CodeParkACID stands for Atomicity, Consistency, Isolation and Durability. Take a transfer that debits one account and credits another.
Atomicity: the transaction happens completely or not at all. If the credit fails after the debit, the database rolls back the debit.
Consistency: a transaction moves the database from one valid state to another. After commit, constraints still hold, such as:
- foreign keys
CHECK (balance >= 0)- uniqueness
Business rules that are not declared as constraints are the application's job.
Isolation: concurrent transactions do not see each other's unfinished work. How strict this is depends on the isolation level, from Read Committed to Serializable.
Durability: once COMMIT returns, the transfer survives a crash. PostgreSQL does this by writing the write-ahead log (WAL) to disk before it reports the commit. The WAL is a log of every change.
- Atomicity means all or nothing.
- Consistency means constraints still hold after commit.
- Isolation means concurrent transactions do not interfere, as far as the isolation level allows.
- Durability means committed data survives a crash, thanks to the WAL.