There is often confusion as to how it can be claimed that MySQL Cluster delivers in-memory performance while also providing durability (the “D” in ACID). This post explains how that can be achieved as well as how to mix and match scalability, High Availability and Durability.

MySQL Cluster deployment options

As an aside, the user can specify specific MySQL Cluster tables or columns to be stored on disk rather than in memory – this is a solution for extra capacity but you don’t need to take this performance hit just to have the data persisted to disk. This post focuses on the in-memory approach.

There is a great deal of flexibility in how you deploy MySQL Cluster with in-memory data – allowing the user to decide which features they want to make use of.

The simplest (and least common) topology …

Until now PBXT has been ACId (with a lower-case d). This is soon to change as I have had some weeks to work on a fully durable version of the transactional engine (http://www.primebase.com/xt).

My first concern in making PBXT fully durable was to what extent I would have to abandon the original "write-once" design. While there are a number of ways to implement durability, the only method used by databases (as far as I know) is the write-ahead log.

The obvious advantage of this method is that all changes can be flushed at once. However, this requires that all data be written twice: once to the log and after that, to the database itself.

My solution to this problem is a compromise, but I think it is a good one. In a nutshell: short records are written twice, and long records are written once. When it comes to durability, this compromise, I …