clearlocks: What Good Is It for Anyway?

The ‘clearlocks’ utility has been around, as a part of Teamcenter, for very long time. In my opinion, for Teamcenter DBAs and system administrators there never was (yes, I said ‘was’…read more) any other friendly and helpful utility than clearlocks; especially with –verbose argument.

If the clearlocks utility is still part of Teamcenter, then why did I use past tense? Is clearlocks no more a friendly or helpful utility? What happened? Well, there are some changes to Teamcenter and as a result, clearlocks utility has lost the status of most friendly and helpful utility (at-least in my book). Let’s see what’s happening.

Why Was clearlocks Needed?

When someone logs into Teamcenter, regardless of the client (i.e. 2-tier, 4-tier, SOA, ITK program, web client, etc.), database records that session and locks held by the session in its session and lock tables. Apart from that, Teamcenter has its own session tables to record its own information about the associated Teamcenter session. Teamcenter also has various tables to record locks obtained by those Teamcenter sessions. Those locks are obviously tied to the corresponding Teamcenter session. These Teamcenter session and lock tables are separate from database’s session and lock tables.

If the Teamcenter session dies for some reason (loss of network connectivity, memory error, session crash, etc.), then user doesn’t get chance to log out of the Teamcenter session. As a result the dead session entry and various locks held by the dead session would remain in Teamcenter session and lock tables. Any other user attempting to modify one of the objects locked by such dead session would experience an error.

The clearlocks utility attempted to identify such dead sessions (i.e. the Teamcenter process that created the session no longer exists), remove those sessions from session table, and remove all locks held by those dead sessions from various lock tables.

What I Used To Advice My Clients

Clearly, the clearlocks utility was of much use. I always advised my clients to:

• Setup a cron job or scheduled job to run “clearlocks –verbose” every 15 to 30 minutes.
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• Additionally, edit tcserver.bat (or start_TcServer1.bat) and add “clearlocks –verbose” immediately prior-to and after the call to tcserver.exe. I preferred not to make this optional, because it ensured that each portal session started clean (i.e. removed any dead sessions and locks) and exited clean in case of any abnormal exits.
• Additionally and optionally, add “clearlocks –verbose” in tc_profilevars.bat

What Changed?

New logic was added to Teamcenter to handle part of the clearlocks functionality. Now, if a Teamcenter session requests a lock on an object that is already locked by some other session, then Teamcenter would check if the other session holding the lock is dead. If it finds that the other session is dead then Teamcenter would remove that dead session and all locks held by that dead session. clearlocks is no longer needed to remove locks held by dead sessions.

I am not sure which Teamcenter version implemented this change. But, I can say that I first saw it in version 8.3 (Thanks to our good friend, Scott Pigman from The PLM Dojo, for politely pointing out the missing information).

So, What’s The Catch?

In rare circumstances, clearlocks can actually be detrimental. Now, with so many client options, it is possible for a Teamcenter session to enter into an idle state and get disconnected from database. When such a Teamcenter session comes out of idle state and becomes active again, it will attempt to reconnect to the database and reestablish its Teamcenter session and lock records. Database may mark such an idle Teamcenter session as a dead session. If clearlocks is run when a session is in idle state and disconnected from database, it will actually remove the session and its locks from the database. This will result into problems when such a session becomes active again and tries to reestablish its identity.

Teamcenter will attempt to clean dead process and its locks only when a lock is requested on an object that is already locked by the dead process. As a side effect, if the dead session does not hold any locks, it will never be cleared and will remain in the database indefinitely. These sessions don’t cause any issues as far as proper working of Teamcenter is concerned, except consuming unnecessary space in the database.

Although it was not clear from any official explanation from Siemens, but it would seem that if a Teamcenter session requests a lock on an object that has been locked by an idle but disconnected session then it too would result in clearing session record and locks of the idle session (even without running clearlocks).

My New Advice To My Clients

• There is no need to run clearlocks actively (via cron job, or start_TcServer1.bat file, etc.) anymore.
• At the same time it is essential to run “clearlocks –verbose” periodically to remove all dead session that are not holding any locks. This can be done when there is least amount of active Teamcenter sessions, such as on Sunday night or during scheduled database server shutdown.
• Worst time to run clearlocks is after an unscheduled/unplanned database server restart, or after network outage, after power failure, etc. These are times when there would be lot of idle but disconnected sessions.

Final Thoughts

Until I was aware of this enhancement, I was running clearlocks per my original advice. And every once in a while I would encounter following error:

INTERNAL ERROR: ghost of modified object <ObjectTag> is NULL

Syslog would show following lines:

    **** THIS PROCESS HAS GONE FROM THE PROCESS LIST TABLE IN THE DATABASE ****
         This is probably because someone ran clearlocks on our process
         Please do not report any locking bugs or POM_i_am_a_zombie_process errors from
         this process until you are confident clearlocks abuse does not account for them.

I did not understand the cause of this error and then suddenly light bulb came on when I learned about this Teamcenter enhancement. clearlocks might still be a friend…it just is not as dear as it used to be.

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