
          <item>
            <p>If option <c>{spawn_opt,SOpts}</c> is present,
              <c>SOpts</c> is passed as option list to
              the <c>spawn_opt</c> BIF, which is used to spawn
              the <c>gen_server</c> process; see
              <seealso marker="erts:erlang#spawn_opt/2">
              <c>spawn_opt/2</c></seealso>.</p>
          </item>
        </list>
        <note>
          <p>Using spawn option <c>monitor</c> is not
            allowed, it causes the function to fail with reason
            <c>badarg</c>.</p>
        </note>
        <p>If the <c>gen_server</c> process is successfully created and
          initialized, the function returns <c>{ok,Pid}</c>, where <c>Pid</c>
          is the pid of the <c>gen_server</c> process. If a process with the
          specified <c>ServerName</c> exists already, 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>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}</v>
        <v>&nbsp;&nbsp;| {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 specified <c>Reason</c>
          and waits for it to terminate. The <c>gen_server</c> process calls
          <seealso marker="#Module:terminate/2">
          <c>Module:terminate/2</c></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> causes an
          error report to be issued using
          <seealso marker="kernel:error_logger#format/2">
          <c>error_logger:format/2</c></seealso>.
          The default <c>Reason</c> is <c>normal</c>.</p>
        <p><c>Timeout</c> is an integer greater than zero that
          specifies how many milliseconds to wait for the server to
          terminate, or the atom <c>infinity</c> to wait
          indefinitely. Defaults to <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>
  </funcs>


  <func>
    <name>yield(Promise) -> {reply, Reply}</name>
    <name>yield(Promise, Timeout) -> {reply, Reply} | timeout</name>
    <fsummary>Wait for a reply from a server.</fsummary>
    <type>
      <v>Promise = reference()</v>
      <v>Reply = term()</v>
      <v>Timeout = timeout()</v>
    </type>
    <desc>
      <p>This function is used to wait for a reply on a request made with
      <seealso marker="#async_call/2"><c>async_call/2</c></seealso>
      from the <c>gen_server</c> process.</p>
      <p><c>Timeout</c> is an integer greater then or equal to zero
      that specifies how many milliseconds to wait for an reply, or
      the atom <c>infinity</c> to wait indefinitely. Defaults to
      <c>infinity</c>.  If no reply is received within the specified
      time, the function returns <c>timeout</c> and no cleanup is
      done, and thus the function can be invoked repeatedly until a
      reply is returned.  Otherwise do a cleanup the request with
      <c>erlang:demonitor(Promise, [flush])</c>.</p>
      <p>The return value <c>Reply</c> is defined in the return value
      of <c>Module:handle_call/3</c>.</p>
      <p>This function can fail for many reasons, including that the
      called <c>gen_server</c> process dying before or during the call.</p>
    </desc>
  </func>

  <section>
    <title>Callback Functions</title>
    <p>The following functions
      are to be exported from a <c>gen_server</c> callback module.</p>
  </section>

  <funcs>
    <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>
        <note>
          <p>This callback is optional, so callback modules need not export it.
