EventStore: Open-Source, Functional Database with Complex Event Processing in JS

"Functional Database" is contradictory. Honestly this sounds like a bunch of buzzwords concatenated together. Is there an example or a purpose for this?

Personally, I've heard of it in the context of https://github.com/commanded/commanded, a CQRS framework for Elixir that supports EventStore as a storage backend.

I think the idea is that it's something simpler than a database. It's more like an append-only file that has on-insert triggers/durable running queries. (Sort of like https://www.pipelinedb.com/ does, or like blockchain nodes do.)

Or, you could think of it as message queue like Kafka, with permanent durability of all "messages", and a single fixed subscriber bolted onto the queue server, where that subscriber is exposed through the queue server's API, allowing users to reprogram it arbitrarily.

It isn’t that contradictory. You can have data in a functional language. One of the common features of a functional language that separates it from imperative is data immutability. In this case it’s a database of immutable data. I personally understand exactly what they’re talking about when they call it a functional database.

We use this heavily in production and to good effect. It has its downsides (doesn't every product) but we have found it works well once you've grasped them. If you have a primarily .NET focused team, have plenty of event-driven applications and want to do CQRS / Event Sourcing for some of your applications I would recommend it as a starter step towards that. It gives you a lot 'out of the box' which is great when starting out but over-reliance on some of its features can make it hard to replace. Make sure you have a good handle on retention and an archival strategy. Storage maybe financially cheap but it's certainly not maintenance friendly once your database grows beyond certain sizes (we're currently running our own fork to solve some of these issues whilst the PR process is going on).