filipesilva

xitdb-clj is a new immutable database for Clojure. It can time travel like Datomic, and store data in a single file like SQLite. On its own, though, it has no query language. Enter Datascript...

Datascript is normally an in-memory database, but it lets you use anything as backing storage. What if we combined them?

Datascript's storage feature doesn't tell us exactly what changed, so I added one more trick: Editscript, the diffing library. I use it to figure out how to update the database with the minimal number of changes.

The end result is a sort of mini Datomic that writes to a single file. And yet, this setup can do something Datomic can't: it can branch off of any old copy of the database. The example below reverts to an older copy of the database.

Keep in mind that all reads and writes are incremental. The database is not being loaded into memory at once, so you can use it to

filipesilva

(ns pie-a-la-mode
  (:require [clojure.core.async :as a]))

;; Concurrency example from The Pragmatic Programmers. Waiters check if there's
;; enough pie and ice cream before they sell an order, but if they don't
;; coordinate it's still possible to sell more than we have.
;;
;; The below code should return either :ok or :not-available for a given order.

(def recipes