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<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
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    </legalnotice>

    <title>gen_server Behaviour</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>gen_server_concepts.xml</file>
  </header>
  <marker id="gen_server"></marker>
  <p>This section is to be read with the
    <seeerl marker="stdlib:gen_server">gen_server(3)</seeerl>
    manual page in <c>stdlib</c>, where all interface functions and
    callback functions are described in detail.</p>

  <section>
    <title>Client-Server Principles</title>
    <p>The client-server model is characterized by a central server
      and an arbitrary number of clients. The client-server model is
      used for resource management operations, where several
      different clients want to share a common resource. The server is
      responsible for managing this resource.</p>
    <marker id="clientserver"></marker>
    <image file="../design_principles/clientserver.gif">
      <icaption>Client-Server Model</icaption>
    </image>
  </section>

  <section>
    <title>Example</title>
    <p>An example of a simple server written in plain Erlang is
      provided in
      <seeguide marker="des_princ#ch1">Overview</seeguide>.
      The server can be reimplemented using <c>gen_server</c>,
      resulting in this callback module:</p>
    <marker id="ex"></marker>
    <code type="none">
-module(ch3).
-behaviour(gen_server).

-export([start_link/0]).
-export([alloc/0, free/1]).
-export([init/1, handle_call/3, handle_cast/2]).

start_link() ->
    gen_server:start_link({local, ch3}, ch3, [], []).

alloc() ->
    gen_server:call(ch3, alloc).

free(Ch) ->
    gen_server:cast(ch3, {free, Ch}).

init(_Args) ->
    {ok, channels()}.

handle_call(alloc, _From, Chs) ->
    {Ch, Chs2} = alloc(Chs),
    {reply, Ch, Chs2}.

handle_cast({free, Ch}, Chs) ->
    Chs2 = free(Ch, Chs),
    {noreply, Chs2}.</code>
    <p>The code is explained in the next sections.</p>
  </section>

  <section>
    <title>Starting a Gen_Server</title>
    <p>In the example in the previous section, <c>gen_server</c> is
    started by calling <c>ch3:start_link()</c>:</p>
    <code type="none">
start_link() ->
    gen_server:start_link({local, ch3}, ch3, [], []) => {ok, Pid}</code>
    <p><c>start_link</c> calls function <c>gen_server:start_link/4</c>.
    This function spawns and links to a new process, a
    <c>gen_server</c>.</p>
    <list type="bulleted">
      <item>
        <p>The first argument, <c>{local, ch3}</c>, specifies the name.
          The gen_server is then locally registered as <c>ch3</c>.</p>
        <p>If the name is omitted, the <c>gen_server</c> is not registered.
          Instead its pid must be used. The name can also be given
          as <c>{global, Name}</c>, in which case the <c>gen_server</c> is
          registered using <c>global:register_name/2</c>.</p>
      </item>
      <item>
        <p>The second argument, <c>ch3</c>, is the name of the callback
          module, that is, the module where the callback functions are
          located.</p>
        <p>The interface functions (<c>start_link</c>, <c>alloc</c>,
	  and <c>free</c>) are then located in the same module
          as the callback functions (<c>init</c>, <c>handle_call</c>, and
          <c>handle_cast</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>[]</c>, is a term that is passed as is
	  to the callback function <c>init</c>. Here, <c>init</c> does not
          need any indata and ignores the argument.</p>
      </item>
      <item>
        <p>The fourth argument, <c>[]</c>, is a list of options. See the
          <c>gen_server(3)</c> manual page for available options.</p>
      </item>
    </list>
    <p>If name registration succeeds, the new <c>gen_server</c> process
      calls the callback function <c>ch3:init([])</c>. <c>init</c> is
      expected to return <c>{ok, State}</c>, where <c>State</c> is the
      internal state of the <c>gen_server</c>. In this case, the state
      is the available channels.</p>
    <code type="none">
init(_Args) ->
    {ok, channels()}.</code>
    <p><c>gen_server:start_link</c> is synchronous. It does not return
      until the <c>gen_server</c> has been initialized and is ready
      to receive requests.</p>
    <p><c>gen_server:start_link</c> must be used if the <c>gen_server</c>
      is part of a supervision tree, that is, started by a supervisor.
      There is another function, <c>gen_server:start</c>, to start a
      standalone <c>gen_server</c>, that is, a <c>gen_server</c> that
      is not part of a supervision tree.</p>
  </section>

