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2.4 Basic Concepts and Structures

To work with Apache as a development platform, we need an overview of the basic units of webserver operation and the core objects that represent them within Apache. The most important are the server, the TCP connection, and the HTTP request. A fourth basic Apache object, the process, is a unit of the operating system rather than the application architecture. Each of these basic units is represented by a core data structure defined in the header file httpd.h and, like other core objects we encounter in applications development, is completely independent of the MPM in use.

Before describing these core data structures, we need to introduce some further concepts used throughout Apache and closely tied to the architecture:

  • APR pools (apr_pool_t) are the core of resource management in Apache. Whenever a resource is allocated dynamically, a cleanup is registered with a pool, ensuring that system resources are freed when they are no longer required. Pools tie resources to the lifetime of one of the core objects. We will describe pools in depth in Chapter 3.
  • Configuration records are used by each module to tie its own data to one of the core objects. The core data structures include configuration vectors (ap_conf_vector_t), with each module having its own entry in the vector. They are used in two ways: to set and retrieve permanent configuration data, and to store temporary data associated with a transient object. They are often essential to avoid use of unsafe static or global data in a module, as discussed in Chapters 4 and 9.

Having introduced pools and configuration records, we are now ready to look at the Apache core objects. In order of importance to most modules, they are

  • request_rec
  • server_rec
  • conn_rec
  • process_rec

The first two are by far the most commonly encountered in application development.

2.4.1 request_rec

A request_rec object is created whenever Apache accepts an HTTP request from a client, and is destroyed as soon as Apache finishes processing the request. The request_rec object is passed to every handler implemented by any module in the course of processing a request (as discussed in Chapters 5 and 6). It holds all of the internal data relevant to processing an HTTP request. It also includes a number of fields used internally to maintain state and client information by Apache:

  • A request pool, for management of objects having the lifetime of the request. It is used to manage resources allocated while processing the request.
  • A vector of configuration records for static request configuration (per-directory data specified in httpd.conf or .htaccess).
  • A vector of configuration records for transient data used in processing.
  • Tables of HTTP input, output, and error headers.
  • A table of Apache environment variables (the environment as seen in scripting extensions such as SSI, CGI, mod_rewrite, and PHP), and a similar "notes" table for request data that should not be seen by scripts.
  • Pointers to all other relevant objects, including the connection, the server, and any related request objects.
  • Pointers to the input and output filter chains (discussed in Chapter 8).
  • The URI requested, and the internal parsed representation of it, including the handler (see Chapter 5) and filesystem mapping (see Chapter 6).

Here is the full definition, from httpd.h:

/** A structure that represents the current request */
struct request_rec {
    /** The pool associated with the request */
    apr_pool_t *pool;
    /** The connection to the client */
    conn_rec *connection;
    /** The virtual host for this request */
    server_rec *server;

    /** Pointer to the redirected request if this is an external redirect */
    request_rec *next;
    /** Pointer to the previous request if this is an internal redirect */
    request_rec *prev;
    /** Pointer to the main request if this is a sub-request
     * (see http_request.h) */
    request_rec *main;

    /* Info about the request itself... we begin with stuff that only
     * protocol.c should ever touch...
     */
    /** First line of request */
    char *the_request;
    /** HTTP/0.9, "simple" request (e.g., GET /foo\n w/no headers) */
    int assbackwards;
    /** A proxy request (calculated during post_read_request/translate_name)
     *  possible values PROXYREQ_NONE, PROXYREQ_PROXY, PROXYREQ_REVERSE,
     *                  PROXYREQ_RESPONSE
     */
    int proxyreq;
    /** HEAD request, as opposed to GET */
    int header_only;
    /** Protocol string, as given to us, or HTTP/0.9 */
    char *protocol;
    /** Protocol version number of protocol; 1.1 = 1001 */
    int proto_num;
    /** Host, as set by full URI or Host: */
    const char *hostname;

    /** Time when the request started */
    apr_time_t request_time;

    /** Status line, if set by script */
    const char *status_line;
    /** Status line */
    int status;

