How The Connection Pool Works

How the Connection Pool Works

Every successful connection is associated with a pool, which maintains two lists — one for idle connections and one for active connections.When a connection in the “active” list has no active requests and no active transactions, it is assumed to be “unused”.A reset of the unused connection is attempted using an [fblangref50-management-session-reset-alter] statement and,

if the reset succeeds (no errors occur) the connection is moved into the “idle” list;
if the reset fails, the connection is closed;
if the pool has reached its maximum size, the oldest idle connection is closed.
When the lifetime of an idle connection expires, it is deleted from the pool and closed.

New Connections

When the engine is asked to create a new external connection, the pool first looks for a candidate in the “idle” list.The search, which is case-sensitive, involves four parameters:

connection string
username
password
role

If a suitable connection is found, it is tested to check that it is still alive.

If it fails the check, it is deleted, and the search is repeated, without reporting any error to the client
Otherwise, the live connection is moved from the “idle” list to the “active” list and returned to the caller
If there are multiple suitable connections, the most recently used one is chosen
If there is no suitable connection, a new one is created and added to the “active” list.