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<chapter>
  <header>
    <copyright>
      <year>1998</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      Licensed under the Apache License, Version 2.0 (the "License");
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    <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>


