DBMS Permanency: Understanding ACID Properties Quiz and Flashcards

Study Notes

Desirable Properties of Transactions

Atomicity: a transaction is performed in its entirety or not at all
Consistency preservation: takes database from one consistent state to another
Isolation: not interfered with by other transactions
Durability or permanency: changes must persist in the database

Transaction States

The Lost Update Problem

If T1 transfers 5 seat reservations from X to Y and T2 reserves 4 seats on X, the final result should be X = 79, but in the interleaving of operations, X = 84 because the update in T1 that removed the five seats from X was lost

The Temporary Update Problem

The Incorrect Summary Problem

The Unrepeatable Read Problem

Transaction T reads the same item twice, value is changed by another transaction T between the two reads, T receives different values for the two reads of the same item

Concurrency Control Based on Timestamp Ordering

Strict Timestamp Ordering:
- If TS(T) > read_TS(X), then delay T until the transaction T’ that wrote or read X has terminated
- If TS(T) > write_TS(X), then delay T until the transaction T’ that wrote or read X has terminated
Thomas’s Write Rule:
- If read_TS(X) > TS(T) then abort and roll-back T and reject the operation
- If write_TS(X) > TS(T), then just ignore the write operation and continue execution
- Else, execute write_item(X) of T and set write_TS(X) to TS(T)

Guaranteeing Serializability by Two-Phase Locking

Two-phase locking protocol:
- Expanding (growing) phase: new locks can be acquired but none can be released, lock conversion upgrades must be done during this phase
- Shrinking phase: existing locks can be released but none can be acquired, downgrades must be done during this phase
The two phases must be mutually exclusive

Variations of Two-Phase Locking

Basic 2PL: technique described above
Conservative (static) 2PL: requires a transaction to lock all the items it accesses before the transaction begins
Strict 2PL: transaction does not release exclusive locks until after it commits or aborts
Rigorous 2PL: transaction does not release any locks until after it commits or aborts

Deadlock and Starvation

Deadlock: occurs when each transaction T in a set is waiting for some item locked by some other transaction T’
Starvation: occurs if a transaction cannot proceed for an indefinite period of time while other transactions continue normally
Solution: first-come-first-served queue

Description

Learn about the desirable properties of transactions in a database management system, including atomicity, consistency preservation, isolation, and durability. Understanding these ACID properties is crucial for ensuring data integrity.