Concurrency Control:
----------------------

Each transaction must obtain a lock for each operation, read (shared
lock) to read an item and write (exclusive) lock to write an item.

Read/Shared locks: Multiple transactions can hold shared lock on the
same item


Write/Exclusive locks: Only one transaction can hold a write lock on a
specific item



2PL: Two phase locking:
-------------------------

Any transaction can be in only one of the two phases:

Growing phase: A transaction can get new locks. As soon as xact
releases a lock, then it enters shrinking phase.

Shrinking phase: A transaction can only release locks (but cannot get
new locks).


If a transaction manager implements two phase locking, then the resulting
schedules are serializable.


Proof: showing no cycles are possible using Two phase locking.
------------


Strict 2PL:

No shrinking phase: Get locks and keep them until commit time.

All schedules resulting from strict 2PL are also guaranteed to be
serializable.

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 shared_lock1(x) r1(x)  ....  upgrade_lock1(x) w1(x)