SpacetimeDB Core Concepts

SpacetimeDB is a relational database that is also a server. It lets you upload application logic directly into the database via WebAssembly modules, eliminating the traditional web/game server layer entirely.

Critical Rules

1. Reducers are transactional. They do not return data to callers. Use subscriptions to read data.
2. Reducers must be deterministic. Do not use filesystem, network, external clocks, or external random sources in reducers. Use ReducerContext for SpacetimeDB-provided timestamp and deterministic random values.
3. Read data via tables/subscriptions, not reducer return values. Clients get data through subscribed queries.
4. Auto-increment IDs are not sequential. Gaps are normal, do not use for ordering. Use timestamps or explicit sequence columns.
5. ctx.sender is the authenticated principal. Never trust identity passed as arguments.

Feature Implementation Checklist