Riak is horribly unfriendly as a database: no SQL, it exposes eventual consistency directly to the developer, it’s relatively slow, and Erlang is a fairly unusual language.
While you can run Riak on a single server, you’d have to really want to.
Its strength is the ability to scale massively, but not many projects need that scale, and by the time you do, you’re probably already using some friendlier database and you’d rather make that one work.
Though it had a couple years head start when there really no other options for people wanting that kind of kit.
Ohhh, this brings memories of developers hitting the wall... Between different SQL databases!
Back in 2016 I was delegated at work to do ops on a project that had big data ambitions in Threat Intelligence space.
Part of how they intended to support that was Apache Phoenix, an SQL database backed by HBase, running on top of Hadoop that also provided object storage (annoyingly through WebHDFS gateway).
Constant problems with hung Phoenix queries and instability of Hadoop in entirety led me to propose moving over to PostgreSQL, which generally went quite well... Except several cases of "basic SQL operations" that turned to have wildly different performance compared to Phoenix and most importantly, to MySQL in MyISAM mode, like doing SELECT (*) on huge tables.
Fun times, got to meet a postgres core team member thanks to it.
One of our biggest disappointments: we had plans to add a way to enforce strong consistency leveraging (IIRC) something akin to multi-paxos, but couldn't get it to work.
The engineering exodus around that time sorta killed the project though, and we never were able to do the big follow-up work to make it really shine.
(Disclaimer: Former Basho Principal Engineer, primary author of strong consistency work, lead riak_core dev from 2011-2015)
I think another 18 months would have been enough too. But it just wasn't the right environment after the hostile take-over / leadership transition.
I apologise if we do eventually cut it. Having worked through the code when chasing unstable tests, I developed an appreciation for the quality of the work.
In the end more and more data was offloaded to MariaDB, until one day the last remaining data couldn't justify the cost of the Riak cluster. I think we swapped out an eight node Riak cluster for two largish MariaDB database (one being a hot-standby).
For one of the other clients it was the exact same scenario, only we had been contracted in to help run the Riak cluster, which we didn't do well. Once they had migrate of it, to Oracle I think, the client left.
To me it always felt like it was just the wrong tool for that particular job. Someone really wanted to be able to jump on the NoSQL hype and sell something. They picked Riak, because it honestly looked really good, and probably was, compared to MongoDB, CouchDB or whatever else happened to float around at the time. It just wasn't the right tool for the problems it was applied to.
(I can't, of course, speak to the truth of this, only that over a couple decades of knowing the dude in question and working with him on and off he had sufficient Clue that I expect he did put in the effort before coming to that conclusion)