Strict 2pl

Strict 2PL: Ensuring Database Consistency Through Serializability

Database concurrency control is crucial for managing simultaneous access to data while maintaining data integrity. One prominent technique used to achieve this is Strict Two-Phase Locking (Strict 2PL), a powerful but potentially performance-limiting method that guarantees serializability. This article will delve into the intricacies of Strict 2PL, exploring its mechanisms, advantages, disadvantages, and practical applications.

Understanding Two-Phase Locking (2PL)

Before delving into Strict 2PL, it's essential to understand its predecessor: Two-Phase Locking (2PL). 2PL is a concurrency control method that employs locks to manage access to data items. Each transaction follows two phases:

1. Growing Phase: The transaction acquires locks on data items as needed. It cannot release any locks during this phase.
2. Shrinking Phase: Once the transaction has acquired all necessary locks, it performs its operations. After this, it releases all its locks. No new locks can be acquired during this phase.

This two-phase structure prevents deadlocks – situations where two or more transactions are blocked indefinitely, waiting for each other to release locks.

Strict 2PL: Enhancing 2PL with Strictness

Strict 2PL takes the basic 2PL concept and adds a crucial constraint: all locks held by a transaction are released only after the transaction commits or aborts. This "strictness" eliminates the possibility of cascading rollbacks and ensures serializability.

Let's illustrate with an example. Consider two transactions, T1 and T2, operating on two data items, A and B:

| Transaction | Operation | Data Item |
|---|---|---|
| T1 | Read | A |
| T1 | Write | A |
| T2 | Read | A |
| T2 | Write | B |

In a standard 2PL, T1 could release its lock on A before committing. If T2 reads A after T1 releases its lock but before T1 commits, and T1 subsequently aborts, T2's read of A would be based on an invalid value. This requires T2 to also rollback, creating a cascading rollback.

Strict 2PL prevents this. T1 holds its lock on A until it commits or aborts. This ensures that any other transaction attempting to access A during T1's execution will be blocked until T1 finishes, maintaining data consistency.

Guaranteeing Serializability

The primary advantage of Strict 2PL is its guarantee of serializability. Serializability ensures that the concurrent execution of multiple transactions produces the same result as if they were executed sequentially in some order. This is crucial for maintaining database integrity and consistency, preventing anomalies that could occur with less restrictive concurrency control methods. The strict holding of locks until commit or abort ensures that the execution effectively mimics a sequential execution, fulfilling the serializability requirement.

Disadvantages of Strict 2PL

While Strict 2PL offers strong guarantees, it comes with performance trade-offs:

Increased Concurrency Restrictions: The strict holding of locks can lead to increased blocking and reduced concurrency. Transactions might have to wait longer for locks to be released, potentially impacting overall system throughput.
Potential for Deadlocks: Although less likely than in less restrictive locking schemes, deadlocks are still possible if transactions are not carefully designed and managed.
Higher Overhead: The mechanisms required to enforce strict locking add overhead to the database system.

Practical Applications of Strict 2PL

Strict 2PL is often used in applications demanding high data integrity, where the cost of cascading rollbacks outweighs the performance penalty. Examples include:

Financial Transactions: Ensuring the accuracy of financial records is paramount. Strict 2PL helps prevent inconsistencies in account balances due to concurrent transactions.
Inventory Management: Tracking inventory levels requires consistent data. Strict 2PL helps prevent discrepancies arising from simultaneous updates.
Airline Booking Systems: Preventing double booking of seats requires a high degree of data consistency, making Strict 2PL a suitable choice.

Summary

Strict 2PL is a robust concurrency control method guaranteeing serializability by strictly holding locks until transaction completion. This eliminates cascading rollbacks and ensures data consistency, making it suitable for applications requiring high data integrity. However, its strictness can lead to reduced concurrency and performance overhead, necessitating careful consideration of its trade-offs. The choice of whether to use Strict 2PL depends on the specific requirements of the application, balancing the need for data integrity with performance considerations.

Frequently Asked Questions (FAQs)

1. What is the difference between Strict 2PL and 2PL? The key difference lies in when locks are released. In 2PL, locks can be released before the transaction commits, potentially leading to cascading rollbacks. Strict 2PL holds locks until the transaction commits or aborts, preventing this.

2. Can deadlocks occur with Strict 2PL? Yes, although less frequently than with less restrictive locking schemes. Deadlocks can occur if there is a circular dependency between transactions waiting for each other to release locks.

3. Is Strict 2PL always the best choice for concurrency control? No. While it guarantees serializability, it can impact performance due to increased blocking and overhead. Other methods like optimistic concurrency control might be more suitable for applications where performance is paramount and the risk of data inconsistency is lower.

4. How does Strict 2PL handle updates? Strict 2PL would acquire exclusive locks on updated data items, preventing other transactions from accessing or modifying them until the updating transaction commits or aborts.

5. What are some alternatives to Strict 2PL? Alternatives include standard 2PL (less strict), optimistic concurrency control, multiversion concurrency control (MVCC), and timestamp ordering. Each offers a different trade-off between concurrency and consistency.

Search Results:

Lecture #16: Two-Phase Locking - CMU 15-445/645 Two-Phase locking (2PL) is a pessimistic concurrency control protocol that uses locks to determine whether a transaction is allowed to access an object in the database on the fly. The protocol does not need to know all of the queries that a transaction will execute ahead of time.

