A database connection forms a channel through which all activity from the client application takes place. Client applications cannot interact with the database server until a connection is made. When the database server connection is made, a user’s ID determines what actions they are authorized to perform on the database server.

When a user connects to a database, the database server assigns the user’s connection a unique connection ID. For each new connection to the database server, the server increments the connection ID value by 1. These connection IDs are logged in the -z server output. The connection ID can be used to filter request logging information, identify which connection has a lock on the database, or track the total number of connections to a server since it started and the order in which those connections were made. See Request logging and How locking works.

You can use the CONNECTION_PROPERTY function to obtain a user's connection-id. See CONNECTION_PROPERTY function [System].