Concurrency control protocols ensure transactions executed simultaneously in a database system uphold the ACID properties of atomicity, consistency, isolation, and durability. There are two main categories of concurrency control protocols: lock-based and timestamp-based. Lock-based protocols use shared and exclusive locks to control access to data, following a two-phase locking protocol where locks are acquired in a growing phase and released in a shrinking phase. Timestamp-based protocols order transactions based on assigned timestamps to determine a serializable execution schedule.

1. Concurrency Control
K.Kasthuri

2. ￮ Concurrency is a multiprogramming
environment where multiple
transactions can be executed
simultaneously.
￮ Concurrency control to control the
concurrency of transactions.
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3. ￮ Concurrency control protocols to ensure
the ACID properties.
￮ The ACID properties are atomicity,
consistency, isolation, and durability of
concurrent transactions
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4. ￮ Concurrency control protocols can be
broadly divided into two categories
1. Lock based protocols
2. Time stamp based protocols
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5. 1. Lock-based Protocols
￮ Database systems equipped with
lock-based protocols use a
mechanism by which any transaction
cannot read or write data until it
acquires an appropriate lock on it.
Locks are of two kinds
1. Shared Lock
2. Exclusive Lock
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6. 1. Shared Lock (S):
• It is also known as Read-only lock.
• As the name suggests it can be shared
between transactions because while holding
this lock the transaction does not have the
permission to update data on the data item.
• S-lock is requested using lock-S instruction.
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7. 2. Exclusive Lock (X):
o Data item can be both read as well as written.
o This is Exclusive and cannot be held
simultaneously on the same data item.
o X-lock is requested using lock-X instruction.
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8. Lock Compatibility Matrix –
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• A transaction may be granted a lock on an item if the requested lock is
compatible with locks already held on the item by other transactions.

9. ￮ Two Phase Locking –
￮ A transaction is said to follow Two Phase
Locking protocol if Locking and Unlocking can
be done in two phases.
￮ 1. Growing Phase
￮ 2. Shrinking Phase
￮ 1. Growing Phase: New locks on data items may
be acquired but none can be released.
￮ 2. Shrinking Phase: Existing locks may be
released but no new locks can be acquired.
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10. Timestamp Ordering Protocol
o The main idea for this protocol is to order the
transactions based on their Timestamps.
o A schedule in which the transactions participate is
then serializable and the only equivalent serial
schedule permitted has the transactions in the
order of their Timestamp Values.
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