The canonical backend datastore exposes a microservice-like HTTP API, which could be extended with gRPC or GraphQL interfaces. The initial datastore is a transactional SQL database, but this implementation detail is abstracted by the API.
As little "application logic" as possible should be embedded in this back-end; as much as possible would be pushed to bots which could be authored and operated by anybody. A separate web interface project talks to the API backend and can be developed more rapidly with less concern about data loss or corruption.
A cronjob will create periodic database dumps, both in "full" form (all tables and all edit history, removing only authentication credentials) and "flattened" form (with only the most recent version of each entity).
A goal is to be linked-data/RDF/JSON-LD/semantic-web "compatible", but not necessarily "first". It should be possible to export the database in a relatively clean RDF form, and to fetch data in a variety of formats, but internally fatcat will not be backed by a triple-store, and will not be bound to a rigid third-party ontology or schema.
Microservice daemons should be able to proxy between the primary API and standard protocols like ResourceSync and OAI-PMH, and third party bots could ingest or synchronize the database in those formats.