MEPX
Chapter 6 of 10All chapters
  1. 1Why a database
  2. 2Tables, rows and types
  3. 3Keys
  4. 4Relationships
  5. 5Normalisation
  6. 6Transactions
  7. 7Indexes
  8. 8When not to use tables
  9. 9Changing the schema
  10. 10Backups and growth

Chapter 6 of 10

Transactions

All or nothing, even under load.

ACID

A transaction is atomic, so it either fully happens or not at all. It leaves the data consistent, runs isolated from others, and once committed is durable through a crash.

  • Moving money is the classic case: both sides of the transfer or neither.
  • Isolation levels trade strictness against concurrency, and the default varies by database.

Concurrency

Two users editing the same row is normal, not exceptional. Optimistic locking with a version column is usually enough, and it beats silently overwriting someone's work.