# Concurrency Control

Process of managing simultaneous operations on the database without interference.
Prevents interference when >2 users access and update the db

# Lost Update Problem

A successful update is overwritten by another user - a read operation of a tuple happens at the same time as a value update of the tuple. This causes the next value update to access the incorrect value, as the read value is incorrect too.
This would be avoided if any reads are prevented until all updates are completed.

# Uncommitted Dependency Problem

A transaction reads the value of a tuple before another transaction is completed / committed. If the 2nd transaction aborts / rolls back, the value read by the 1st is inconsistent.
This would be avoided by preventing the 1st transaction from reading until the 2nd has completed ( commit or abort ).

# Inconsistent Analysis Problem

A transaction reads several values, a second transaction updates some of them during the execution of the first. This creates inconsistent values across the effected tuples.
This would be avoided by preventing reads of targeted tuples until the first transaction has committed or aborted updates.

# Conflicting Actions - General Rules

- Two transactions do not conflict if they:
	- Are only reading data items
	- Operate on completely different data items
- Two operations conflict if all of these are true:
	- Belong to different transactions
	- Access same data item
	- At least on writes the item