Dead Connection Detection (DCD) - Oracle

Oracle Database : Understanding Dead Connection Detection, Resource Limits, V$SESSION, V$PROCESS and OS processes

Dead Connection Detection (DCD)

These are previously valid connections with the database but the connection between the client and server processes has terminated abnormally.

Examples of a dead connection:
– A user reboots/turns-off their machine without logging off or disconnecting from the database.
– A network problem prevents communication between the client and the server.

In these cases, the shadow process running on the server and the session in the database may not terminate.

Implemented by

* adding SQLNET.EXPIRE_TIME = [MINUTES] to the sqlnet.ora file 

With DCD is enabled, the Server-side process sends a small 10-byte packet to the client process after the duration of the time interval specified in minutes by the SQLNET.EXPIRE_TIME parameter.

If the client-side connection is still connected and responsive, the client discards the probe packet, ..and another packet will be sent when next interval expires (assuming no other activity on the connection).

If the client fails and client OS sends back an error in responding to the DCD probe packet

• the Server side process is marked as a dead connection and
• PMON performs the clean up of the database processes / resources
• The client OS processes are terminated

NOTE: SQLNET.RECV_TIMEOUT can be set on the SERVER side sqlnet.ora file. This will set a timeout for the server process to wait for data from the client process.

Inactive Sessions

These are sessions that remain connected to the database with status in v$session of INACTIVE. For inactive sessions and not dead connection, use below profile or resource manager method to control.

Example of an INACTIVE session:
– A user starts a program/session, then leaves it running and idle for an extended period of time.

Database Resource Limits (using user profiles)

Implemented by:

• Setting RESOURCE_LIMIT = TRUE in the database startup parameter file (spfile or pfile)
• Creating or modifying existing user profiles (DBA_PROFILES) to have one or more resource limit
• Assigning a profile to a user whose resources are wished to be limited

It could happen that if the idle_time has been set on the DEFAULT profile, this can lead to an MTS dispatcher being set to ‘sniped’ and then getting ‘cleaned up’ via the shell script. The removal of the dispatcher will result in other sessions ‘dying’. In that case, If you are to implement resource limits, may be advisable to create new profiles that be assigned to users and not to change the characteristics of DEFAULT. Alternatively, if you do change DEFAULT, ensure that all the properties that you have affected have been fully tested in a development environment.

When a resource limit is exceeded (for example IDLE_TIME) … PMON does the following

• Mark the V$SESSION as SNIPED (Older Versions) or KILLED (Newer Versions)
• Clean up the database resources for the session
• Remove the V$SESSION entry

When a resource limit is exceeded (for example IDLE_TIME) … PMON marks the session as SNIPED in V$SESSION. Then, AFTER the SNIPED session tries to execute any SQL statement, its database resources are cleaned up and its V$SESSION entry is removed.

Resource Manager

Resource manager plans can be created to kill inactive sessions with high idle time. Refer to How To Automatic Kill Inactive Sessions using Resource Manager (Doc ID 1935739.1). This document contains the details with an example. You can customize the plan directives as per your requirement.

In this case, once the inactive time goes beyond the specified MAX_IDLE_TIME, PMON marks the session as KILLED. When this KILLED later tries to execute any SQL statement, its database resources are cleaned up and its V$SESSION entry is removed.

It is strongly recommended that both DCD and Resource Limits with Profiles be implemented in order to clean up resources at both the database and OS level.

This combination will not clean up IDLE / ABANDONED / INACTIVE connections (OS processes) as these sessions still have active clients

For this case we will see that :

• PMON has cleaned up the V$SESSION entries .. but both the OS processes and the V$PROCESS entries will still exist
• SQLNET will continue to be able to send the 10 byte packet successfully until the session is logged off

This condition can be a major problem as

• The database exhausts PROCESSES and gives ORA-20 maximum number of processes [num] exceeded
• The OS can become exhausted due to the un needed resources consumed by the abandoned processes

The SYMPTOMS of this condition are:

• The database view V$PROCESS will have no corresponding V$SESSION entry
• An OS process / thread still exists for the SNIPED session

The solutions to this scenario can are to clean up the OS processes … after which the V$PROCESS entries should be removed automatically.

Conclusion

1) DCD initiates clean up of OS and database processes that have disconnected/terminated abnormally.

2) DCD will not initiate clean up sessions that are still connected … but are idle / abandoned / inactive.

3) Database Resource Limits + user Profiles clean up database resources for user sessions that exceed resource limits.

4) Database Resource Limits + user Profiles will not clean up OS processes.

5) If DCD and Database Resource Limits + user Profiles are used in combination .. Dead Connections OS and Database Resources will be cleaned up.

6) IDLE / ABANDONED / INACTIVE sessions OS processes will not be cleaned up even if DCD and Database Resource Limits + user Profiles are used in combination … these must be cleaned up manually.

7) Resource Manager can be used to kill an inactive session, which exceeds the idle time mentioned in the plan directives.