Patterns of Distributed Systems

Chapter 3

Write-Ahead Log

Provide durability guarantee without the storage data structures to be flushed to disk, by persisting every state change as a command to the append only log.

Also known as: Commit Log

Problem

Strong durability guarantee is needed even in the case of the server machines storing data failing. Once a server agrees to perform an action, it should do so even if it fails and restarts losing all of its in-memory state.

Solution

Store each state change as a command in a file on a hard disk (Figure 3.1). A single log is maintained for each server process. It is sequentially appended. A single log, appended sequentially, simplifies handling of logs at restart and the subsequent online operations (when the log is appended with ...