Eventual consistency in plain English

Question

I often hear about eventual consistency in different speeches about NoSQL, data grids etc. It seems that definition of eventual consistency varies in many sources (and mayb

Answer 1

Eventual consistency is more like a spectrum. On one end you have strong consistency and on other you have eventual consistency. In between there are levels like Snapshot, read my writes, bounded staleness. Doug Terry has a beautiful explanation in his paper on eventual consistency thru baseball .

As per me eventual consistency is basically toleration to random data in random order every time you read from a data store. Anything better than that is a stronger consistency model. For example, a snapshot has stale data but will return same data if read again so it is predictable. Sometimes application can tolerate data which is stale for a given amount of time beyond which it demands consistent data.

If you look at meaning of consistency it relates more to uniformity or lack of deviation. So in non computer system terms it could mean toleration for unexpected variations. It could be very well explained thru ATM. An ATM could be offline hence divergent from account balance from core systems. However there is a toleration for showing different balances for a window of time. Once the ATM comes online, it can sync with core systems and reflect same balance. So an ATM could be said to be eventually consistent.