Reducer Context

SpacetimeDB: Reducer Context ↗

Reducer context is an Effect service yielded from the assembled module.

import * as Effect from "effect/Effect"
import * as Stdb from "effect-spacetimedb"

const audit = Stdb.table("audit", {
  columns: {
    id: Stdb.u64().primaryKey().autoInc(),
    sender: Stdb.identity(),
    message: Stdb.string(),
  },
})

const Audit = Stdb.StdbGroup.make("Audit").add(
  Stdb.StdbFn.reducer("audit_write", {
    params: Stdb.struct({ message: Stdb.string() }),
  }),
)

const Module = Stdb.StdbModule.make("app").addTables(audit).add(Audit)
const { Db, ReducerCtx } = Module

const AuditLive = Stdb.StdbBuilder.group(Module, "Audit", {
  audit_write: Effect.fn(function* ({ message }) {
    const ctx = yield* ReducerCtx
    const db = yield* Db
    yield* db.audit.insert({
      id: 0n,
      sender: ctx.sender,
      message,
    })
  }),
})

ReducerCtx exposes the sender identity, authenticated sender metadata, timestamp, connection id, deterministic random stream, UUID helpers, and the same writable db view available through Db.

effect-spacetimedb exposes context as scoped services rather than a positional argument: ReducerCtx for reducers, TxCtx inside explicit transaction bodies, and MutationCtx for helpers that must work in both scopes.

Caution

Reducers are deterministic synchronous host calls. async/Promises, timers, microtasks, global Math.random, and wall-clock reads such as Date.now() or no-argument new Date() throw typed errors. Use Random.* from effect/Random, context randomness, and the call timestamp instead. See Randomness & Determinism.