Aurora is actually not a database but is a scalable storage layer that operates over the network and is decoupled from the query engine (compute). The architecture has been used to implement vastly different query engines on top of it (PgSQL, MySQL, DocumentDB – a MongoDB alternative, and Neptune – a property graph database / triple store).

The closest abstraction I can think of to describe Aurora is a VAX/VMS cluster – where the consumer sees a single entity, regardless of size, whilst the scaling (out or back in) remains entirely opaque.

Aurora does not support RDS Proxy for PostgreSQL or its equivalents for other query engine types because it addresses cluster access through cluster endpoints. There are two types of endpoints: one for read-only queries («reader endpoints» in Aurora parlance) and one for read-mutate queries («writer endpoint»). Aurora supports up to 15 reader endpoints, but there can be only one writer endpoint.

Reader endpoints improve the performance of non-mutating queries by distributing the load across read replicas. The default Aurora cluster endpoint always points to the writer instance. Consumers can either default to the writer endpoint for all queries or segregate non-mutating queries to reader endpoints for faster execution.

This behaviour is consistent across all supported query engines, such as PostgreSQL, Neptune, and DocumentDB.

I do not think it is correct to state that Aurora does not use the OS page cache – it does, as there is still a server with an operating system somewhere, despite the «serverless» moniker. In fact, due to its layered distributed architecture, there is now more than one OS page cache, as described in [0].

Since Aurora is only accessible over the network, it introduces unique peculiarities where the standard provisions of storage being local do not apply.

Now, onto the subject of costs. A couple of years ago, an internal client who ran provisioned RDS clusters in three environments (dev, uat, and prod) reached out to me with a request to create infrastructure clones of all three clusters. After analysing their data access patterns, peak times, and other relevant performance metrics, I figured that they did not need provisioned RDS and would benefit from Aurora Serverless instead – which is exactly what they got (unbeknownst to them, which I consider another net positive for Aurora). The dev and uat environments were configured with lower upper ACU's, whilst production had a higher upper ACU configuration, as expected.

Switching to Aurora Serverless resulted in a 30% reduction in the monthly bill for the dev and uat environments right off the bat and nearly a 50% reduction in production costs compared to a provisioned RDS cluster of the same capacity (if we use the upper ACU value as the ceiling). No code changes were required, and the transition was seamless.

Ironically, I have discovered that the AWS cost calculator consistently overestimates the projected costs, and the actual monthly costs are consistently lower. The cost calculator provides a rough estimate, which is highly useful for presenting the solution cost estimate to FinOps or executives. Unintentionally, it also offers an opportunity to revisit the same individuals later and inform them that the actual costs are lower. It is quite amusing.

[0] https://muratbuffalo.blogspot.com/2024/07/understanding-perf...