Data models and transactions

Decide what a row means and who may change it

Several people can read the same bookmark while each has their own read/unread state. Two buyers can also race for the last item in stock. These examples show why a correct-looking table and a transaction keyword do not by themselves enforce the product’s rules.

Model identity and access patterns first. Then inspect query plans and drive conflicting writes in two PostgreSQL sessions. Keep the supplied PostgreSQL lab separate from the starter API’s SQLite database. Explain both returned rows and rejected operations.