<?xml version="1.0" encoding="latin1" ?> <!DOCTYPE chapter SYSTEM "chapter.dtd"> <chapter> <header> <copyright> <year>1998</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>Using the Erlang Generic Server Back-end</title> <prepared></prepared> <docno></docno> <date>98-08-06</date> <rev>B</rev> <file>ch_erl_genserver.xml</file> </header> <section> <title>Introduction</title> <p>The mapping of OMG IDL to the Erlang programming language when Erlang generic server is the back-end of choice is similar to the one used in the chapter 'OMG IDL Mapping'. The only difference is in the generated code, a client stub and server skeleton to an Erlang <c>gen_server</c>. Orber's User's Guide contain a more detailed description of IDL to Erlang mapping.</p> </section> <section> <title>Compiling the Code</title> <p>The <c>ic:gen/2</c> function can be called from the command line as follows:</p> <p></p> <code type="none"> shell> erlc "+{be, erl_genserv}" MyFile.idl </code> </section> <section> <title>Writing the Implementation File</title> <p>For each IDL interface <c><![CDATA[<interface name>]]></c> defined in the IDL file :</p> <list type="bulleted"> <item>Create the corresponding Erlang file that will hold the Erlang implementation of the IDL definitions. </item> <item>Call the implementation file after the scope of the IDL interface, followed by the suffix <c>_impl</c>.</item> <item>Export the implementation functions.</item> </list> <p>For each function defined in the IDL interface :</p> <list type="bulleted"> <item>Implement an Erlang function that uses as arguments in the same order, as the input arguments described in the IDL file, and returns the value described in the interface.</item> <item>When using the function, follow the mapping described in chapter 2.</item> </list> </section> <section> <title>An Example</title> <p>In this example, a file <c>random.idl</c> generates code for the Erlang gen_server back-end:</p> <code type="none"> // Filename random.idl module rmod { interface random { // Generate a new random number double produce(); // Initialize random generator oneway void init(in long seed1, in long seed2, in long seed3); }; }; </code> <p>When the file "random.idl" is compiled (e.g., <c>shell> erlc "+{be, erl_genserv}" random.idl</c>) five files are produced; two for the top scope, two for the interface scope, and one for the module scope. The header files for top scope and interface are empty and not shown here. In this case, the stub/skeleton file <c>rmod_random.erl</c> is the most important. This module exports two kinds of operations:</p> <list type="bulleted"> <item><em>Administrative</em> - used when, for example, creating and terminating the server.</item> <item><em>IDL dependent</em> - operations defined in the IDL specification. In this case, <c>produce</c> and <c>init</c>.</item> </list> <section> <title>Administrative Operations</title> <p>To create a new server instance, one of the following functions should be used:</p> <list type="bulleted"> <item><em>oe_create/0/1/2</em> - create a new instance of the object. Accepts <c>Env</c> and <c>RegName</c>, in that order, as parameters. The former is passed uninterpreted to the initialization operation of the call-back module, while the latter must be as the <c>gen_server</c> parameter <c>ServerName</c>. If <c>Env</c> is left out, an empty list will be passed.</item> <item><em>oe_create_link/0/1/2</em> - similar to <c>oe_create/0/1/2</c>, but create a linked server.</item> <item><em>typeID/0</em> - returns the scooped id compliant with the OMG standard. In this case the string <c>"IDL:rmod/random:1.0"</c>.</item> <item><em>stop/1</em> - asynchronously terminate the server. The required argument is the return value from any of the start functions.</item> </list> </section> <section> <title>IDL Dependent Operations</title> <p>Operations can either be synchronous or asynchronous (i.e., <c>oneway</c>). These are, respectively, mapped to <c>gen_server:call/2/3</c> and <c>gen_server:cast/2</c>. Consult the <c>gen_server</c> documentation for valid return values.</p> <p>The IDL dependent operations in this example are listed below. The first argument must be the whatever the create operation returned.</p> <list type="bulleted"> <item><em>init(ServerReference, Seed1, Seed2, Seed3)</em> - initialize the random number generator.</item> <item><em>produce(ServerReference)</em> - generate a new random number.</item> </list> </section> <p>If the compile option <c>timeout</c> is used a timeout must be added (e.g., <c>produce(ServerReference, 5000)</c>). For more information, see the <c>gen_server</c> documentation.</p> <section> <title>Implementation Module</title> <p>The implementation module shall, unless the compile option <c>impl</c> is used, be named <c>rmod_random_impl.erl</c>. and could look like this:</p> <code type="none"> -module('rmod_random_impl'). %% Mandatory gen_server operations -export([init/1, terminate/2, code_change/3]). %% Add if 'handle_info' compile option used -export([handle_info/2]). %% API defined in IDL specification -export([produce/1,init/4]). %% Mandatory operations init(Env) -> {ok, []}. terminate(From, Reason) -> ok. code_change(OldVsn, State, Extra) -> {ok, State}. %% Optional handle_info(Info, State) -> {noreply, NewState}. %% IDL specification produce(State) -> case catch random:uniform() of {'EXIT',_} -> {stop, normal, "random:uniform/0 - EXIT", State}; RUnif -> {reply, RUnif, State} end. init(State, S1, S2, S3) -> case catch random:seed(S1, S2, S3) of {'EXIT',_} -> {stop, normal, State}; _ -> {noreply, State} end. </code> <p>Compile the code and run the example:</p> <code type="none"><![CDATA[ 1> make:all(). Recompile: rmod_random Recompile: oe_random Recompile: rmod_random_impl up_to_date 2> {ok,R} = rmod_random:oe_create(). {ok,<0.30.0>} 3> rmod_random:init(R, 1, 2, 3). ok 4> rmod_random:produce(R). 1.97963e-4 5> ]]></code> </section> </section> </chapter>