Sorry, I'm not trying to be snarky, but that sounds like "it's good for everything" and doesn't help me determine when I should look at Fauna and when I shouldn't.
Perhaps alternately - when is Fauna NOT a good choice?
Perhaps alternately - when is Fauna NOT a good choice?
FaunaDB is a unique database: its architecture offers linear scalability for transaction throughput (limited, of course, by contention on common records and predicates). That sets it aside from databases which use a single coordinator for all, or just cross-shard, transactions, like Datomic and VoltDB, respectively.
It's also intended for geographic replication, which is a scenario many databases don't try to handle with the same degree of transactional safety--lots of folks support, say, snapshot isolation or linearizability inside one DC, but between DCs all bets are off.
FaunaDB also does not depend on clocks for safety, which sets it aside from other georeplicated databases that assume clocks are well-synchronized. Theoretically, relying on clocks can make you faster, so those kinds of systems might outperform Fauna--at the potential cost of safety issues when clocks misbehave. In practice, there are lots of factors that affect performance, and it's easy to introduce extra round trips into a protocol which could theoretically be faster. There's a lot of room for optimization! I can't speak very well to performance numbers, because Jepsen isn't designed as a performance benchmark, and its workloads are intentionally pathological, with lots of contention.
One of the things you lose with FaunaDB's architecture is interactive transactions--as in VoltDB, you submit transactions all at once. That means FaunaDB can make some optimizations that interactive session transactions can't do! But it also means you have to fit your transactional logic into FaunaDB's query language. The query language is (IMO) really expressive, but if you needed to do, e.g. a complex string-munging operation inside a transaction to decide what to do, it might not be expressible in a single FaunaDB transaction; you might have to say, read data, make the decision in your application, then execute a second CaS transaction to update state.
FaunaDB's not totally available--it uses unanimous agreement over majority quorums, which means some kinds of crashes or partitions can pause progress. If you're looking for a super low latency CRDT-style database where every node can commit even if totally isolated, it's not the right fit. It is a good fit if you need snapshot isolation to strict serializability, which are strong consistency models!
What is actually a good for this lately?
It's true. The community is keeping Riak alive, and pushing it forward. The CRDT stuff has not been worked on for a long time, despite there being numerous tabled improvements in that area SINCE 2014. It was neglected by the "new" management that crashed Basho into the ground as being "complicated computer science nonsense". Since then I've been unable to do the CRDT work for Riak I wanted, due to lack of time, and no one willing to pay for it (despite numerous bait-and-switch offers for completing the bigsets work.)
The CRDT work in riak needs: 1. the smaller/more efficient map merging 2. some new types from Sam merging (range reg etc) 3. the bigsets work completing and integrating (this enables delta-CRDTs) 4. big maps (built on the above work)
When it has all that it would be at the place I'd envisioned for it, before everything went super bad at Basho. I work at Joyent now, so can't dedicate any time to taking the CRDT work forward, though I really wish I could. I still have a deep interest in the EC datatypes world.
IMO, at the time it was released (2014) Riak had ground breaking support for convergent datatypes, since then it has only lost ground. Am I bitter? Yes, a little.
WRT other systems with CRDT support, https://www.antidotedb.eu/ came out of the syncfree research project. Chistopher Meiklejohn's work is industrial grade, state of the art, actively developed, and highly recommended. https://github.com/lasp-lang As others have mentioned, REDIS also uses CRDTs for MDC replication, and has CRDT co-inventor Carlos Baquero on the tech board.
http://cs.yale.edu/homes/thomson/publications/calvin-sigmod1...
Currently it supports its own native DSL similar to LINQ or an ORM, and if you want compatibility with existing query languages from other databases, we will be rolling those out over time.
It's not a good choice for analytics or time series data which is redundant or aggregated, and doesn't need the high availability or performance overhead of transactional isolation.
Additionally, FaunaDB gives you the tools to work within a global environment with _safe tradeoffs_. For example, reads at SI/Serializable can be fast and uncoordinated. You choose per-query.
That said, I don't think it's reasonable to infer that because failures occur more often than we'd like, systems based on consensus are always inappropriate for latency and performance sensitive systems. While there are minimum bounds on latency in consensus systems, there are also plenty of human-scale problems for which that bound isn't a dealbreaker. Moreover, some types of computation (e.g. updates in sequentially consistent or snapshot isolated systems) simply can't be performed without paying the same costs as consensus. Consensus can be an appropriate, and in some cases the only possible solution, to some problems.
For example, a 7 replica FaunaDB cluster can lose 3 replicas and maintain availability for reads and writes; better than Aurora in your example.