            If a release upgrade/downgrade with <c>Change={advanced,Extra}</c>
            specified in the <c>appup</c> file is made when <c>code_change/3</c>
            isn't implemented the process will crash with an <c>undef</c> exit
            reason.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process when it is
          to update its internal state during a release upgrade/downgrade,
          that is, when the instruction <c>{update,Module,Change,...}</c>,
          where <c>Change={advanced,Extra}</c>, is specifed in
          the <c>appup</c> file. For more information, see section
          <seealso marker="doc/design_principles:release_handling#instr">
          Release Handling Instructions</seealso> in OTP Design Principles.</p>
        <p>For an upgrade, <c>OldVsn</c> is <c>Vsn</c>, and
          for 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 <c>gen_server</c>
          process.</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 must return the updated
          internal state.</p>
        <p>If the function returns <c>{error,Reason}</c>, the ongoing
          upgrade fails and rolls 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 <c>gen_server</c> 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 <c>gen_server</c> module provides a default
            implementation of this function that returns the callback
            module state.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process in the
          following situations:</p>
        <list type="bulleted">
          <item>
            <p>One of <seealso marker="sys#get_status/1">
              <c>sys:get_status/1,2</c></seealso>
              is invoked to get the <c>gen_server</c> status. <c>Opt</c> is set
              to the atom <c>normal</c>.</p>
          </item>
          <item>
            <p>The <c>gen_server</c> process terminates abnormally and logs an
              error. <c>Opt</c> is set to the atom <c>terminate</c>.</p>
          </item>
        </list>
        <p>This function is useful for changing the form and
          appearance of the <c>gen_server</c> status for these cases. A
          callback module wishing to change
          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 <c>gen_server</c> process.</p>
        <p><c>PDict</c> is the current value of the process dictionary of
          the <c>gen_server</c> process..</p>
        <p><c>State</c> is the internal state of the <c>gen_server</c>
          process.</p>
        <p>The function is to return <c>Status</c>, a term that
          changes the details of the current state and status of
          the <c>gen_server</c> process. 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 <c>gen_server</c> state. Following this recommendation is not
          required, but it makes 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 that large state terms are
          printed in log files.</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;| {reply,Reply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState} | {noreply,NewState,Timeout}</v> 
        <v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</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;Continue = term()</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a <c>gen_server</c> process receives a request sent using
          <seealso marker="#call/2"><c>call/2,3</c></seealso> or
          <seealso marker="#multi_call/2"><c>multi_call/2,3,4</c></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 <c>gen_server</c>
          process.</p>
        <list type="bulleted">
          <item>
            <p>If <c>{reply,Reply,NewState}</c> is returned, 
              <c>{reply,Reply,NewState,Timeout}</c> or 
              <c>{reply,Reply,NewState,hibernate}</c>, <c>Reply</c> is
              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 <c>gen_server</c> process then
              continues executing with the possibly updated internal state
              <c>NewState</c>.</p>
            <p>For a description of <c>Timeout</c> and <c>hibernate</c>, see
              <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>.</p>
          </item>
          <item>
            <p>If <c>{noreply,NewState}</c> is returned,
              <c>{noreply,NewState,Timeout}</c>, or
              <c>{noreply,NewState,hibernate}</c>, the <c>gen_server</c>
              process continues executing with <c>NewState</c>. Any reply to
              <c>From</c> must be specified explicitly using
              <seealso marker="#reply/2"><c>reply/2</c></seealso>.</p>
          </item>
          <item>
            <p>If <c>{stop,Reason,Reply,NewState}</c> is returned,
              <c>Reply</c> is given back to <c>From</c>.</p>
          </item>
          <item>
            <p>If <c>{stop,Reason,NewState}</c> is returned, any reply
              to <c>From</c> must be specified explicitly using
              <seealso marker="#reply/2"><c>reply/2</c></seealso>.
              The <c>gen_server</c> process then calls
              <c>Module:terminate(Reason,NewState)</c> and terminates.</p>
          </item>
        </list>
      </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;| {noreply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Continue = term()</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a <c>gen_server</c> process receives a request sent using
          <seealso marker="#cast/2"><c>cast/2</c></seealso> or
          <seealso marker="#abcast/2"><c>abcast/2,3</c></seealso>,
          this function is called to handle the request.</p>
        <p>For a description of the arguments and possible return values, see
          <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>Module:handle_continue(Continue, State) -> Result</name>
      <fsummary>Handle a continue instruction.</fsummary>
      <type>
        <v>Continue = term()</v>
        <v>State = term()</v>
        <v>Result = {noreply,NewState} | {noreply,NewState,Timeout}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,hibernate}</v>
        <v>&nbsp;&nbsp;| {noreply,NewState,{continue,Continue}}</v>
        <v>&nbsp;&nbsp;| {stop,Reason,NewState}</v>
        <v>&nbsp;NewState = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Continue = term()</v>
        <v>&nbsp;Reason = normal | term()</v>
      </type>
      <desc>
        <note>
          <p>This callback is optional, so callback modules need to
            export it only if they return <c>{continue,Continue}</c>
            from another callback. If continue is used and the callback
            is not implemented, the process will exit with <c>undef</c>
            error.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process whenever
          a previous callback returns <c>{continue, Continue}</c>.