  <section>
    <title>Synchronous Requests - Call</title>
    <p>The synchronous request <c>alloc()</c> is implemented using
      <c>gen_server:call/2</c>:</p>
    <code type="none">
alloc() ->
    gen_server:call(ch3, alloc).</code>
    <p><c>ch3</c> is the name of the <c>gen_server</c> and must agree
      with the name used to start it. <c>alloc</c> is the actual
      request.</p>
    <p>The request is made into a message and sent to the
      <c>gen_server</c>. When the request is received, the
      <c>gen_server</c> calls
      <c>handle_call(Request, From, State)</c>, which is expected to
      return a tuple <c>{reply,Reply,State1}</c>. <c>Reply</c> is
      the reply that is to be sent back to the client, and
      <c>State1</c> is a new value for the state of the
      <c>gen_server</c>.</p>
    <code type="none">
handle_call(alloc, _From, Chs) ->
    {Ch, Chs2} = alloc(Chs),
    {reply, Ch, Chs2}.</code>
    <p>In this case, the reply is the allocated channel <c>Ch</c> and
      the new state is the set of remaining available channels
      <c>Chs2</c>.</p>
    <p>Thus, the call <c>ch3:alloc()</c> returns the allocated channel
      <c>Ch</c> and the <c>gen_server</c> then waits for new requests,
      now with an updated list of available channels.</p>
  </section>

  <section>
    <title>Asynchronous Requests - Cast</title>
    <p>The asynchronous request <c>free(Ch)</c> is implemented using
      <c>gen_server:cast/2</c>:</p>
    <code type="none">
free(Ch) ->
    gen_server:cast(ch3, {free, Ch}).</code>
    <p><c>ch3</c> is the name of the <c>gen_server</c>.
      <c>{free, Ch}</c> is the actual request.</p>
    <p>The request is made into a message and sent to the
      <c>gen_server</c>.
      <c>cast</c>, and thus <c>free</c>, then returns <c>ok</c>.</p>
    <p>When the request is received, the <c>gen_server</c> calls
      <c>handle_cast(Request, State)</c>, which is expected to
      return a tuple <c>{noreply,State1}</c>. <c>State1</c> is a new
      value for the state of the <c>gen_server</c>.</p>
    <code type="none">
handle_cast({free, Ch}, Chs) ->
    Chs2 = free(Ch, Chs),
    {noreply, Chs2}.</code>
    <p>In this case, the new state is the updated list of available
      channels <c>Chs2</c>. The <c>gen_server</c> is now ready for new
      requests.</p>
  </section>

  <section>
    <title>Stopping</title>

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

...

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

    <section>
      <title>Standalone Gen_Servers</title>
      <p>If the <c>gen_server</c> is not part of a supervision tree, a
        stop function can be useful, for example:</p>
      <code type="none">
...
export([stop/0]).
...

stop() ->
    gen_server:cast(ch3, stop).
...

handle_cast(stop, State) ->
    {stop, normal, State};
handle_cast({free, Ch}, State) ->
    ....

...

terminate(normal, State) ->
    ok.</code>
      <p>The callback function handling the <c>stop</c> request returns
        a tuple <c>{stop,normal,State1}</c>, where <c>normal</c>
        specifies that it is a normal termination and <c>State1</c> is
        a new value for the state of the <c>gen_server</c>. This causes
        the <c>gen_server</c> to call <c>terminate(normal, State1)</c>
	and then it terminates gracefully.</p>
    </section>
  </section>

  <section>
    <title>Handling Other Messages</title>
    <p>If the <c>gen_server</c> is to be able to receive other messages
      than requests, the callback function <c>handle_info(Info, State)</c>
      must be implemented to handle them. Examples of other messages
      are exit messages, if the <c>gen_server</c> is linked to other
      processes (than the supervisor) and trapping exit signals.</p>
    <code type="none">
handle_info({'EXIT', Pid, Reason}, State) ->
    ..code to handle exits here..
    {noreply, State1}.</code>
    <p>The <c>code_change</c> method must also be implemented.</p>
    <code type="none">
code_change(OldVsn, State, Extra) ->
    ..code to convert state (and more) during code change
    {ok, NewState}.</code>
  </section>
</chapter>

