path: root/docs/manual/developer/modules.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>

<manualpage metafile="modules.xml.meta">
<parentdocument href="./">Developer Documentation</parentdocument>

<title>Converting Modules from Apache 1.3 to Apache 2.0</title>

<summary>
    <p>This is a first attempt at writing the lessons I learned
    when trying to convert the <code>mod_mmap_static</code> module to Apache
    2.0. It's by no means definitive and probably won't even be
    correct in some ways, but it's a start.</p>
</summary>

<section id="easy"><title>The easier changes ...</title>

    <section id="cleanup"><title>Cleanup Routines</title>
      <p>These now need to be of type <code>apr_status_t</code> and return a
      value of that type. Normally the return value will be
      <code>APR_SUCCESS</code> unless there is some need to signal an error in
      the cleanup. Be aware that even though you signal an error not all code
      yet checks and acts upon the error.</p>
    </section>

    <section id="init"><title>Initialisation Routines</title>
      <p>These should now be renamed to better signify where they sit
      in the overall process. So the name gets a small change from
      <code>mmap_init</code> to <code>mmap_post_config</code>. The arguments
      passed have undergone a radical change and now look like</p>

      <ul>
        <li><code>apr_pool_t *p</code></li>
        <li><code>apr_pool_t *plog</code></li>
        <li><code>apr_pool_t *ptemp</code></li>
        <li><code>server_rec *s</code></li>
      </ul>
    </section>

    <section id="datatypes"><title>Data Types</title>
      <p>A lot of the data types have been moved into the <a
      href="http://apr.apache.org/">APR</a>. This means that some have had
      a name change, such as the one shown above. The following is a brief
      list of some of the changes that you are likely to have to make.</p>

      <ul>
        <li><code>pool</code> becomes <code>apr_pool_t</code></li>
        <li><code>table</code> becomes <code>apr_table_t</code></li>
      </ul>
    </section>
</section>

<section id="messy"><title>The messier changes...</title>

    <section id="register-hooks"><title>Register Hooks</title>
      <p>The new architecture uses a series of hooks to provide for
      calling your functions. These you'll need to add to your module
      by way of a new function, <code>static void register_hooks(void)</code>.
      The function is really reasonably straightforward once you
      understand what needs to be done. Each function that needs
      calling at some stage in the processing of a request needs to
      be registered, handlers do not. There are a number of phases
      where functions can be added, and for each you can specify with
      a high degree of control the relative order that the function
      will be called in.</p>

      <p>This is the code that was added to <code>mod_mmap_static</code>:</p>
      <example><pre>
static void register_hooks(void)
{
    static const char * const aszPre[]={ "http_core.c",NULL };
    ap_hook_post_config(mmap_post_config,NULL,NULL,HOOK_MIDDLE);
    ap_hook_translate_name(mmap_static_xlat,aszPre,NULL,HOOK_LAST);
};</pre>
      </example>

      <p>This registers 2 functions that need to be called, one in
      the <code>post_config</code> stage (virtually every module will need this
      one) and one for the <code>translate_name</code> phase. note that while
      there are different function names the format of each is
      identical. So what is the format?</p>

      <example>
        ap_hook_<var>phase_name</var>(<var>function_name</var>,
        <var>predecessors</var>, <var>successors</var>, <var>position</var>);
      </example>

      <p>There are 3 hook positions defined...</p>

      <ul>
        <li><code>HOOK_FIRST</code></li>
        <li><code>HOOK_MIDDLE</code></li>
        <li><code>HOOK_LAST</code></li>
      </ul>

      <p>To define the position you use the position and then modify
      it with the predecessors and successors. Each of the modifiers
      can be a list of functions that should be called, either before
      the function is run (predecessors) or after the function has
      run (successors).</p>

      <p>In the <code>mod_mmap_static</code> case I didn't care about the
      <code>post_config</code> stage, but the <code>mmap_static_xlat</code>
      <strong>must</strong> be called after the core module had done it's name
      translation, hence the use of the aszPre to define a modifier to the
      position <code>HOOK_LAST</code>.</p>
    </section>

    <section id="moddef"><title>Module Definition</title>
      <p>There are now a lot fewer stages to worry about when
      creating your module definition. The old defintion looked
      like</p>

