Editing Two-phase locking (section)

==Read and write locks==
Locks are used to guarantee [[serializability]]. A transaction is ''holding'' a [[Lock (computer science)|lock]] on an object if that transaction has acquired a lock on that object which has not yet been released.

For 2PL, the only used data-access locks are '''read-locks''' ('''shared locks''') and '''write-locks''' ('''exclusive locks'''). Below are the rules for ''read-locks'' and ''write-locks'':

* A transaction is allowed to read an object if and only if it is holding a ''read-lock'' or ''write-lock'' on that object.
* A transaction is allowed to write an object if and only if it is holding a ''write-lock'' on that object.
* A [[Database transaction schedule|schedule]] (i.e., a set of transactions) is allowed to hold multiple locks on the same object simultaneously if and only if none of those locks are write-locks. If a disallowed lock attempts on being held simultaneously, it will be blocked.

{| class="wikitable" style="text-align:center;"
|+Lock compatibility table
|-
! Lock type !! read-lock  !! write-lock
|-
! read-lock
| '''✔''' || '''X'''
|-
! write-lock
| '''X''' || '''X'''
|}