Kevac Marko (mkevac) wrote,
Kevac Marko

Apache mod_dbd and more than one connection pool

Apache mod_dbd and more than one connection pool


Apache mod_dbd is a module for popular web server Apache HTTPD, which provides connection pooling to popular RDBMs for other modules. mod_dbd can provide only one connection pool per server (virtual server).

This article will explain how multiple pools per server capability was implemented and how it works.
Patch and testing module is in bug.


mod_dbd configuration looks like this:
       DBDriver mysql
       DBDParams "host=, port=..."
       DBDPersist On

In order to provide multiple pools capability, container was introduces, so configuration now looks like this:

    <DBDPool "marko1">
        DBDriver mysql
        DBDParams "host=, port=..."
        DBDPersist On

    <DBDPool "marko2">
        DBDriver mysql
        DBDParams "host=, port=..."
        DBDPersist On

In order to maintain compability, one can still configure pool without container, in which case this pool will have default name.


mod_dbd had this functions defined:

ap_dbd_open(apr_pool_t*, server_rec*);
ap_dbd_close(server_rec*, ap_dbd_t*);
ap_dbd_prepare(server_rec*, const char*, const char*);

Now all these functions have siblings with pool_name variable:

ap_dbd_open_pool(apr_pool_t*, server_rec*, const char*);
ap_dbd_close_pool(server_rec*, ap_dbd_t*, const char*);
ap_dbd_acquire_pool(request_rec*, const char*);
ap_dbd_cacquire_pool(conn_rec*, const char*);
ap_dbd_prepare_pool(server_rec*, const char*, const char*, const char*);

When old functions are used, pool with default pool name will be used. For named pools, one must use new functions.

Internal implementation

Module config

Module configuration structure svr_cfg, which enabled only one pool

 typedef struct {
     dbd_cfg_t *cfg;
     dbd_group_t *group;
 } svr_cfg;

was changed to hash of structures dbd_pool_t:

 typedef struct {
     dbd_cfg_t *cfg;
     dbd_group_t *group;
 } dbd_pool_t;

 typedef struct {
     apr_hash_t *pools; /* hash of dbd_pool_t structures */
 } svr_cfg;

New functions

Each of old functions without '_pool' is just wrapper to '_pool' functions. For example:

 DBD_DECLARE_NONSTD(void) ap_dbd_prepare(server_rec *s, const char *query,
                                         const char *label)
     ap_dbd_prepare_pool(s, DBD_DEFAULT_POOL_NAME, query, label);
Tags: apache, devel, opensource

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