qcast-front/.agents/skills/vercel-react-best-practices/rules/server-parallel-nested-fetching.md

---
title: Parallel Nested Data Fetching
impact: CRITICAL
impactDescription: eliminates server-side waterfalls
tags: server, rsc, parallel-fetching, promise-chaining
---

## Parallel Nested Data Fetching

When fetching nested data in parallel, chain dependent fetches within each item's promise so a slow item doesn't block the rest.

**Incorrect (a single slow item blocks all nested fetches):**

```tsx
const chats = await Promise.all(
  chatIds.map(id => getChat(id))
)

const chatAuthors = await Promise.all(
  chats.map(chat => getUser(chat.author))
)
```

If one `getChat(id)` out of 100 is extremely slow, the authors of the other 99 chats can't start loading even though their data is ready.

**Correct (each item chains its own nested fetch):**

```tsx
const chatAuthors = await Promise.all(
  chatIds.map(id => getChat(id).then(chat => getUser(chat.author)))
)
```

Each item independently chains `getChat` → `getUser`, so a slow chat doesn't block author fetches for the others.