week 10

Learning Outcomes / Objectives

By the end of this week, students should be able to:

1. Define a transaction and explain the ACID properties.

2. Describe the concurrency problems that arise when transactions execute simultaneously.

3. Explain lock-based and timestamp-based concurrency control protocols.

4. Describe transaction isolation levels and their trade-offs.

5. Explain the concept of database failure and recovery.

6. Describe log-based recovery and the ARIES algorithm concepts.

The four ACID properties guarantee reliable transaction processing in a DBMS:

ACID Property	Definition and Banking Example
ATOMICITY	A transaction is treated as a single indivisible unit — it either completes fully (COMMIT) or has no effect (ROLLBACK). In the bank transfer: if the credit to Ngozi fails, the debit from Amina is automatically rolled back.
CONSISTENCY	A transaction brings the database from one valid state to another valid state. All integrity constraints must be satisfied before and after. E.g., bank balances must never go negative (if that's a constraint), and total money in the system is conserved.
ISOLATION	Concurrent transactions execute as if they were serial (one at a time). One transaction's intermediate results are not visible to others. E.g., while the transfer is in progress, neither Amina nor Ngozi's balances should show half-updated values.
DURABILITY	Once a transaction is committed, its changes are permanent — even if the system crashes immediately after. The ₦10,000 transfer is permanent once COMMIT executes. A power cut cannot undo it.

Teaching and Learning Methods to be Employed