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Apache 1.3 API notes

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This document has not been updated to take into account changes made in the 2.0 version of the Apache HTTP Server. Some of the information may still be relevant, but please use it with care.

These are some notes on the Apache API and the data structures you have to deal with, etc. They are not yet nearly complete, but hopefully, they will help you get your bearings. Keep in mind that the API is still subject to change as we gain experience with it. (See the TODO file for what might be coming). However, it will be easy to adapt modules to any changes that are made. (We have more modules to adapt than you do).

A few notes on general pedagogical style here. In the interest of conciseness, all structure declarations here are incomplete -- the real ones have more slots that I'm not telling you about. For the most part, these are reserved to one component of the server core or another, and should be altered by modules with caution. However, in some cases, they really are things I just haven't gotten around to yet. Welcome to the bleeding edge.

Finally, here's an outline, to give you some bare idea of what's coming up, and in what order:


Basic concepts

We begin with an overview of the basic concepts behind the API, and how they are manifested in the code.

Handlers, Modules, and Requests

Apache breaks down request handling into a series of steps, more or less the same way the Netscape server API does (although this API has a few more stages than NetSite does, as hooks for stuff I thought might be useful in the future). These are:

These phases are handled by looking at each of a succession of modules, looking to see if each of them has a handler for the phase, and attempting invoking it if so. The handler can typically do one of three things:

Most phases are terminated by the first module that handles them; however, for logging, `fixups', and non-access authentication checking, all handlers always run (barring an error). Also, the response phase is unique in that modules may declare multiple handlers for it, via a dispatch table keyed on the MIME type of the requested object. Modules may declare a response-phase handler which can handle any request, by giving it the key */* (i.e., a wildcard MIME type specification). However, wildcard handlers are only invoked if the server has already tried and failed to find a more specific response handler for the MIME type of the requested object (either none existed, or they all declined).

The handlers themselves are functions of one argument (a request_rec structure. vide infra), which returns an integer, as above.

A brief tour of a module

At this point, we need to explain the structure of a module. Our candidate will be one of the messier ones, the CGI module -- this handles both CGI scripts and the ScriptAlias config file command. It's actually a great deal more complicated than most modules, but if we're going to have only one example, it might as well be the one with its fingers in every place.

Let's begin with handlers. In order to handle the CGI scripts, the module declares a response handler for them. Because of ScriptAlias, it also has handlers for the name translation phase (to recognize ScriptAliased URIs), the type-checking phase (any ScriptAliased request is typed as a CGI script).

The module needs to maintain some per (virtual) server information, namely, the ScriptAliases in effect; the module structure therefore contains pointers to a functions which builds these structures, and to another which combines two of them (in case the main server and a virtual server both have ScriptAliases declared).

Finally, this module contains code to handle the ScriptAlias command itself. This particular module only declares one command, but there could be more, so modules have command tables which declare their commands, and describe where they are permitted, and how they are to be invoked.

A final note on the declared types of the arguments of some of these commands: a pool is a pointer to a resource pool structure; these are used by the server to keep track of the memory which has been allocated, files opened, etc., either to service a particular request, or to handle the process of configuring itself. That way, when the request is over (or, for the configuration pool, when the server is restarting), the memory can be freed, and the files closed, en masse, without anyone having to write explicit code to track them all down and dispose of them. Also, a cmd_parms structure contains various information about the config file being read, and other status information, which is sometimes of use to the function which processes a config-file command (such as ScriptAlias). With no further ado, the module itself:

/* Declarations of handlers. */

int translate_scriptalias (request_rec *);
int type_scriptalias (request_rec *);
int cgi_handler (request_rec *);

/* Subsidiary dispatch table for response-phase
 * handlers, by MIME type */

handler_rec cgi_handlers[] = {
{ "application/x-httpd-cgi", cgi_handler },
{ NULL }

/* Declarations of routines to manipulate the
 * module's configuration info. Note that these are
 * returned, and passed in, as void *'s; the server
 * core keeps track of them, but it doesn't, and can't,
 * know their internal structure.

void *make_cgi_server_config (pool *);
void *merge_cgi_server_config (pool *, void *, void *);

/* Declarations of routines to handle config-file commands */

extern char *script_alias(cmd_parms *, void *per_dir_config, char *fake, char *real);

command_rec cgi_cmds[] = {
{ "ScriptAlias", script_alias, NULL, RSRC_CONF, TAKE2,
"a fakename and a realname"},
{ NULL }

module cgi_module = {

  NULL,                     /* initializer */
  NULL,                     /* dir config creator */
  NULL,                     /* dir merger */
  make_cgi_server_config,   /* server config */
  merge_cgi_server_config,  /* merge server config */
  cgi_cmds,                 /* command table */
  cgi_handlers,             /* handlers */
  translate_scriptalias,    /* filename translation */
  NULL,                     /* check_user_id */
  NULL,                     /* check auth */
  NULL,                     /* check access */
  type_scriptalias,         /* type_checker */
  NULL,                     /* fixups */
  NULL,                     /* logger */
  NULL                      /* header parser */

How handlers work

The sole argument to handlers is a request_rec structure. This structure describes a particular request which has been made to the server, on behalf of a client. In most cases, each connection to the client generates only one request_rec structure.

A brief tour of the request_rec

The request_rec contains pointers to a resource pool which will be cleared when the server is finished handling the request; to structures containing per-server and per-connection information, and most importantly, information on the request itself.

The most important such information is a small set of character strings describing attributes of the object being requested, including its URI, filename, content-type and content-encoding (these being filled in by the translation and type-check handlers which handle the request, respectively).

Other commonly used data items are tables giving the MIME headers on the client's original request, MIME headers to be sent back with the response (which modules can add to at will), and environment variables for any subprocesses which are spawned off in the course of servicing the request. These tables are manipulated using the ap_table_get and ap_table_set routines.

Note that the Content-type header value cannot be set by module content-handlers using the ap_table_*() routines. Rather, it is set by pointing the content_type field in the request_rec structure to an appropriate string. e.g.,

r->content_type = "text/html";

Finally, there are pointers to two data structures which, in turn, point to per-module configuration structures. Specifically, these hold pointers to the data structures which the module has built to describe the way it has been configured to operate in a given directory (via .htaccess files or <Directory> sections), for private data it has built in the course of servicing the request (so modules' handlers for one phase can pass `notes' to their handlers for other phases). There is another such configuration vector in the server_rec data structure pointed to by the request_rec, which contains per (virtual) server configuration data.

Here is an abridged declaration, giving the fields most commonly used:

struct request_rec {

pool *pool;
conn_rec *connection;
server_rec *server;

/* What object is being requested */

char *uri;
char *filename;
char *path_info;

char *args;           /* QUERY_ARGS, if any */
struct stat finfo;    /* Set by server core;
                       * st_mode set to zero if no such file */

char *content_type;
char *content_encoding;

/* MIME header environments, in and out. Also,
 * an array containing environment variables to
 * be passed to subprocesses, so people can write
 * modules to add to that environment.
 * The difference between headers_out and
 * err_headers_out is that the latter are printed
 * even on error, and persist across internal
 * redirects (so the headers printed for
 * ErrorDocument handlers will have them).

table *headers_in;
table *headers_out;
table *err_headers_out;
table *subprocess_env;

/* Info about the request itself... */

int header_only;     /* HEAD request, as opposed to GET */
char *protocol;      /* Protocol, as given to us, or HTTP/0.9 */
char *method;        /* GET, HEAD, POST, etc. */
int method_number;   /* M_GET, M_POST, etc. */

/* Info for logging */

char *the_request;
int bytes_sent;