5.6.3.How it Works
If OCI connection pooling is enabled, a separate connection pool is created for each Oracle instance connected to. All connections specifying the same server instance (i.e. the same TNS service name) share the same connection pool. The connection pool is reference counted and only destroyed when the last Lite/agent connection using it is closed down.
Each agent connection holds a reference to an OCI connection pool record or OCPR. The OCPR contains a handle to the OCI connection pool associated with the Oracle instance connected to. The agent maintains a hash table of OCPRs, keyed by instance name. Each OCPR is reference counted. The reference count for an OCPR is incremented each time an agent uses the connection pool to connect to the associated Oracle instance, and decremented when the agent connection is closed. The OCPR is destroyed, and the connection pool closed, when the last agent connection using it is closed.
When an agent is about to establish a connection to an Oracle instance, it searches for an OCPR for the instance. If one exists, a connection pool for that instance already exists. The agent requests an OCI connection from this pool and increments the reference count of the OCPR. If one doesn't exist, the agent creates an OCI connection pool for the instance by registering a new OCPR.
Note: different ODBC DSNs, using this driver and the same server instance, may specify different pool settings (max pool size, min pool size etc.) The settings of the first DSN to connect fixes the characteristics of the pool.
The maximum number of connection pools supported by an Oracle agent or Lite driver is 100. Each connection pool allows up to 500 connections, depending on the value set in the environment variable OCI_CONNPOOL_MAX or the Lite driver setup dialog. The maximum linger time is 300 seconds.