Agreement across a system.
WHAT STRONG CONSISTENCY MEANS
Every read returns the most recent write.
WHAT EVENTUAL CONSISTENCY MEANS
Reads may return stale data, converging over time.
WHY THAT TRADE-OFF EXISTS
In a distributed system, when parts cannot communicate, you must choose between remaining consistent and remaining available.
WHAT THAT MEANS PRACTICALLY
You cannot have both during a network partition.
WHAT REQUIRES STRONG CONSISTENCY
Money Inventory, where overselling matters Anything where a stale read causes a wrong decision
WHAT TOLERATES EVENTUAL CONSISTENCY
Counts and statistics Feeds and timelines Search results Most read-heavy content
WHAT TRANSACTIONS PROVIDE
A group of changes that all succeed or all fail.
WHY THAT MATTERS
Otherwise a partial failure leaves data in an invalid state.
WHAT TO BE CAREFUL WITH
Transactions spanning services, which is far harder than within one database.
WHAT TO PREFER
Designing so that a transaction is needed within one boundary only.
WHAT TO DECIDE EXPLICITLY
Which data requires which guarantee.
