Patterns of Distributed Systems

Chapter 18

Version Vector

Maintain a list of counters, one per cluster node, to detect concurrent updates.

Problem

If multiple servers allow the same key to be updated, it’s important to detect when the values are concurrently updated across a set of replicas.

Solution

Each key value is associated with a version vector that maintains a number for each cluster node.

In essence, a version vector is a set of counters, one for each node. A version vector for three nodes (blue, green, black) would look something like [blue: 43, green: 54, black: 12]. Each time a node has an internal update, it updates its own counter, so an update in the green node would change the vector to [blue: 43, green: 55, black: 12]. Whenever two nodes communicate, they ...