Seven Databases in Seven Weeks, 2nd Edition

Eventual Consistency

Distributed databases must be partition tolerant, so the choice between availability and consistency can be difficult. However, while CAP dictates that if you pick availability you cannot have true consistency, you can still provide eventual consistency.

The idea behind eventual consistency is that each node is always available to serve requests. As a trade-off, data modifications are propagated in the background to other nodes. This means that at any time the system may be inconsistent, but the data is still largely accurate.

The Internet’s Domain Name Service (DNS) is a prime example of an eventually consistent system. You register a domain, and it may take a few days to propagate to all DNS servers across the Internet. ...