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diff --git a/system/doc/design_principles/des_princ.xml b/system/doc/design_principles/des_princ.xml index e8f289b905..77c61eafb0 100644 --- a/system/doc/design_principles/des_princ.xml +++ b/system/doc/design_principles/des_princ.xml @@ -28,50 +28,52 @@ <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> + <marker id="otp design principles"></marker> + <p>The <em>OTP design principles</em> define 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> + <em>workers</em> and <em>supervisors</em>:</p> <list type="bulleted"> - <item>Workers are processes which perform computations, that is, + <item>Workers are processes that perform computations, that is, they do the actual work.</item> - <item>Supervisors are processes which monitor the behaviour of + <item>Supervisors are processes that 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 + code into supervisors and workers, which makes it possible to design and program fault-tolerant software.</item> </list> + <p>In the following figure, square boxes represents supervisors and + circles represent workers:</p> <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> + the supervisors are similar in structure. The only difference + between them is which child processes they supervise. 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> + (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 + the callback module which is to 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 + <p>The following example 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>, @@ -149,7 +151,7 @@ loop(Mod, State) -> State2 = Mod:handle_cast(Req, State), loop(Mod, State2) end.</code> - <p>and a callback module <c>ch2.erl</c>:</p> + <p>And a callback module <c>ch2.erl</c>:</p> <code type="none"> -module(ch2). -export([start/0]). @@ -173,27 +175,27 @@ handle_call(alloc, Chs) -> handle_cast({free, Ch}, Chs) -> free(Ch, Chs). % => Chs2</code> - <p>Note the following:</p> + <p>Notice the following:</p> <list type="bulleted"> - <item>The code in <c>server</c> can be re-used to build many + <item>The code in <c>server</c> can be reused 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 server name, in this example the atom + <c>ch2</c>, is hidden from the users of the client functions. This + means that 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 + is also hidden. This is good programming practice and allows + one to change the protocol without changing the code using the interface functions.</item> - <item>We can extend the functionality of <c>server</c>, without + <item>The functionality of <c>server</c> can be extended 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 + <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 + below. This is an example only, a realistic implementation must be able to handle situations like running out - of channels to allocate etc.)</p> + of channels to allocate, and so on.</p> <code type="none"> channels() -> {_Allocated = [], _Free = lists:seq(1,100)}. @@ -208,30 +210,30 @@ free(Ch, {Alloc, Free} = Channels) -> 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 + <p>Code written without using behaviours can be more + efficient, but the increased efficiency is at the expense of generality. The ability to manage all applications in the system - in a consistent manner is very important.</p> + in a consistent manner is important.</p> <p>Using behaviours also makes it easier to read and understand - code written by other programmers. Ad hoc programming structures, + code written by other programmers. Improvised programming structures, while possibly more efficient, are always more difficult to understand.</p> - <p>The module <c>server</c> corresponds, greatly simplified, + <p>The <c>server</c> module 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> + <list type="bulleted"> + <item><p><seealso marker="gen_server_concepts">gen_server</seealso></p> + <p>For implementing the server of a client-server relation</p></item> + <item><p><seealso marker="fsm">gen_fsm</seealso></p> + <p>For implementing finite-state machines</p></item> + <item><p><seealso marker="events">gen_event</seealso></p> + <p>For implementing event handling functionality</p></item> + <item><p><seealso marker="sup_princ">supervisor</seealso></p> + <p>For implementing a supervisor in a supervision tree</p></item> + </list> <p>The compiler understands the module attribute <c>-behaviour(Behaviour)</c> and issues warnings about - missing callback functions. Example:</p> + missing callback functions, for example:</p> <code type="none"> -module(chs3). -behaviour(gen_server). @@ -248,13 +250,17 @@ free(Ch, {Alloc, Free} = Channels) -> 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> + programming database services, and Debugger, which is used + to debug Erlang programs. The minimal system based on Erlang/OTP + consists of the following two applications:</p> + <list type="bulleted"> + <item>Kernel - Functionality necessary to run Erlang</item> + <item>STDLIB - Erlang standard libraries</item> + </list> <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 + <p>The simplest applications do not have any processes, + but consist 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 @@ -266,12 +272,11 @@ free(Ch, {Alloc, Free} = Channels) -> <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> + 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> + in the section about target systems in Section 2 System Principles.</p> </section> <section> |