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<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Overview</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>des_princ.xml</file>
  </header>
  <p>The <em>OTP Design Principles</em> is a set of principles for how
    to structure Erlang code in terms of processes, modules and
    directories.</p>

  <section>
    <title>Supervision Trees</title>
    <p>A basic concept in Erlang/OTP is the <em>supervision tree</em>.
      This is a process structuring model based on the idea of
      <em>workers</em> and <em>supervisors</em>.</p>
    <list type="bulleted">
      <item>Workers are processes which perform computations, that is,
       they do the actual work.</item>
      <item>Supervisors are processes which monitor the behaviour of
       workers. A supervisor can restart a worker if something goes
       wrong.</item>
      <item>The supervision tree is a hierarchical arrangement of
       code into supervisors and workers, making it possible to
       design and program fault-tolerant software.</item>
    </list>
    <marker id="sup6"></marker>
    <image file="../design_principles/sup6.gif">
      <icaption>Supervision Tree</icaption>
    </image>
    <p>In the figure above, square boxes represents supervisors and
      circles represent workers.</p>
  </section>

  <section>
    <title>Behaviours</title>
    <p>In a supervision tree, many of the processes have similar
      structures, they follow similar patterns. For example,
      the supervisors are very similar in structure. The only difference
      between them is which child processes they supervise. Also, many
      of the workers are servers in a server-client relation, finite
      state machines, or event handlers such as error loggers.</p>
    <p><em>Behaviours</em> are formalizations of these common patterns.
      The idea is to divide the code for a process in a generic part
      (a behaviour module) and a specific part (a <em>callback module</em>).</p>
    <p>The behaviour module is part of Erlang/OTP. To implement a
      process such as a supervisor, the user only has to implement
      the callback module which should export a pre-defined set of
      functions, the <em>callback functions</em>.</p>
    <p>An example to illustrate how code can be divided into a generic
      and a specific part: Consider the following code (written in
      plain Erlang) for a simple server, which keeps track of a number
      of "channels". Other processes can allocate and free the channels
      by calling the functions <c>alloc/0</c> and <c>free/1</c>,
      respectively.</p>
    <marker id="ch1"></marker>
    <code type="none">
-module(ch1).
-export([start/0]).
-export([alloc/0, free/1]).
-export([init/0]).

start() ->
    spawn(ch1, init, []).

alloc() ->
    ch1 ! {self(), alloc},
    receive
        {ch1, Res} ->
            Res
    end.

free(Ch) ->
    ch1 ! {free, Ch},
    ok.

init() ->
    register(ch1, self()),
    Chs = channels(),
    loop(Chs).

loop(Chs) ->
    receive
        {From, alloc} ->
            {Ch, Chs2} = alloc(Chs),
            From ! {ch1, Ch},
            loop(Chs2);
        {free, Ch} ->
            Chs2 = free(Ch, Chs),
            loop(Chs2)
    end.</code>
    <p>The code for the server can be rewritten into a generic part
      <c>server.erl</c>:</p>
    <code type="none">
-module(server).
-export([start/1]).
-export([call/2, cast/2]).
-export([init/1]).

start(Mod) ->
    spawn(server, init, [Mod]).

call(Name, Req) ->
    Name ! {call, self(), Req},
    receive
        {Name, Res} ->
            Res
    end.

cast(Name, Req) ->
    Name ! {cast, Req},
    ok.

init(Mod) ->
    register(Mod, self()),
    State = Mod:init(),
    loop(Mod, State).

loop(Mod, State) ->
    receive
        {call, From, Req} ->
            {Res, State2} = Mod:handle_call(Req, State),
            From ! {Mod, Res},
            loop(Mod, State2);
        {cast, Req} ->
            State2 = Mod:handle_cast(Req, State),
            loop(Mod, State2)
    end.</code>
    <p>and a callback module <c>ch2.erl</c>:</p>
    <code type="none">
-module(ch2).
-export([start/0]).
-export([alloc/0, free/1]).
-export([init/0, handle_call/2, handle_cast/2]).

start() ->
    server:start(ch2).

alloc() ->
    server:call(ch2, alloc).

free(Ch) ->
    server:cast(ch2, {free, Ch}).

init() ->
    channels().

