Poor man's bitemporal data system in SQLite and Clojure

I've been absolutely waist deep in a bitemporal system on top of PostgreSQL using tstzrange fields. We manage an enormous portfolio of hundreds of thousands of domain names. Every time our core db is modified, before/after states are emitted to a change table. We've been doing this since 2022. Those changes get lightly transformed via trigger into a time travel record, with the valid from/to range and a gist index to make asking questions about the state of the world at a particular point in time easy. For perspective our change table has 90M rows.

All of it works quite well and is decently performant. We can ask questions like, how many domains did we own on March 13th, 2024? Or look at the entire lifecycle of a domains ownership (owned, released, re-acquired, transfered, etc).

The big challenge and core issue we discovered though is that our data sucks. QAing this new capability has been a moving target. Tons of mistakes over time that were partially undone or manually undone without proper audit trail. Ghost records. Rapid changes by our bulk editor tool a->b->a->b that need to get squashed into just a->b. The schema of our database has evolved over time, too, which has made this tough to view a consistent representation of things even if the fields storing that data were renamed. When the system was first introduced, we had ~5 columns to track. Now we have over 30.

Suffice to say if I were to do things over again, I would implement a much better change tracking system that bakes in tools to clean/erase/undo/soft-delete/hard-delete mistakes so that future me (now) wouldn't have so many edge cases to deal with in this time traveling system. I'd also like to just make the change tracking capable of time travel itself, versus building that as a bolt-on side table that tracks and works from the change table. Transitioning to an EAV (entity-attr-value) approach is on my spike list, too. Makes it easier to just reduce (key,val) tuples down into an up to date representation versus looking at diffs of before/after.

Really interesting stuff. I learned a lot about this from Clojure/Datomic and think its quite neat that so many Clojurists are interested in and tackling this problem. As the author notes in this post, XTDB is another one.

For my little system (blog author here) I've decided that all tables will be append-only logs of facts. In the post, I tried keeping traditional schema alongside a "main" facts table. Of course, the problem of audit logs comes up for any change made to traditional "current-database-view" tables. And then who audit logs the audit log?

I feel like "two systems" is the problem.

Writes should either be traditional schema -> standard log of all changes, OR, should be "everything is a log", and the system keeps the "current" view updated, which is just a special case of the "as of" query, where "as of" is always "now".

How badly my schema will behave (in my SQLite-based architecture) is to be discovered. I will hopefully be in a position to run a reasonable benchmark next week.

A follow-up blog post is likely :sweat-smile: