ACID transactions

Atomicity, Consistency, Isolation, Durability: a transaction happens entirely or not at all, leaves data valid, does not see half-finished work of others and survives a crash once committed.

Where it shows up in real systems

  • Relational database

    The primary handles every write; read replicas add read capacity but lag slightly behind, so a user may not see their own write. A Multi-AZ standby takes over in about a minute if the primary fails. Sharding multiplies write capacity, but a bad shard key creates a hot shard. Without connection pooling, many app instances exhaust the connection limit.

    In production: PostgreSQL (Amazon RDS, Aurora), MySQL, Azure SQL Database, Cloud SQL, Citus, Vitess

See it break, then fix it

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