Apache HTTP Server Version 2.4

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# Request Processing in the Apache HTTP Server 2.x

### Warning

Warning - this is a first (fast) draft that needs further revision!

Several changes in 2.0 and above affect the internal request processing mechanics. Module authors need to be aware of these changes so they may take advantage of the optimizations and security enhancements.

The first major change is to the subrequest and redirect mechanisms. There were a number of different code paths in the Apache HTTP Server 1.3 to attempt to optimize subrequest or redirect behavior. As patches were introduced to 2.0, these optimizations (and the server behavior) were quickly broken due to this duplication of code. All duplicate code has been folded back into `ap_process_request_internal()` to prevent the code from falling out of sync again.

This means that much of the existing code was 'unoptimized'. It is the Apache HTTP Project's first goal to create a robust and correct implementation of the HTTP server RFC. Additional goals include security, scalability and optimization. New methods were sought to optimize the server (beyond the performance of 1.3) without introducing fragile or insecure code.

## The Request Processing Cycle

All requests pass through `ap_process_request_internal()` in `server/request.c`, including subrequests and redirects. If a module doesn't pass generated requests through this code, the author is cautioned that the module may be broken by future changes to request processing.

To streamline requests, the module author can take advantage of the hooks offered to drop out of the request cycle early, or to bypass core hooks which are irrelevant (and costly in terms of CPU.)

## The Request Parsing Phase

### Unescapes the URL

The request's `parsed_uri` path is unescaped, once and only once, at the beginning of internal request processing.

This step is bypassed if the proxyreq flag is set, or the `parsed_uri.path` element is unset. The module has no further control of this one-time unescape operation, either failing to unescape or multiply unescaping the URL leads to security repercussions.

### Strips Parent and This Elements from the URI

All `/../` and `/./` elements are removed by `ap_getparents()`. This helps to ensure the path is (nearly) absolute before the request processing continues.

This step cannot be bypassed.

### Initial URI Location Walk

Every request is subject to an `ap_location_walk()` call. This ensures that `<Location>` sections are consistently enforced for all requests. If the request is an internal redirect or a sub-request, it may borrow some or all of the processing from the previous or parent request's ap_location_walk, so this step is generally very efficient after processing the main request.

### translate_name

Modules can determine the file name, or alter the given URI in this step. For example, `mod_vhost_alias` will translate the URI's path into the configured virtual host, `mod_alias` will translate the path to an alias path, and if the request falls back on the core, the `DocumentRoot` is prepended to the request resource.

If all modules `DECLINE` this phase, an error 500 is returned to the browser, and a "couldn't translate name" error is logged automatically.

### Hook: map_to_storage

After the file or correct URI was determined, the appropriate per-dir configurations are merged together. For example, `mod_proxy` compares and merges the appropriate `<Proxy>` sections. If the URI is nothing more than a local (non-proxy) `TRACE` request, the core handles the request and returns `DONE`. If no module answers this hook with `OK` or `DONE`, the core will run the request filename against the `<Directory>` and `<Files>` sections. If the request 'filename' isn't an absolute, legal filename, a note is set for later termination.

### URI Location Walk

Every request is hardened by a second `ap_location_walk()` call. This reassures that a translated request is still subjected to the configured `<Location>` sections. The request again borrows some or all of the processing from its previous `location_walk` above, so this step is almost always very efficient unless the translated URI mapped to a substantially different path or Virtual Host.

### Hook: header_parser

The main request then parses the client's headers. This prepares the remaining request processing steps to better serve the client's request.

## The Security Phase

Needs Documentation. Code is:

    if ((access_status = ap_run_access_checker(r)) != 0) {
        return decl_die(access_status, "check access", r);
    }

    if ((access_status = ap_run_check_user_id(r)) != 0) {
        return decl_die(access_status, "check user", r);
    }

    if ((access_status = ap_run_auth_checker(r)) != 0) {
        return decl_die(access_status, "check authorization", r);
    }

## The Preparation Phase

### Hook: type_checker

The modules have an opportunity to test the URI or filename against the target resource, and set mime information for the request. Both `mod_mime` and `mod_mime_magic` use this phase to compare the file name or contents against the administrator's configuration and set the content type, language, character set and request handler. Some modules may set up their filters or other request handling parameters at this time.

If all modules `DECLINE` this phase, an error 500 is returned to the browser, and a "couldn't find types" error is logged automatically.

### Hook: fixups

Many modules are 'trounced' by some phase above. The fixups phase is used by modules to 'reassert' their ownership or force the request's fields to their appropriate values. It isn't always the cleanest mechanism, but occasionally it's the only option.

## The Handler Phase

This phase is not part of the processing in `ap_process_request_internal()`. Many modules prepare one or more subrequests prior to creating any content at all. After the core, or a module calls `ap_process_request_internal()` it then calls `ap_invoke_handler()` to generate the request.

### Hook: insert_filter

Modules that transform the content in some way can insert their values and override existing filters, such that if the user configured a more advanced filter out-of-order, then the module can move its order as need be. There is no result code, so actions in this hook better be trusted to always succeed.

### Hook: handler

The module finally has a chance to serve the request in its handler hook. Note that not every prepared request is sent to the handler hook. Many modules, such as `mod_autoindex`, will create subrequests for a given URI, and then never serve the subrequest, but simply lists it for the user. Remember not to put required teardown from the hooks above into this module, but register pool cleanups against the request pool to free resources as required.