Concurrency on index structures. The Problem Maintain isolation. Produce serializable schedules. For now, ignore recovery.
Locks restrict access to objects.
Used for deletes and inserts as well. If lock is compatible with all existing locks, lock can be granted. If lock conflicts with any existing lock, requester either waits or obtains an error return. Maintaining Serializability Not always OK to release a lock when you are done with the object. T1 could acquire the lock on A. T2 could then acquire the lock on B. T1 will now wait until T2 completes.