the.com/data consistency
every copy of the truth agrees, even when it's lying in five places at once.
means the guarantee that data stays accurate and matches across all the places it's stored or read, no matter who's writing or reading it.
from grew out of database theory in the 1970s, formalized as the C in ACID by Jim Gray's transaction work, then got messier once systems went distributed and CAP theorem made everyone pick their poison.
CAP tradeoffyou cannot have consistency, availability, and partition tolerance all at once
eventual vs strongeventual just means consistent later, not never
bank testyour balance should never show two different numbers on two atms
coststronger consistency usually means slower writes
for instance
google spanner — uses atomic clocks and gps to fake global strong consistency, 2012
amazon dynamodb — defaults to eventual consistency, offers strong reads for a price
postgresql mvcc — gives each transaction a consistent snapshot since the 1990s
cassandra tunable — lets you pick consistency level per query, quorum or bust