Eventual Consistency - Revisited

Werner Vogels, the CTO of Amazon, wrote a fascinating piece on eventual consistency. If you recall, consistency, is one the three properties of the CAP theorem. In his post Werner discusses an ideal world where there is only a single consistency model:

when an update is made all observers would see that update

Werner provides us with an historical perspective of how folks discovered that this was actually something that was pretty hard to do. Werner dives in more and discusses how consistency is viewed from the clients perspective and from the server perspective.

Werner, describes eventual consistency as:

a specific form of weak consistency; the storage system guarantees that if no new updates are made to the object, eventually all accesses will return the last updated value. If no failures occur, the maximum size of the inconsistency window can be determined based on factors such as communication delays, the load on the system, and the number of replicas involved in the replication scheme.

There are more details about various types of weak consistencies and a brief mention of Amazon's Dynamo key/value data store. It's a great read.

Eventually Consistent - Revisited