- Berkeley DB Reference Guide:
- Transaction Protected Applications
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The first component of the infrastructure, deadlock detection, is not so much a requirement specific to transaction protected applications, but rather is necessary for almost all applications where more than a single thread of control will be accessing the database at one time. While Berkeley DB automatically handles database locking, it is normally possible for deadlock to occur. It is not required by all such applications, for example read-only applications cannot deadlock.
When the deadlock occurs, what happens is that two (or more) threads of control holding locks request other locks that cannot be granted, and will never be granted because one of the threads of control waiting holds the requested resource.
For example, consider two processes A and B. Let's say that A obtains an exclusive lock on item X, and B obtains an exclusive lock on item Y. Then, A requests a lock on Y and B requests a lock on X. A will wait until resource Y becomes available and B will wait until resource X becomes available. Unfortunately, since both A and B are waiting, neither will release the locks they hold and neither will ever obtain the resource on which it is waiting. In order to detect that deadlock has happened, a separate process or thread must review the locks currently held in the database. If deadlock has occurred, a victim must be selected, and that victim will then return the error DB_LOCK_DEADLOCK from whatever Berkeley DB call it was making.
Berkeley DB provides a separate UNIX-style utility which can be used to perform this deadlock detection, named db_deadlock. Alternatively, applications can create their own deadlock utility or thread using the underlying lock_detect function, or specify that Berkeley DB run the deadlock detector internally whenever there is a conflict over a lock (see DBENV->set_lk_detect for more information).
Deciding how often to run the deadlock detector and which of the deadlocked transactions will be forced to abort when the deadlock is detected is a common tuning parameter for Berkeley DB applications.
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