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docs: clarify deterministic reducer randomness #5322

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7 changes: 4 additions & 3 deletions docs/docs/00200-core-concepts/00100-databases/00500-cheat-sheet.md
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Original file line number Diff line number Diff line change
Expand Up @@ -753,6 +753,7 @@ ctx.sender // Identity of caller
ctx.connectionId // ConnectionId | undefined
ctx.timestamp // Timestamp
ctx.identity // Module's identity
ctx.random // Deterministic random generator
```

</TabItem>
Expand All @@ -764,7 +765,7 @@ ctx.Sender // Identity of caller
ctx.ConnectionId // ConnectionId?
ctx.Timestamp // Timestamp
ctx.Identity // Module's identity
ctx.Rng // Random number generator
ctx.Rng // Deterministic random generator
```

</TabItem>
Expand All @@ -776,7 +777,7 @@ ctx.sender() // Identity of caller
ctx.connection_id() // Option<ConnectionId>
ctx.timestamp // Timestamp
ctx.identity() // Module's identity
ctx.rng() // Random number generator
ctx.rng() // Deterministic random generator
```

</TabItem>
Expand All @@ -788,7 +789,7 @@ ctx.sender() // Identity of caller (Identity type)
ctx.connection_id // std::optional<ConnectionId>
ctx.timestamp // Timestamp of current transaction (Timestamp type)
ctx.identity() // Module's own identity (Identity type)
ctx.rng() // Random number generator (for seeded randomness)
ctx.rng() // Deterministic random generator
```

</TabItem>
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2 changes: 1 addition & 1 deletion skills/concepts/SKILL.md
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Expand Up @@ -20,7 +20,7 @@ SpacetimeDB is a relational database that is also a server. It lets you upload a
## Critical Rules

1. **Reducers are transactional.** They do not return data to callers. Use subscriptions to read data.
2. **Reducers must be deterministic.** No filesystem, network, timers, or random. All state must come from tables.
2. **Reducers must be deterministic.** No filesystem, network, timers, wall-clock time, or ambient randomness. Use `ReducerContext` APIs such as `ctx.timestamp` / `ctx.Timestamp` and the language-specific deterministic RNG instead.
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.
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