Transactions and concurrency

Vocabulary

All or nothing

A transaction is all-or-nothing, and ACID names the four promises: atomicity (all or nothing), consistency (constraints keep true), isolation (interleaving looks serial), durability (commits survive the crash).

{exercise:uni-cs0805-acid}

Practice

The A in ACID stands for —

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T1 reads 100, T2 reads 100, T1 writes 90, T2 writes 120: the 10-unit withdrawal vanished — the same race as unit 7.3, now with money.
R1(A) R2(A) W2(A) W1(A) ends 90 with T2's deposit lost; R1(A) W1(A) R2(A) W2(A) ends 110 — equal to serial T1-then-T2. Serializability means: equal to some serial order.

{exercise:uni-cs0805-schedule}

Practice

R1(A) R2(A) W2(A) W1(A) from 100 with -10/+20 ends —

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Two-phase locking: reads and writes lock, locks held to commit — one transaction waits and the outcome equals a serial order. Deadlock is the price: the detector kills a victim, which retries.
Isolation levels name the anomalies you accept: read-uncommitted allows dirty reads (T2 sees T1's uncommitted 90 that later rolls back); read-committed removes them; repeatable-read stabilises re-reads; serializable removes phantoms. Keep transactions short — locks held through user input are deadlock bait.

{exercise:uni-cs0805-levels}

Practice

Reading another transaction uncommitted rollback requires fixing to at least —

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