Resource drain with idle sessions/connection to the database

Summary:

Idle sessions connected to an Oracle Database consume various resources, including memory (PGA, UGA), database locks, SGA, session state, network, and system resources. This can degrade performance and reduce available connections. Mitigation strategies include setting IDLE_TIME limits, session sniping, connection pooling, monitoring and alerts, and periodic cleanup to maintain database performance and scalability.

Yes, even an idle session connected to an Oracle Database can still consume and hold onto certain resources. Here are the key resources that an idle session may impact:

1. Memory:

• PGA (Program Global Area): An idle session consumes memory in the PGA, which includes session-specific data, cursor information, and other session-related memory structures.

• UGA (User Global Area): If using a shared server configuration, the UGA is allocated in the shared pool, and idle sessions can hold onto memory within the UGA.

2. Database Locks:

• Row-Level Locks: Although rare, if an idle session has performed DML operations and hasn’t committed or rolled back, it could still be holding row-level locks. These locks can block other transactions from accessing those rows.

• Resource Locks: An idle session might also hold onto locks on other database objects, such as tables, preventing schema changes or causing blocking issues.

3. System Global Area (SGA):

• Shared Pool: Idle sessions might have cursors or SQL statements stored in the shared pool. While these cursors might be aged out over time, they still consume memory until that happens.

• Library Cache: Any parsed SQL or PL/SQL code associated with the session can be held in the library cache, consuming resources.

4. Session State:

• Temporary Segments: An idle session might have temporary segments in use (e.g., for sorts or hash joins) that aren't released until the session ends.

• Session State: The session state, including session variables, PL/SQL package states, and context areas, is maintained in memory, which can be significant depending on the session’s workload before becoming idle.

5. Network Resources:

• Connections: Even if idle, a session keeps a connection open to the database, consuming network sockets and possibly affecting connection pooling mechanisms if the application is using connection pools.

6. System Resources:

• CPU Usage: While minimal, an idle session can still consume CPU cycles, especially if it periodically pings the database to maintain the connection (common in some applications or middle-tier setups).

• File Handles: Sessions might have open file handles, such as log files or trace files, which can impact system resources.

Why It Matters:

Idle sessions, especially in environments with a high number of connections, can lead to unnecessary resource consumption, which could degrade overall database performance, increase the likelihood of contention, and reduce the number of available connections for active users.

Mitigation Strategies:

1. Implement IDLE_TIME Limits:

   • As discussed earlier, use the IDLE_TIME parameter in profiles to disconnect idle sessions automatically after a certain period.

2. Session Sniping:

   • Implement session sniping to mark and later terminate sessions that have been idle for too long.

3. Connection Pooling:

   • Use connection pooling in applications to minimize the number of idle sessions. This ensures that connections are returned to the pool when not in use and only active sessions consume resources.

4. Monitoring and Alerts:

   • Regularly monitor the database for idle sessions and resource usage. Set up alerts for sessions that have been idle for too long or are consuming excessive resources.

5. Periodic Cleanup:

   • Use database jobs or scripts to review and terminate long-idle sessions periodically.

By addressing idle sessions effectively, you can help maintain the performance and scalability of your Oracle Database.