    /* Request method, two ways; also, protocol, etc. Outside of protocol.c,
     * look, but don't touch.
     */

    /** Request method (e.g., GET, HEAD, POST, etc.) */
    const char *method;
    /** M_GET, M_POST, etc. */
    int method_number;

    /**
     *  'allowed' is a bit-vector of the allowed methods.
     *
     *  A handler must ensure that the request method is one that
     *  it is capable of handling.  Generally modules should DECLINE
     *  any request methods they do not handle.  Prior to aborting the
     *  handler like this, the handler should set r->allowed to the list
     *  of methods that it is willing to handle. This bitvector is used
     *  to construct the "Allow:" header required for OPTIONS requests,
     *  and HTTP_METHOD_NOT_ALLOWED and HTTP_NOT_IMPLEMENTED status codes.
     *
     *  Since the default_handler deals with OPTIONS, all modules can
     *  usually decline to deal with OPTIONS.  TRACE is always allowed;
     *  modules don't need to set it explicitly.
     *
     *  Since the default_handler will always handle a GET, a
     *  module which does *not* implement GET should probably return
     *  HTTP_METHOD_NOT_ALLOWED.  Unfortunately this means that a Script GET
     *  handler can't be installed by mod_actions.
     */
    apr_int64_t allowed;
    /** Array of extension methods */
    apr_array_header_t *allowed_xmethods;
    /** List of allowed methods */
    ap_method_list_t *allowed_methods;

    /** byte count in stream is for body */
    apr_off_t sent_bodyct;
    /** body byte count, for easy access */
    apr_off_t bytes_sent;
    /** Last modified time of the requested resource */
    apr_time_t mtime;

    /* HTTP/1.1 connection-level features */

    /**Sending chunked transfer-coding */
    int chunked;
    /** The Range: header */
    const char *range;
    /** The "real" content length */
    apr_off_t clength;

    /** Remaining bytes left to read from the request body */
    apr_off_t remaining;
    /** Number of bytes that have been read  from the request body */
    apr_off_t read_length;
    /** Method for reading the request body
     * (e.g., REQUEST_CHUNKED_ERROR, REQUEST_NO_BODY,
     *  REQUEST_CHUNKED_DECHUNK, etc.) */
    int read_body;
    /** reading chunked transfer-coding */
    int read_chunked;
    /** is client waiting for a 100 response? */
    unsigned expecting_100;

    /* 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).
    *
    * The 'notes' apr_table_t is for notes from one module to another, with no
    * other set purpose in mind...
    */

   /** MIME header environment from the request */
   apr_table_t *headers_in;
   /** MIME header environment for the response */
   apr_table_t *headers_out;
   /** MIME header environment for the response, printed even on errors and
    * persist across internal redirects */
   apr_table_t *err_headers_out;
   /** Array of environment variables to be used for subprocesses */
   apr_table_t *subprocess_env;
   /** Notes from one module to another */
   apr_table_t *notes;

  /* content_type, handler, content_encoding, and all content_languages
    * MUST be lowercased strings.  They may be pointers to static strings;
    * they should not be modified in place.
    */
   /** The content-type for the current request */
   const char *content_type;   /* Break these out -- we dispatch on 'em */
   /** The handler string that we use to call a handler function */
   const char *handler;        /* What we *really* dispatch on */

   /** How to encode the data */
   const char *content_encoding;
   /** Array of strings representing the content languages */
   apr_array_header_t *content_languages;

   /** variant list validator (if negotiated) */
   char *vlist_validator;

   /** If an authentication check was made, this gets set to the user name. */
   char *user;
   /** If an authentication check was made, this gets set to the auth type. */
   char *ap_auth_type;

   /** This response cannot be cached */
   int no_cache;
   /** There is no local copy of this response */
   int no_local_copy;