CS 186 Spring 2016 Midterm 2 Solutions I. Locking and Serializability Note that the converse is true 2PL guarantees conflict serializability. d. Schedules produced by twophase locking are guaranteed to prevent cascading aborts. False. Strict 2PL is needed to guarantee this. e. Strict twophase locking is both necessary and sufficient to …

Strict two-phase locking (2PL) - Vanderbilt University In cases where transactions involve the same objects, Strict 2PL can radically limit opportunities for parallelism/interleaving …. But Strict 2PL makes interleaving safe, and the good news is that in practice, there are many transactions that do not involve the same objects and that can be interleaved to improve throughput

Database Concurrency Control and Recovery - University of … Usually, some form of two-phase locking ( 2PL ) is used: 1. Non-strict 2PL: a) phase of acquiring locks: locks are acquired as the objects are needed. b) phase of releasing locks: once all locks have been acquired, locks are released when the object operations complete. - ensures a serialisable execution schedule

Concurrency Control - University of Wisconsin–Madison Review: Strict 2PL Strict Two-phase Locking (Strict 2PL) Protocol: Each Xact must obtain a S (shared) lock on object before reading, and an X (exclusive) lock on object before writing. All locks held by a transaction are released when the transaction completes If an Xact holds an X lock on an object, no other

Lecture #16: Two-Phase Locking - CMU 15-445/645 Two-Phase locking (2PL) is a pessimistic concurrency control protocol that uses locks to determine whether a transaction is allowed to access an object in the database on the ﬂy. The protocol does not need to know all of the queries that a transaction will execute ahead of time.

CSE 444: Database Internals - University of Washington –Strict 2PL •“Short duration” READ locks –Only acquire lock while reading (not 2PL) CSE 444 -Winter 2019 52 Unrepeatable reads When reading same element twice, may get two different values 3. Isolation Level: Repeatable Read •“Long duration” WRITE locks –Strict 2PL •“Long duration” READ locks –Strict 2PL CSE 444 ...

Strong consistency is not hard to get: Two-Phase Locking and … We focus on strong consistency in the form of strict serializability implemented through strict 2PL, but also explore read-committed and repeatable-read, two common isolation levels used in many systems. We do not leverage any known optimizations in the locking or commit parts of …

Lecture 22: Intro to 2. Conflict Serializability, Locking Transactions ... Strict Two‐Phase Locking •We consider locking‐specifically, strict two‐phase locking‐as a way to deal with concurrency, because is guarantees conflict serializability (if it completes‐see upcoming…) •Also (conceptually) straightforward to implement, and transparent to the user!

13 Two Phase Locking - databaselecture.com • Strict 2PL: release all locks at transaction end • Can also combine the two (conservative strict 2PL) • Plain 2PL makes no restrictions on locking periods

Lecture #17: Two-Phase Locking Two-Phase locking (2PL) is a pessimistic concurrency control protocol that uses locks to determine whether a transaction is allowed to access an object in the database on the ﬂy. The protocol does not need to know all of the queries that a transaction will execute ahead of time.

Lecture 6: Isolation vs. Strict Isolation, 2-Phase Locking (2PL), … • Two ways: Strict Isolation and Non-Strict Isolation – Strict Isolation: guarantee we never experience lost updates, dirty reads, or unrepeatable reads – Non-Strict Isolation: let transaction continue to execute despite potential problems (i.e., more optimistic) • Non-strict isolation usually allows more concurrency but

CSE 444: Database Internals - University of Washington You need to implement strict two-phase locking transactions should acquire the appropriate type of lock on any object before accessing that object transaction shouldn’t release any locks until after the transaction commits.

CSE 444 Practice Problems Transactions: Concurrency Control (b) Show that strict 2PL disallows this schedule. Solution: Strict 2PL has two two rules: i. If a transaction T wants to read (respectively, modify) an object, it rst requests a shared (respectively, exclusive) lock on the object. ii. All locks held by a transaction …

Two-Phase Locking (2PL) - Kent transaction acquires a lock when it needs to access a data object. If it releases the lock after that access, but before the transactions ends, data could become visible to other transactions.

Strict 2 Phase Locking in Organizational Data Protection - IJERA The strict two-phase locking (S2PL) class of schedules is the intersection of the 2PL class with the class of schedules possessing the Strictness property. To comply with the S2PL protocol a transaction needs to comply with 2PL, and release its write (exclusive) locks only after it has ended, i.e., being either committed or aborted.

Lecture #17: Two-Phase Locking Strong Strict 2PL (SSPL, also known as Rigorous 2PL) is a variant of 2PL where the transaction only re-leases locks when it finishes. A schedule is strict if a value written by a transaction is not read or overwritten by other transactions until that transaction finishes. Thus, there is not a shrinking phase in SS2PL like in regular 2PL.

Lecture #17: Two-Phase Locking Two-Phase locking (2PL) is a pessimistic concurrency control protocol that uses locks to determine whether a transaction is allowed to access an object in the database on the fly. The protocol does not need to know all of the queries that a transaction will execute ahead of time.

Review of Schedules Scheduler - University of Washington Strict 2PL • Strict 2PL: All locks held by a transaction are released when the transaction is completed; release happens at the time of COMMIT or ROLLBACK • Schedule is recoverable • Schedule avoids cascading aborts • Schedule is strict: read book CSE 444 -Spring 2016 20 Strict 2PL T1 T2 L1(A); READ(A) A :=A+100 WRITE(A); L2(A); DENIED…

Lecture 13: Two Phase Locking - gatech.edu Strong Strict Two-Phase Locking • A schedule is strict if a value written by a txn is not read or overwritten by other txns until that txn finishes. • Advantages: Does not incur cascading aborts. Aborted txns can be undone by just restoring original values of modified tuples.