Transactions and Isolation

Start with the idea

Atomicity, consistency, and durability are easy to state. Isolation is a dial: read committed still allows non-repeatable reads, repeatable read still allows phantoms in some engines, and serializable costs the most because it must behave as if transactions ran one at a time. Naming your database default is what separates a real answer from a memorised one.

ACID is a set of transaction properties, not a promise that every concurrent read sees the same snapshot. Atomicity means the transaction's changes commit together or are rolled back together under the database's contract. Consistency means the declared constraints and application invariants remain valid across a committed transaction. Isolation describes how concurrent transactions interact; durability describes what survives an acknowledged commit according to the storage system's guarantees. Each property depends on the database and configuration in use.

Use a concrete isolation example.