Database principle--Transaction concurrency control

transactional concurrency : means that transactions can be executed concurrently at the same time

Problems that exist:

1, loss of modification (T1 and T2 simultaneously modified, T2 results will be T1 results covered)

2, read dirty data (T1 modified, T2 read, but T1 undo modification)

3, cannot repeat reads (T1 before and after reading the data different)

Concurrency control:

lock : Mutex (X), shared Lock (S), U lock

2-Segment Lock protocol : In two stages, the first stage can only apply for the lock, the second stage can only release the lock

Note : The 2-segment lock is not the same as a one-time block, locking all data at once, so there may be a deadlock

Database principle--Transaction concurrency control