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2.7 Request Processing in Apache

Most, though by no means all, modules are concerned with some aspect of processing an HTTP request. But there is rarely, if ever, a reason for a module to concern itself with every aspect of HTTP—that is the business of the httpd. The advantage of a modular approach is that a module can easily focus on a particular task but ignore aspects of HTTP that are not relevant to it.

2.7.1 Content Generation

The simplest possible formulation of a webserver is a program that listens for HTTP requests and returns a response when it receives one (Figure 2-2). In Apache, this job is fundamentally the business of a content generator, the core of the webserver.

Figure 2-2

Figure 2-2 Minimal webserver

Exactly one content generator must be run for every HTTP request. Any module may register content generators, normally by defining a function referenced by a handler that can be configured using the SetHandler or AddHandler directives in httpd.conf. The default generator, which is used when no specific generator is defined by any module, simply returns a file, mapped directly from the request to the filesystem. Modules that implement content generators are sometimes known as "content generator" or "handler" modules.

2.7.2 Request Processing Phases

In principle, a content generator can handle all the functions of a webserver. For example, a CGI program gets the request and produces the response, and it can take full control of what happens between them. Like other webservers, Apache splits the request into different phases. For example, it checks whether the user is authorized to do something before the content generator does that thing.

Several request phases precede the content generator (Figure 2-3). These serve to examine and perhaps manipulate the request headers, and to determine what to do with the request. For example:

  • The request URL will be matched against the configuration, to determine which content generator should be used.
  • The request URL will normally be mapped to the filesystem. The mapping may be to a static file, a CGI script, or whatever else the content generator may use.
  • If content negotiation is enabled, mod_negotiation will find the version of the resource that best matches the browser's preference. For example, the Apache manual pages are served in the language requested by the browser.
  • Access and authentication modules will enforce the server's access rules, and determine whether the user is permitted what has been requested.
  • mod_alias or mod_rewrite may change the effective URL in the request.
Figure 2-3

Figure 2-3 Request processing in Apache

There is also a request logging phase, which comes after the content generator has sent a reply to the browser.

2.7.2.1 Nonstandard Request Processing

Request processing may sometimes be diverted from the standard processing axis described here, for a variety of reasons:

  • A module may divert processing into a new request or error document at any point before the response has been sent (Chapter 6).
  • A module may define additional phases and enable other modules to hook their own processing in (Chapter 10).
  • There is a quick_handler hook that bypasses normal processing, used by mod_cache (not discussed in this book).

2.7.3 Processing Hooks

The mechanism by which a module can influence or take charge of some aspect of processing in Apache is through a sequence of hooks. The usual hooks for processing a request in Apache 2.0 are described next.

post_read_requestThis is the first hook available to modules in normal request processing. It is available to modules that need to hook very early into processing a request.

translate_nameApache maps the request URL to the filesystem. A module can insert a hook here to substitute its own logic—for example, mod_alias.

map_to_storageSince the URL has been mapped to the filesystem, we are now in a position to apply per-directory configuration (<Directory> and <Files> sections and their variants, including any relevant .htaccess files if enabled). This hook enables Apache to determine the configuration options that apply to this request. It applies normal configuration directives for all active modules, so few modules should ever need to apply hooks here. The only standard module to do so is mod_proxy.

header_parserThis hook inspects the request headers. It is rarely used, as modules can perform that task at any point in the request processing, and they usually do so within the context of another hook. mod_setenvif is a standard module that uses a header_parser to set internal environment variables according to the request headers.

access_checkerApache checks whether access to the requested resource is permitted according to the server configuration (httpd.conf). A module can add to or replace Apache's standard logic, which implements the Allow/Deny From directives in mod_access (httpd 1.x and 2.0) or mod_authz_host (httpd 2.2).

check_user_idIf any authentication method is in use, Apache will apply the relevant authentication and set the username field r->user. A module may implement an authentication method with this hook.

auth_checkerThis hook checks whether the requested operation is permitted to the authenticated user.

