Simon Willison’s Weblog

DSQL Vignette: Reads and Compute. Marc Brooker is one of the engineers behind AWS's new Aurora DSQL horizontally scalable database. Here he shares all sorts of interesting details about how it works under the hood.

The system is built around the principle of separating storage from compute: storage uses S3, while compute runs in Firecracker:

Each transaction inside DSQL runs in a customized Postgres engine inside a Firecracker MicroVM, dedicated to your database. When you connect to DSQL, we make sure there are enough of these MicroVMs to serve your load, and scale up dynamically if needed. We add MicroVMs in the AZs and regions your connections are coming from, keeping your SQL query processor engine as close to your client as possible to optimize for latency.

We opted to use PostgreSQL here because of its pedigree, modularity, extensibility, and performance. We’re not using any of the storage or transaction processing parts of PostgreSQL, but are using the SQL engine, an adapted version of the planner and optimizer, and the client protocol implementation.

The system then provides strong repeatable-read transaction isolation using MVCC and EC2's high precision clocks, enabling reads "as of time X" including against nearby read replicas.

The storage layer supports index scans, which means the compute layer can push down some operations allowing it to load a subset of the rows it needs, reducing round-trips that are affected by speed-of-light latency.

The overall approach here is disaggregation: we’ve taken each of the critical components of an OLTP database and made it a dedicated service. Each of those services is independently horizontally scalable, most of them are shared-nothing, and each can make the design choices that is most optimal in its domain.