   /* What object is being requested (either directly, or via include
    * or content-negotiation mapping).
    */
   /** The URI without any parsing performed */
    char *unparsed_uri;
    /** The path portion of the URI */
    char *uri;
    /** The filename on disk corresponding to this response */
    char *filename;
    /** The true filename, we canonicalize r->filename if these don't match */
    char *canonical_filename;
    /** The PATH_INFO extracted from this request */
    char *path_info;
    /** The QUERY_ARGS extracted from this request */
    char *args;
    /**  finfo.protection (st_mode) set to zero if no such file */
    apr_finfo_t finfo;
    /** A struct containing the components of URI */
    apr_uri_t parsed_uri;

    /**
     * Flag for the handler to accept or reject path_info on
     * the current request.  All modules should respect the
     * AP_REQ_ACCEPT_PATH_INFO and AP_REQ_REJECT_PATH_INFO
     * values, while AP_REQ_DEFAULT_PATH_INFO indicates they
     * may follow existing conventions.  This is set to the
     * user's preference upon HOOK_VERY_FIRST of the fixups.
     */
    int used_path_info;

    /* Various other config info which may change with .htaccess files.
     * These are config vectors, with one void* pointer for each module
     * (the thing pointed to being the module's business).
     */

    /** Options set in config files, etc. */
    struct ap_conf_vector_t *per_dir_config;
    /** Notes on *this* request */
    struct ap_conf_vector_t *request_config;

    /**
     * A linked list of the .htaccess configuration directives
     * accessed by this request.
     * N.B.: always add to the head of the list, _never_ to the end.
     * That way, a sub-request's list can (temporarily) point to a parent's list
     */
    const struct htaccess_result *htaccess;

    /** A list of output filters to be used for this request */
    struct ap_filter_t *output_filters;
    /** A list of input filters to be used for this request */
    struct ap_filter_t *input_filters;

    /** A list of protocol level output filters to be used for this
     *  request */
    struct ap_filter_t *proto_output_filters;
    /** A list of protocol level input filters to be used for this
     *  request */
    struct ap_filter_t *proto_input_filters;

    /** A flag to determine if the eos bucket has been sent yet */
    int eos_sent;

/* Things placed at the end of the record to avoid breaking binary
 * compatibility.  It would be nice to remember to reorder the entire
 * record to improve 64-bit alignment the next time we need to break
 * binary compatibility for some other reason.
 */
};

2.4.2 server_rec

The server_rec defines a logical webserver. If virtual hosts are in use,8 each virtual host has its own server_rec, defining it independently of the other hosts. The server_rec is created at server start-up, and it never dies unless the entire httpd is shut down. The server_rec does not have its own pool; instead, server resources need to be allocated from the process pool, which is shared by all servers. It does have a configuration vector as well as server resources including the server name and definition, resources and limits, and logging information.

The server_rec is the second most important structure to programmers, after the request_rec. It will feature prominently throughout our discussion of module programming.

Here is the full definition, from httpd.h:

/** A structure to store information for each virtual server */
struct server_rec {
    /** The process this server is running in */
    process_rec *process;
    /** The next server in the list */
    server_rec *next;

    /** The name of the server */
    const char *defn_name;
    /** The line of the config file that the server was defined on */
    unsigned defn_line_number;
    /* Contact information */

    /** The admin's contact information */
    char *server_admin;
    /** The server hostname */
    char *server_hostname;
    /** for redirects, etc. */
    apr_port_t port;

    /* Log files -- note that transfer log is now in the modules... */

    /** The name of the error log */
    char *error_fname;
    /** A file descriptor that references the error log */
    apr_file_t *error_log;
    /** The log level for this server */
    int loglevel;

    /* Module-specific configuration for server, and defaults... */

    /** true if this is the virtual server */
    int is_virtual;
    /** Config vector containing pointers to modules' per-server config
     *  structures. */
    struct ap_conf_vector_t *module_config;
    /** MIME type info, etc., before we start checking per-directory info */
    struct ap_conf_vector_t *lookup_defaults;

    /* Transaction handling */

    /** I haven't got a clue */
    server_addr_rec *addrs;
    /** Timeout, as an apr interval, before we give up */
    apr_interval_time_t timeout;
    /** The apr interval we will wait for another request */
    apr_interval_time_t keep_alive_timeout;
    /** Maximum requests per connection */
    int keep_alive_max;
    /** Use persistent connections? */
    int keep_alive;

