A Critique of the CAP theorem by Martin Kleppmann.

https://arxiv.org/abs/1509.05393

Basically naive CAP theorem assumes "no latency (delay) allowed", in which case, yeah, sure, the quorum must agree or you are writing to a split-brain quorum somehow. But what if delay sensitivity is a knob that can be tuned, and writes have conflict resolution?

Martin Kleppmann's book Designing Data-Intensive Applications is considered a useful tome for anyone who needs a insight in the complexity involved in distributed systems (hint: getting data in ram flushed to an SSD also involves multiple moving parts)

Basically, many people convinced themselves that the formalization of CAP means that systems need to choose between highly available and strongly consistent, and hence chose eventual consistency. This is, partially, because Gilbert and Lynch define "availability" to mean "available to all clients, even those on a minority side of a partition". The much more useful "available to a majority of clients" is achievable at the same time as strong consistency in presence of a single partition, and this is the common cloud failure mode.

See https://brooker.co.za/blog/2024/07/25/cap-again.html for a longer take.

Other, much more reasonable trade-offs, lead to eventual consistency too. Mostly latency optimizations, but many of those also lead to non-zero RPO and so are undesirable for multiple reasons. We discuss some of this in section 8 of the DSQL paper: https://arxiv.org/pdf/2607.13276

Thank you both! That was really enlightening & made for interesting reading!

Though the note at the bottom of the article to teachers to not start with CAP has me concerned about the quality of other content I’ve seen thus far …