SpiceDB is an open-source database system for managing application permissions, inspired by Google's internal authorization as described in the Zanzibar paper. Unlike Zanzibar, which treats users as a first-class concept and assumes they can always be represented by a unique integer, SpiceDB does not make this assumption due to the diverse range of Identity Providers used by organizations. Instead, SpiceDB allows developers to model their own users, enabling relationships that take the form of (resource, relation, subject) rather than just (resource, relation, user). This approach provides flexibility and enables schemas that can handle various interrelated types, such as users, personal access tokens, and API keys, making it a more adaptable system.