It's custom. I explored a Redis fork months ago, prior to the ability for modules. https://github.com/tidwall/redis-gis

I then tried to make a module but redis modules do not support hooking into the pubsub tooling, so Tile38 live geofencing and webhooks were not possible.

Lol looks like this database needs its own database. Yeah seems like it should be a plugin or library not sure why it's standalone if it's just backed by redis.

It's not backed by Redis. It's fully standalone. I wish I could have made a redis module back when I started the project, but I'm plenty happy how Tile38 has matured.

Eh, fair enough then. If you don't have to setup redis yourself it doesn't really matter