AQA 7517 4.10.5: Client server databases
The free BugBot lessons that teach this point, a robot to try it on, and the past-paper questions that test it.
What it covers
A database change is often several statements that only make sense together. When BugBot carries two balls from the red zone to the blue zone, the database must add 2 to blue and take 2 from red. If the power fails between the two, two balls have appeared from nowhere. This lesson is about transactions, the way a DBMS makes sure that never happens, and the four properties, ACID, that a reliable transaction must have.
- What a transaction is
- Half a change
- ACID
- Consistency: rules the database enforces
- A transaction around a physical action
- Record locking and redundancy
Or do it in the simulator, with tasks that mark themselves
Lessons that teach it
- A11.6 Transactions and ACID Databases and big data
A transaction is all or nothing: atomicity, consistency, isolation and durability, commit and rollback, and redundancy. - A11.7 Client server databases and concurrent access Databases and big data
Many clients, one database: the lost update, and the four ways to prevent it: record locks, serialisation, timestamp ordering and…
Try it on a robot
The task from A11.6 Transactions and ACID, running here. Change the program and press Run.
The specification is AQA's own document; this page says which of our lessons teach point 4.10.5 and is not endorsed by the board.