      <example><pre>
module MODULE_VAR_EXPORT <var>module_name</var>_module =
{
    STANDARD_MODULE_STUFF,
    /* initializer */
    /* dir config creater */
    /* dir merger --- default is to override */
    /* server config */
    /* merge server config */
    /* command handlers */
    /* handlers */
    /* filename translation */
    /* check_user_id */
    /* check auth */
    /* check access */
    /* type_checker */
    /* fixups */
    /* logger */
    /* header parser */
    /* child_init */
    /* child_exit */
    /* post read-request */
};</pre>
      </example>

      <p>The new structure is a great deal simpler...</p>
      <example><pre>
module MODULE_VAR_EXPORT <var>module_name</var>_module =
{
    STANDARD20_MODULE_STUFF,
    /* create per-directory config structures */
    /* merge per-directory config structures  */
    /* create per-server config structures    */
    /* merge per-server config structures     */
    /* command handlers */
    /* handlers */
    /* register hooks */
};</pre>
      </example>

      <p>Some of these read directly across, some don't. I'll try to
      summarise what should be done below.</p>

      <p>The stages that read directly across :</p>

      <dl>
        <dt><code>/* dir config creater */</code></dt>
        <dd><code>/* create per-directory config structures */</code></dd>

        <dt><code>/* server config */</code></dt>
        <dd><code>/* create per-server config structures */</code></dd>

        <dt><code>/* dir merger */</code></dt>
        <dd><code>/* merge per-directory config structures */</code></dd>

        <dt><code>/* merge server config */</code></dt>
        <dd><code>/* merge per-server config structures */</code></dd>

        <dt><code>/* command table */</code></dt>
        <dd><code>/* command apr_table_t */</code></dd>

        <dt><code>/* handlers */</code></dt>
        <dd><code>/* handlers */</code></dd>
      </dl>

      <p>The remainder of the old functions should be registered as
      hooks. There are the following hook stages defined so
      far...</p>

      <dl>
        <dt><code>ap_hook_post_config</code></dt>
        <dd>this is where the old <code>_init</code> routines get
        registered</dd>

        <dt><code>ap_hook_http_method</code></dt>
        <dd>retrieve the http method from a request. (legacy)</dd>

        <dt><code>ap_hook_open_logs</code></dt>
        <dd>open any specified logs</dd>

        <dt><code>ap_hook_auth_checker</code></dt>
        <dd>check if the resource requires authorization</dd>

        <dt><code>ap_hook_access_checker</code></dt>
        <dd>check for module-specific restrictions</dd>

        <dt><code>ap_hook_check_user_id</code></dt>
        <dd>check the user-id and password</dd>

        <dt><code>ap_hook_default_port</code></dt>
        <dd>retrieve the default port for the server</dd>

        <dt><code>ap_hook_pre_connection</code></dt>
        <dd>do any setup required just before processing, but after
        accepting</dd>

        <dt><code>ap_hook_process_connection</code></dt>
        <dd>run the correct protocol</dd>

        <dt><code>ap_hook_child_init</code></dt>
        <dd>call as soon as the child is started</dd>

        <dt><code>ap_hook_create_request</code></dt>
        <dd>??</dd>

        <dt><code>ap_hook_fixups</code></dt>
        <dd>last chance to modify things before generating content</dd>

        <dt><code>ap_hook_handler</code></dt>
        <dd>generate the content</dd>

        <dt><code>ap_hook_header_parser</code></dt>
        <dd>lets modules look at the headers, not used by most modules, because
        they use <code>post_read_request</code> for this</dd>

        <dt><code>ap_hook_insert_filter</code></dt>
        <dd>to insert filters into the filter chain</dd>

        <dt><code>ap_hook_log_transaction</code></dt>
        <dd>log information about the request</dd>

        <dt><code>ap_hook_optional_fn_retrieve</code></dt>
        <dd>retrieve any functions registered as optional</dd>

        <dt><code>ap_hook_post_read_request</code></dt>
        <dd>called after reading the request, before any other phase</dd>

        <dt><code>ap_hook_quick_handler</code></dt>
        <dd>called before any request processing, used by cache modules.</dd>

        <dt><code>ap_hook_translate_name</code></dt>
        <dd>translate the URI into a filename</dd>

        <dt><code>ap_hook_type_checker</code></dt>
        <dd>determine and/or set the doc type</dd>
      </dl>
    </section>
</section>
</manualpage>