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<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2018</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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          http://www.apache.org/licenses/LICENSE-2.0

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      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 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>