type_checkerThis hook applies rules related to the MIME type (where applicable) of the requested resource, and determines the content handler to use (if not already set). Standard modules implementing this hook include mod_negotiation (selection of a resource based on HTTP content negotiation) and mod_mime (setting the MIME type and handler information according to standard configuration directives and conventions such as filename "extensions").

fixupsThis general-purpose hook enables modules to run any necessary processing after the preceding hooks but before the content generator. Like post_read_request, it is something of a catch-all, and is one of the most commonly used hooks.

handlerThis is the content generator hook. It is responsible for sending an appropriate response to the client. If there are input data, the handler is also responsible for reading them. Unlike the other hooks, where zero or many functions may be involved in processing a request, every request is processed by exactly one handler.

log_transactionThis hook logs the transaction after the response has been returned to the client. A module may modify or replace Apache's standard logging.

A module may hook its own handlers into any of these processing phases. The module provides a callback function and hooks it in, and Apache calls the function during the appropriate processing phase. Modules that concern themselves with the phases before content generation are sometimes known as metadata modules; they are described in detail in Chapter 6. Modules that deal with logging are known as logging modules. In addition to using the standard hooks, modules may define further processing hooks, as described in Chapter 10.

2.7.4 The Data Axis and Filters

What we have described so far is essentially similar to the architecture of every general-purpose webserver. There are, of course, differences in the details, but the request processing (metadata → generator → logger) phases are common.

The major innovation in Apache 2, which transforms it from a "mere" webserver (like Apache 1.3 and others) into a powerful applications platform, is the filter chain. The filter chain can be represented as a data axis, orthogonal to the request-processing axis (Figure 2-4). The request data may be processed by input filters before reaching the content generator, and the response may be processed by output filters before being sent to the client. Filters enable a far cleaner and more efficient implementation of data processing than was possible in the past, as well as separating content generation from its transformation and aggregation.

Figure 2-4

Figure 2-4 Apache 2 introduces a new data axis enabling a new range of powerful applications

2.7.4.1 Handler or Filter?

Many applications can be implemented as either a handler or a filter. Sometimes it may be clear that one of these solutions is appropriate and the other would be nonsensical, but between these extremes lies a gray area. How does one decide whether to write a handler or a filter?

When making this decision, there are several questions to consider:

  • Feasibility: Can it be made to work in both cases? If not, there's an instant decision.
  • Utility: Is the functionality it provides more useful in one case than the other? Filters are often far more useful than handlers, because they can be reused with different content generators and chained both with generators and other filters. But every request has to be processed by some handler, even if it does nothing!
  • Complexity: Is one version substantially more complex than the other? Will it take more time and effort to develop, and/or run more slowly? Filter modules are usually more complex than the equivalent handler, because a handler is in full control of its data and can read or write at will, whereas a filter has to implement a callback that may be called several times with partial data, which it must treat as unstructured chunks. We will discuss this issue in detail in Chapter 8.

For example, Apache 1.3 users can do an XSLT transformation by building it into handlers, such as CGI or PHP. Alternatively, they can use an XSLT module, but this is very slow and cumbersome (this author tried an XSLT module for Apache 1.3, but found it many hundreds of times slower than running XSLT in a CGI script operating on temporary files). Running XSLT in a handler works, but loses modularity and reusability. Any nontrivial application that needs it has to reinvent that wheel, using whatever libraries are available for the programming or scripting language used and often resorting to ugly hacks such as temporary files.

Apache 2, by contrast, allows us to run XSLT in a filter. Content handlers requiring XSLT can simply output the XML as is, and leave the transformation to Apache. The first XSLT module for Apache 2, written by Phillip Dunkel and released while Apache 2.0 was still in beta testing, was initially incomplete, but already worked far better than XSLT in Apache 1.3. It is now further improved, and is one of a choice of XSLT modules. This book's author developed another XSLT module.

More generally, if a module has both data inputs and outputs, and if it may be used in more than one application, then it is a strong candidate for implementation as a filter.

