We don't use an underlying storage engine. Everything runs directly on your warehouse. We build the activity stream by running queries against the warehouse and writing to a table that we create inside of it. When people build datasets with Narrator we compile everything down to sql and query the warehouse directly.

The tech stack is Python for the backend scheduling and query engine hosted on AWS. For the frontend it's React. We have some internal data stores for managing our own state and a bit of caching - Postgres, S3, ElasticSearch. We use GraphQL a fair bit.