I actually built my own immutable database which does support branching (see profile), so it seems like a huge miss that these ones don't. It's pretty much the main reason I would want an immutable database.
That said, I’m adding xitdb to the list of tech to try out. Thank you for building it!
Oh, and thanks for linking to my article :-)
The linked article points out that Datomic doesn't support branching from the past. It absolutely does support branching, and I've built entire test suites that way.
From a cursory glance, I'd say Datomic does exactly what the original parent article is discussing. It works great and it's super convenient.
With `datomic.api/with`, you can apply new datoms and get back a new DB value. Repeat this process as many times as you want, in as many directions as you want, switching as you choose. You're building a tree of immutable DB values—seems clearly like branching to me.
If by "read only" you mean that they're not persisted to disk, then that's an important point, but it surely doesn't obviate the utility of the functionality. It's useful in a number of cases, and especially testing scenarios like the Xata article describes.
If you built an immutable database that persists the branches, that is very cool and sounds useful—kudos! That said, I also don't want to downplay the utility of what Datomic does; it's a major help to me.
My database supports persisted branching, but not just at the database level. You can "branch" (i.e., make a fast clone) data at any level, such as data for a specific user. Many production uses for this, not just testing, yet almost no database supports this. It uses the same HAMT algorithm that Clojure uses.