Definition of transaction a group of several reads and writes logical unit - looks like one operation all or nothing (commit or abort/rollback) Who needs transactions when you need certain guarantees Acid Atomicity - abortability (resiliant to faults) Consistency - some invariants about the data must be true Isolation - concurrent transactions are isolated serializability - too strong Durability - committed means data will not be lost there is no perfect guarantee Base Basically Available, Soft state, and Eventual consistency Single-Object vs Multi-Object Operations many writes atomicity : all written or none isolation : one transaction cannot see a partial write of another identify multi-object transaction tie to client’s TCP connection - bad separate transaction id Single object writes Storage engine atomicity : log for crash recovery isolation : using a lock atomic operations increment compare-and-set Multi-object transactions when do you need them?