CS4432: Database Systems II

CS4432: Database Systems II

Concurrency ControlEnforcing Serializability: Locking

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Enforcing Serializable Schedules

• DBMSs use a “Scheduler” that schedules the actions of transactions

• Transactions send their requests (R or W) to Scheduler • The scheduler prevents the formation of cycles

– It grants permission to R or W only if no cycle will be formed

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Locking Protocol

• “Scheduler” uses a locking protocol to enforce serializability

• Two New actions– Lock (exclusive): li(A) Transaction Ti locks item A

– Unlock: Ui(A) Transaction Ti unlocks (releases) item A

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locktable

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Rule #1: Well-Formed Transactions

Ti: … li(A) … pi(A) … ui(A) ...

Any action (R/W) must be after the lock (l) and before the unlock (u)

Rule 1 is at the level of each transaction independent of the others

Rule 1 is at the level of each transaction independent of the others

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Rule #2 Legal Scheduler

S = …….. li(A) ………... ui(A) ……...

no lj(A)

No transaction Tj can lock item A that is already locked by another transaction Ti(Transaction Tj must wait until Ti releases its lock)

Rule 2 is at the level of the complete schedule (Set of interleaving transactions)

Rule 2 is at the level of the complete schedule (Set of interleaving transactions)

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Schedule F: Let’s Add Some Locking!

Does the locking mechanism working? Does it guarantee serializable schedule??

Does the locking mechanism working? Does it guarantee serializable schedule??

Still Something is Missing…

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Still by applying the locks….results is not consistent !!!

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Rule #3 Two Phase Locking (2PL)

Ti = ……. li(A) ………... ui(A) ……...

No unlocks No locks

A transaction never start unlocks until it takes all its locks. In other words: Take all locks before the first unlock

Rule 3 is for each transaction…Rule 3 is for each transaction…

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2 Phase Locking Protocol

# of locks held by a transaction

Growing Phase Shrinking Phase

Collect locksRelease locks

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Schedule F : Does it follow 2PL ?

Both transactions violate 2PL

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Example 2PL: Schedule G

l2(B)

Scheduler will deny this request(T2 will wait…)

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Example 2PL: Schedule G

l2(B)

Scheduler will deny this request(T2 will wait…)

At this point the scheduler can resume T2

T2 Continue processing

Scheduler Forced Schedule G to be Serializable

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Theorem

• Any legal schedule S following the 2PL Is a Conflict Serializable schedule

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Most DBMSs use two-phase locking (2PL) to enforce concurrency

Most DBMSs use two-phase locking (2PL) to enforce concurrency

There is a room for improvement

Improvement 1: Handling Deadlocks

• 2PL protocol does not prevent deadlocks– Transactions waiting for each other indefinitely

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l1(B)

T1 has to wait for T2

l2(A)

T2 has to wait for T1

Deadlock situation

Handling Deadlocks

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• Detection– Wait-for graph

• Prevention– Resource ordering– Timeout– Wait-die– Wound-wait

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Deadlock Detection• Build Wait-For graph

– Ti Tj: means Ti is waiting for Tj

• Use lock table structures• Built incrementally by the scheduler• When cycle found, rollback victim

Cycle (Deadlock)

Select one among T1, T2, and T3 to kill (rollback) A good choice is “the most recent one”

Deadlock Prevention: Timeout

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• If transaction waits more than L sec., roll it back!

• Pros: Simple scheme• Cons: Hard to select L

Check the other techniques in Textbook for your knowledge

Check the other techniques in Textbook for your knowledge

Improvement 2: Avoiding Cascade Rollback

• 2PL protocol does not prevent dirty reads

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commit

Rollback

Now T2 is based on deleted transaction

To avoid this situation, we use Strict Two-Phase Locking (S2PL)

To avoid this situation, we use Strict Two-Phase Locking (S2PL)

Strict 2PL (S2PL)

• A transaction releases its lock only at commit or rollback time

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# of locks held by a transaction

Growing Phase Shrinking Phase

Collect locksRelease locks

Commit or rollback event

S2PL: Example

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U1(A)

Unlock A has to wait until the end (even if T1 is done

processing A)

Summary of Concurrency Control

• How to manage multiple transactions at the same time and still ensure consistency

• Schedules (order of actions)– Serial, Conflict Serializable, Legal

• Precedence Graphs for Conflict Serializable

• Locking Protocol – 2PL , S2PL, Handling of deadlocks

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