          <c>handle_continue/2</c> is invoked immediately after the previous
          callback, which makes it useful for performing work after
          initialization or for splitting the work in a callback in
          multiple steps, updating the process state along the way.</p>
        <p>For a description of the other arguments and possible return values,
          see <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</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;| {noreply,NewState,{continue,Continue}}</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>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The <c>gen_server</c> module provides a default
            implementation of this function that logs about the unexpected
            <c>Info</c> message, drops it and returns <c>{noreply, State}</c>.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process when a
          time-out 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 time-out
          has occurred, or the received message.</p>
        <p>For a description of the other arguments and possible return values,
          see <seealso marker="#Module:handle_call/3">
          <c>Module:handle_call/3</c></seealso>.</p>
      </desc>
    </func>

    <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;| {ok,State,{continue,Continue}} | {stop,Reason} | ignore</v>
        <v>&nbsp;State = term()</v>
        <v>&nbsp;Timeout = int()>=0 | infinity</v>
        <v>&nbsp;Reason = term()</v>
      </type>
      <desc>
        <p>Whenever a <c>gen_server</c> process is started using
          <seealso marker="#start/3"><c>start/3,4</c></seealso> or
          <seealso marker="#start_link/3"><c>start_link/3,4</c></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 is to
          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 <c>gen_server</c> process.</p>
        <p>If an integer time-out value is provided, a time-out occurs
          unless a request or a message is received within
          <c>Timeout</c> milliseconds. A time-out is represented by
          the atom <c>timeout</c>, which is to be handled by the
          <seealso marker="#Module:handle_info/2">
          <c>Module:handle_info/2</c></seealso> 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 time-out value,
          the process goes into
          hibernation when waiting for the next message to arrive (by calling 
          <seealso marker="proc_lib#hibernate/3">
          <c>proc_lib:hibernate/3</c></seealso>).</p>
        <p>If the initialization fails, the function is to return
          <c>{stop,Reason}</c>, where <c>Reason</c> is any term, or
          <c>ignore</c>.</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>
        <note>
          <p>This callback is optional, so callback modules need not
            export it. The <c>gen_server</c> module provides a default
            implementation without cleanup.</p>
        </note>
        <p>This function is called by a <c>gen_server</c> process when it is
          about to terminate. It is to be the opposite of
          <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>
          and do any necessary cleaning up. When it returns,
          the <c>gen_server</c> process 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 <c>gen_server</c> process.</p>
        <p><c>Reason</c> depends on why the <c>gen_server</c> process is
          terminating. If it is because another callback function has returned
          a stop tuple <c>{stop,..}</c>, <c>Reason</c> has
          the value specified in that tuple. If it is because of a failure,
          <c>Reason</c> is the error reason.</p>
        <p>If the <c>gen_server</c> process is part of a supervision tree and
          is ordered by its supervisor to terminate, this function is
          called with <c>Reason=shutdown</c> if the following
          conditions apply:</p>
        <list type="bulleted">
          <item>
            <p>The <c>gen_server</c> process has been set to trap exit
              signals.</p>
          </item>
          <item>
            <p>The shutdown strategy as defined in the child specification
              of the supervisor is an integer time-out value, not
              <c>brutal_kill</c>.</p>
           </item>
        </list>
        <p>Even if the <c>gen_server</c> process is <em>not</em> part of a
          supervision tree, this function is called if it receives an
          <c>'EXIT'</c> message from its parent. <c>Reason</c> is the same
          as in the <c>'EXIT'</c> message.</p>
        <p>Otherwise, the <c>gen_server</c> process terminates immediately.</p>
        <p>Notice that for any other reason than <c>normal</c>,
          <c>shutdown</c>, or <c>{shutdown,Term}</c>, the <c>gen_server</c>
          process is assumed to terminate because of an error and
          an error report is issued using
          <seealso marker="kernel:error_logger#format/2">
          <c>error_logger:format/2</c></seealso>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seealso marker="gen_event"><c>gen_event(3)</c></seealso>,
      <seealso marker="gen_statem"><c>gen_statem(3)</c></seealso>,
      <seealso marker="proc_lib"><c>proc_lib(3)</c></seealso>,
      <seealso marker="supervisor"><c>supervisor(3)</c></seealso>,
      <seealso marker="sys"><c>sys(3)</c></seealso></p>
  </section>
</erlref>