what’s acid?

notes from Martin Kleppman’s amazing “Designing Data-Intensive Applications” book.

ACID is about safety guarantees for database transactions.

Atomicity

NOT about concurrent writes, that’s “isolation”

application: do these 5 writes
atomic DB: omg there was an error in the middle, rolling them all back!

Consistency

super overloaded term. This sense of “consistency” is actually an application property not a DB property.

not linearizability
not as in “eventual consistency”

About preserving application invariants like “every sale gets an invoice”

Isolation

app 1: I’m selling a watch
app 2: I’m selling the same watch

Isolation is about preventing rare conditions like this.

Some isolation levels:

• serializability
• snapshot isolation
• read committed

Durability

Durable DB: I committed your writes
app: phew my data won’t get lost even if the DB crashes/there’s a hardware failure

Perfect durability doesn’t exist.

Can involve:

• write-ahead log (usually)
• replication