Oracle deadly embrace tips

Question:  What is the deadly embrace, or the perpetual embrace?  Does Oracle prevent the deadly embrace, and if so, what errors do I see and how can I prevent them?

Answer:  The deadly embrace often manifests itself as ORA-04020 deadlock detected error.  This is how a deadly embrace happens in Oracle:

1 - Assume that SQL 1 is "holding rows" (via the select for update clause) and is waiting to get more rows that are held by SQL 2.

2 - SQL 2 attempts to update rows that are being held by SQL 1.

3 - Oracle aborts SQL 2 and issues one of these deadlock messages:

Also see these notes on ORA-00054 error, resource busy and NOWAIT specified.

Here are several Oracle deadlock error messages:

ORA-00060: deadlock detected while waiting for resource

Cause: Transactions deadlocked one another while waiting for resources.

Action: Look at the trace file to see the transactions and resources involved. Retry if necessary.

ORA-04020 deadlock detected while trying to lock object

Cause: While trying to lock a library object, a deadlock is detected.  

Action: Retry the operation later.


As we see the perpetual embrace happens when another DML are waiting on resources that are requested by the other, we have the condition knows as the "deadly embrace", and Oracle terminates the DML that causes the embrace.  This perpetual embrace occurs when a session is locking rows using the "for update" clause", which is not a good practice.

Whenever you have competing DML running against the same data, you run the risk of a deadlock. This deadlock condition is an age-old issue known as the "perpetual embrace"!

If your ORA-00060 is caused by competing resources, the perpetual embrace happens when the aborted task (Task B) attempts to lock a row which is being held by another task (Task A), which, in-turn, is waiting for task B to release a lock. To prevent a perpetual wait, Oracle aborts the transaction that caused the deadlock.

The select for update is not a good locking strategy because there are many things that can go wrong.  Instead of select for update, savvy Oracle developers will adopt alternatives mechanisms like a re-read upon update commit where the transaction re-read the rows and ensure that it has not changed since the original read.

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