Apache Module mod_dbd

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Description:	Manages SQL database connections
Status:	Extension
Module Identifier:	dbd_module
Source File:	mod_dbd.c
Compatibility:	Version 2.1 and later

Summary

mod_dbd manages SQL database connections using apr_dbd. It provides database connections on request to modules requiring SQL database functions, and takes care of managing databases with optimal efficiency and scalability for both threaded and non-threaded MPMs.

Topics

Connection Pooling
Apache DBD API
SQL Prepared Statements

Connection Pooling

This module manages database connections, in a manner optimised for the platform. On non-threaded platforms, it provides a persistent connection in the manner of classic LAMP (Linux, Apache, Mysql, Perl/PHP/Python). On threaded platform, it provides an altogether more scalable and efficient connection pool, as described in this article at ApacheTutor. mod_dbd supersedes the modules presented in that article.

Apache DBD API

mod_dbd exports five functions for other modules to use. The API is as follows:

typedef struct {
    apr_dbd_t *handle;
    apr_dbd_driver_t *driver;
    apr_hash_t *prepared;
} ap_dbd_t;

/* Export functions to access the database */

/* acquire a connection that MUST be explicitly closed.
 * Returns NULL on error
 */
AP_DECLARE(ap_dbd_t*) ap_dbd_open(apr_pool_t*, server_rec*);

/* release a connection acquired with ap_dbd_open */
AP_DECLARE(void) ap_dbd_close(server_rec*, ap_dbd_t*);

/* acquire a connection that will have the lifetime of a request
 * and MUST NOT be explicitly closed.  Return NULL on error.
 * This is the preferred function for most applications.
 */
AP_DECLARE(ap_dbd_t*) ap_dbd_acquire(request_rec*);

/* acquire a connection that will have the lifetime of a connection
 * and MUST NOT be explicitly closed.  Return NULL on error.
 */
AP_DECLARE(ap_dbd_t*) ap_dbd_cacquire(request_rec*);

/* Prepare a statement for use by a client module */
AP_DECLARE(void) ap_dbd_prepare(server_rec*, const char*, const char*);

/* Also export them as optional functions for modules that prefer it */
APR_DECLARE_OPTIONAL_FN(ap_dbd_t*, ap_dbd_open, (apr_pool_t*, server_rec*));
APR_DECLARE_OPTIONAL_FN(void, ap_dbd_close, (server_rec*, ap_dbd_t*));
APR_DECLARE_OPTIONAL_FN(ap_dbd_t*, ap_dbd_acquire, (request_rec*));
APR_DECLARE_OPTIONAL_FN(ap_dbd_t*, ap_dbd_cacquire, (conn_rec*));
APR_DECLARE_OPTIONAL_FN(void, ap_dbd_prepare, (server_rec*, const char*, const char*));

SQL Prepared Statements

mod_dbd supports SQL prepared statements on behalf of modules that may wish to use them. Each prepared statement must be assigned a name (label), and they are stored in a hash: the prepared field of an ap_dbd_t. Hash entries are of type apr_dbd_prepared_t and can be used in any of the apr_dbd prepared statement SQL query or select commands.

It is up to dbd user modules to use the prepared statements and document what statements can be specified in httpd.conf, or to provide their own directives and use ap_dbd_prepare.

DBDExptime Directive

Description:	Keepalive time for idle connections
Syntax:	`DBDExptime time-in-seconds`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

Set the time to keep idle connections alive where the number of connections specified in DBDKeep has been exceeded (threaded platforms only).

DBDKeep Directive

Description:	Maximum sustained number of connections
Syntax:	`DBDKeep number`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

Set the maximum number of connections per process to be sustained, other than for handling peak demand (threaded platforms only).

DBDMax Directive

Description:	Maximum number of connections
Syntax:	`DBDMax number`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

Set the hard maximum number of connections per process (threaded platforms only).

DBDMin Directive

Description:	Minimum number of connections
Syntax:	`DBDMin number`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

Set the minimum number of connections per process (threaded platforms only).

DBDParams Directive

Description:	Parameters for database connection
Syntax:	`DBDParams param1=value1[,param2=value2]`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

As required by the underlying driver. Typically this will be used to pass whatever cannot be defaulted amongst username, password, database name, hostname and port number for connection.

Connection string parameters for current drivers include:

MySQL: host, port, user, pass, dbname, sock
Oracle: user, pass, dbname, server
PostgreSQL: The connection string is passed straight through to PQconnectdb
SQLite2: The connection string is split on a colon, and part1:part2 is used as sqlite_open(part1, atoi(part2), NULL)
SQLite3: The connection string is passed straight through to sqlite3_open

DBDPersist Directive

Description:	Whether to use persistent connections
Syntax:	`DBDPersist 0\|1`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

If set to 0, persistent and pooled connections are disabled. A new database connection is opened when requested by a client, and closed immediately on release. This option is for debugging and low-usage servers.

The default is to enable a pool of persistent connections (or a single LAMP-style persistent connection in the case of a non-threaded server), and should almost always be used in operation.

DBDPrepareSQL Directive

Description:	Define an SQL prepared statement
Syntax:	`DBDPrepareSQL "SQL statement" label`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

For modules such as authentication that use repeatedly use a single SQL statement, optimum performance is achieved by preparing the statement at startup rather than every time it is used. This directive prepares an SQL statement and assigns it a label.

DBDriver Directive

Description:	Specify an SQL driver
Syntax:	`DBDriver name`
Context:	server config, virtual host
Status:	Extension
Module:	mod_dbd

Selects an apr_dbd driver by name. The driver must be installed on your system (on most systems, it will be a shared object or dll). For example, DBDriver mysql will select the MySQL driver in apr_dbd_mysql.so.