2.7.4.2 Content Generator Examples

  • The default handler sends a file from the local disk under the DocumentRoot. Although a filter could do that, there's nothing to be gained.
  • CGI, the generic API for server-side programming, is a handler. Because CGI scripts expect the central position in the webserver architecture, it has to be a handler. However, a somewhat similar framework for external filters is also provided by mod_ext_filter.
  • The Apache proxy is a handler that fetches contents from a back-end server.
  • Any form-processing application will normally be implemented as a handler—particularly those that accept POST data, or other operations that can alter the state of the server itself. Likewise, applications that generate a report from any back end are usually implemented as handlers. However, when the handler is based on HTML or XML pages with embedded programming elements, it can usefully be implemented as a filter.

2.7.4.3 Filter Examples

  • mod_include implements server-side includes, a simple scripting language embedded in pages. It is implemented as a filter, so it can post-process content from any content generator, as discussed earlier with reference to XSLT.
  • mod_ssl implements secure transport as a connection-level filter, thereby enabling all normal processing in the server to work with unencrypted data. This represents a major advance over Apache 1.x, where secure transport was complex and required a lot of work to combine it with other applications.
  • Markup parsing modules are used to post-process and transform XML or HTML in more sophisticated ways, from simple link rewriting9 through XSLT and Xinclude processing,10 to a complete API for markup filtering,11 to a security filter that blocks attempts to attack vulnerable applications such as PHP scripts.12 Examples will be introduced in Chapter 8.
  • Image processing can take place in a filter. This author developed a custom proxy for a developer of mobile phone browsers. Because the browser tells the proxy its capabilities, images can be reduced to fit within the screen space and, where appropriate, translated to gray scale, thereby reducing the volume of data sent and accelerating browsing over slow connections.
  • Form-processing modules need to decode data sent from a web browser. Input filter modules, such as mod_form and mod_upload,13 spare applications from reinventing that wheel.
  • Data compression and decompression are implemented in mod_deflate. The filter architecture allows this module to be much simpler than mod_gzip (an Apache 1.3 compression module) and to dispense with any use of temporary files.

2.7.5 Order of Processing

Before moving on to discuss how a module hooks itself into any of the stages of processing a request/data, we should pause to clear up a matter that often causes confusion—namely, the order of processing.

The request processing axis is straightforward, with phases happening strictly in order. But confusion arises in the data axis. For maximum efficiency, this axis is pipelined, so the content generator and filters do not run in a deterministic order. For example, you cannot in general set something in an input filter and expect it to apply in the generator or output filters.

The order of processing centers on the content generator, which is responsible for pulling data from the input filter stack and pushing data onto the output filters (where applicable, in both cases). When a generator or filter needs to set something affecting the request as a whole, it must do so before passing any data down the chain (generator and output filters) or before returning data to the caller (input filters).

2.7.6 Processing Hooks

Now that we have an overview of request processing in Apache, we can show how a module hooks into it to play a part.

The Apache module structure declares several (optional) data and function members:

module AP_MODULE_DECLARE_DATA my_module = {
    STANDARD20_MODULE_STUFF,  /* macro to ensure version consistency */
    my_dir_conf,              /* create per-directory configuration record */
    my_dir_merge,             /* merge per-directory configuration records */
    my_server_conf,           /* create per-server configuration record */
    my_server_merge,          /* merge per-server configuration records */
    my_cmds,                  /* configuration directives */
    my_hooks                  /* register modules functions with the core */
};

The configuration directives are presented as an array; the remaining module entries are functions. The relevant function for the module to create request processing hooks is the final member:

static void my_hooks(apr_pool_t *pool) {
   /* create request processing hooks as required */
}

Which hooks we need to create here depend on which part or parts of the request our module is interested in. For example, a module that implements a content generator (handler) will need a handler hook, looking something like this:

ap_hook_handler(my_handler, NULL, NULL, APR_HOOK_MIDDLE) ;

Now my_handler will be called when a request reaches the content generation phase. Hooks for other request phases are similar.

The following prototype applies to a handler for any of these phases:

static int my_handler(request_rec *r) {
    /* do something with the request */
}

Details and implementation of this prototype are discussed in Chapters 5 and 6.

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