    /** Pathname for ServerPath */
    const char *path;
    /** Length of path */
    int pathlen;

    /** Normal names for ServerAlias servers */
    apr_array_header_t *names;
    /** Wildcarded names for ServerAlias servers */
    apr_array_header_t *wild_names;
    /** limit on size of the HTTP request line    */
    int limit_req_line;
    /** limit on size of any request header field */
    int limit_req_fieldsize;
    /** limit on number of request header fields  */
    int limit_req_fields;
};

2.4.3 conn_rec

The conn_rec object is Apache's internal representation of a TCP connection. It is created when Apache accepts a connection from a client, and later it is destroyed when the connection is closed. The usual reason for a connection to be made is to serve one or more HTTP requests, so one or more request_rec structures will be instantiated from each conn_rec. Most applications will focus on the request and ignore the conn_rec, but protocol modules and connection-level filters will need to use the conn_rec, and modules may sometimes use it in tasks such as optimizing the use of resources over the lifetime of an HTTP Keepalive (persistent connection).

The conn_rec has no configuration information, but has a configuration vector for transient data associated with a connection as well as a pool for connection resources. It also has connection input and output filter chains, plus data describing the TCP connection.

It is important to distinguish clearly between the request and the connection—the former is always a subcomponent of the latter. Apache cleanly represents each as a separate object, with one important exception, which we will deal with in discussing connection filters in Chapter 8.

Here is the full definition from httpd.h:

/** Structure to store things which are per connection */
struct conn_rec {
    /** Pool associated with this connection */
    apr_pool_t *pool;
    /** Physical vhost this conn came in on */
    server_rec *base_server;
    /** used by http_vhost.c */
    void *vhost_lookup_data;

    /* Information about the connection itself */
    /** local address */
    apr_sockaddr_t *local_addr;
    /** remote address */
    apr_sockaddr_t *remote_addr;
    /** Client's IP address */
    char *remote_ip;
    /** Client's DNS name, if known.  NULL if DNS hasn't been checked;
     *  "" if it has and no address was found.  N.B.: Only access this through
     * get_remote_host() */
    char *remote_host;
    /** Only ever set if doing rfc1413 lookups.  N.B.: Only access this through
     *  get_remote_logname() */
    char *remote_logname;

    /** Are we still talking? */
    unsigned aborted:1;

    /** Are we going to keep the connection alive for another request?
     * @see ap_conn_keepalive_e */
    ap_conn_keepalive_e keepalive;

    /** Have we done double-reverse DNS? -1 yes/failure, 0 not yet,
     *  1 yes/success */
    signed int double_reverse:2;

    /** How many times have we used it? */
    int keepalives;
    /** server IP address */
    char *local_ip;
    /** used for ap_get_server_name when UseCanonicalName is set to DNS
     *  (ignores setting of HostnameLookups) */
    char *local_host;

    /** ID of this connection; unique at any point in time */
    long id;
    /** Config vector containing pointers to connections per-server
     *  config structures */
    struct ap_conf_vector_t *conn_config;
    /** Notes on *this* connection: send note from one module to
     *  another. Must remain valid for all requests on this conn. */
    apr_table_t *notes;
    /** A list of input filters to be used for this connection */
    struct ap_filter_t *input_filters;
    /** A list of output filters to be used for this connection */
    struct ap_filter_t *output_filters;
    /** Handle to scoreboard information for this connection */
    void *sbh;
    /** The bucket allocator to use for all bucket/brigade creations */
    struct apr_bucket_alloc_t *bucket_alloc;
    /** The current state of this connection */
    conn_state_t *cs;
    /** Is there data pending in the input filters? */
    int data_in_input_filters;
};

2.4.4 process_rec

Unlike the other core objects discussed earlier, the process_rec is an operating system object rather than a web architecture object. The only time applications need concern themselves with it is when they are working with resources having the lifetime of the server, when the process pool serves all of the server_rec objects (and is accessed from a server_rec as s->process->pool). The definition appears in httpd.h, but is not reproduced here.

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