Patterns of Distributed Systems

Problem

In a distributed system, nodes need to be able to tell which value for a key is the most recent. Sometimes they need to know past values so they can react properly to changes in a value.

Solution

Store a version number with each value. The version number is incremented for every update. This allows every update to be converted to a new write without blocking a read. Clients can read historical values at a specific version number.

Consider a simple example of a replicated key-value store. The leader of the cluster handles all the writes to the key-value store. It saves the write requests in a Write-Ahead Log. The write-ahead ...