# Apache模块 mod_dbd
| [说明](#calibre_link-11) | 管理SQL数据库连接，为需要数据库功能的模块提供支持 |
| --- | --- |
| [状态](#calibre_link-12) | 扩展(E) |
| [模块名](#calibre_link-13) | dbd_module |
| [源文件](#calibre_link-14) | mod_dbd.c |
| [兼容性](#calibre_link-58) | Version 2.1 及以后的版本中可用 |
### 概述
`mod_dbd` manages SQL database connections using [apr_dbd](http://people.apache.org/~niq/dbd.html). 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.
## 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](http://www.apachetutor.org/dev/reslist). `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 指令
| [说明](#calibre_link-18) | Keepalive time for idle connections |
| --- | --- |
| [语法](#calibre_link-19) | `DBDExptime time-in-seconds` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | 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 指令
| [说明](#calibre_link-18) | Maximum sustainednumber of connections |
| --- | --- |
| [语法](#calibre_link-19) | `DBDKeep number` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | mod_dbd |
Set the maximum number of connections per process to be sustained, other than for handling peak demand (threaded platforms only).
## DBDMax 指令
| [说明](#calibre_link-18) | Maximum number of connections |
| --- | --- |
| [语法](#calibre_link-19) | `DBDMax number` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | mod_dbd |
Set the hard maximum number of connections per process (threaded platforms only).
## DBDMin 指令
| [说明](#calibre_link-18) | Minimum number of connections |
| --- | --- |
| [语法](#calibre_link-19) | `DBDMin number` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | mod_dbd |
Set the minimum number of connections per process (threaded platforms only).
## DBDParams 指令
| [说明](#calibre_link-18) | Parameters for database connection |
| --- | --- |
| [语法](#calibre_link-19) | `DBDParams param1=value1[,param2=value2]` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | 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.
## DBDPersist 指令
| [说明](#calibre_link-18) | Whether to use persistent connections |
| --- | --- |
| [语法](#calibre_link-19) | `DBDPersist 0|1` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | 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 指令
| [说明](#calibre_link-18) | Define an SQL prepared statement |
| --- | --- |
| [语法](#calibre_link-19) | `DBDPrepareSQL "SQL statement" label` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | 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 指令
| [说明](#calibre_link-18) | Specify an SQL driver |
| --- | --- |
| [语法](#calibre_link-19) | `DBDriver name` |
| [作用域](#calibre_link-20) | server config, virtual host |
| [状态](#calibre_link-21) | 扩展(E) |
| [模块](#calibre_link-22) | 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.
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