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

handle_cast({free, Ch}, Chs) ->
    free(Ch, Chs). % => Chs2</code>
    <p>Note the following:</p>
    <list type="bulleted">
      <item>The code in <c>server</c> can be re-used to build many
       different servers.</item>
      <item>The name of the server, in this example the atom
      <c>ch2</c>, is hidden from the users of the client functions.
       This means the name can be changed without affecting them.</item>
      <item>The protcol (messages sent to and received from the server)
       is hidden as well. This is good programming practice and allows
       us to change the protocol without making changes to code using
       the interface functions.</item>
      <item>We can extend the functionality of <c>server</c>, without
       having to change <c>ch2</c> or any other callback module.</item>
    </list>
    <p>(In <c>ch1.erl</c> and <c>ch2.erl</c> above, the implementation
      of <c>channels/0</c>, <c>alloc/1</c> and <c>free/2</c> has been
      intentionally left out, as it is not relevant to the example.
      For completeness, one way to write these functions are given
      below. Note that this is an example only, a realistic
      implementation must be able to handle situations like running out
      of channels to allocate etc.)</p>
    <code type="none">
channels() ->
   {_Allocated = [], _Free = lists:seq(1,100)}.

alloc({Allocated, [H|T] = _Free}) ->
   {H, {[H|Allocated], T}}.

free(Ch, {Alloc, Free} = Channels) ->
   case lists:member(Ch, Alloc) of
      true ->
         {lists:delete(Ch, Alloc), [Ch|Free]};
      false ->
         Channels
   end.        </code>
    <p>Code written without making use of behaviours may be more
      efficient, but the increased efficiency will be at the expense of
      generality. The ability to manage all applications in the system
      in a consistent manner is very important.</p>
    <p>Using behaviours also makes it easier to read and understand
      code written by other programmers. Ad hoc programming structures,
      while possibly more efficient, are always more difficult to
      understand.</p>
    <p>The module <c>server</c> corresponds, greatly simplified,
      to the Erlang/OTP behaviour <c>gen_server</c>.</p>
    <p>The standard Erlang/OTP behaviours are:</p>
    <taglist>
      <tag><seealso marker="gen_server_concepts">gen_server</seealso></tag>
      <item>For implementing the server of a client-server relation.</item>
      <tag><seealso marker="fsm">gen_fsm</seealso></tag>
      <item>For implementing finite state machines.</item>
      <tag><seealso marker="events">gen_event</seealso></tag>
      <item>For implementing event handling functionality.</item>
      <tag><seealso marker="sup_princ">supervisor</seealso></tag>
      <item>For implementing a supervisor in a supervision tree.</item>
    </taglist>
    <p>The compiler understands the module attribute
      <c>-behaviour(Behaviour)</c> and issues warnings about
      missing callback functions. Example:</p>
    <code type="none">
-module(chs3).
-behaviour(gen_server).
...

3> c(chs3).
./chs3.erl:10: Warning: undefined call-back function handle_call/3
{ok,chs3}</code>
  </section>

  <section>
    <title>Applications</title>
    <p>Erlang/OTP comes with a number of components, each implementing
      some specific functionality. Components are with Erlang/OTP
      terminology called <em>applications</em>. Examples of Erlang/OTP
      applications are Mnesia, which has everything needed for
      programming database services, and Debugger which is used to
      debug Erlang programs. The minimal system based on Erlang/OTP
      consists of the applications Kernel and STDLIB.</p>
    <p>The application concept applies both to program structure
      (processes) and directory structure (modules).</p>
    <p>The simplest kind of application does not have any processes,
      but consists of a collection of functional modules. Such an
      application is called a <em>library application</em>. An example
      of a library application is STDLIB.</p>
    <p>An application with processes is easiest implemented as a
      supervision tree using the standard behaviours.</p>
    <p>How to program applications is described in
      <seealso marker="applications">Applications</seealso>.</p>
  </section>

  <section>
    <title>Releases</title>
    <p>A <em>release</em> is a complete system made out from a subset of
      the Erlang/OTP applications and a set of user-specific
      applications.</p>
    <p>How to program releases is described in
      <seealso marker="release_structure">Releases</seealso>.</p>
    <p>How to install a release in a target environment is described
      in the chapter about Target Systems in System Principles.</p>
  </section>

  <section>
    <title>Release Handling</title>
    <p><em>Release handling</em> is upgrading and downgrading between
      different versions of a release, in a (possibly) running system.
      How to do this is described in
      <seealso marker="release_handling">Release Handling</seealso>.</p>
  </section>
</chapter>

