1---
2title: Parallel Nested Data Fetching
3impact: CRITICAL
4impactDescription: eliminates server-side waterfalls
5tags: server, rsc, parallel-fetching, promise-chaining
6---
7 
8## Parallel Nested Data Fetching
9 
10When fetching nested data in parallel, chain dependent fetches within each item's promise so a slow item doesn't block the rest.
11 
12**Incorrect (a single slow item blocks all nested fetches):**
13 
14```tsx
15const chats = await Promise.all(
16  chatIds.map(id => getChat(id))
17)
18 
19const chatAuthors = await Promise.all(
20  chats.map(chat => getUser(chat.author))
21)
22```
23 
24If 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.
25 
26**Correct (each item chains its own nested fetch):**
27 
28```tsx
29const chatAuthors = await Promise.all(
30  chatIds.map(id => getChat(id).then(chat => getUser(chat.author)))
31)
32```
33 
34Each item independently chains `getChat` → `getUser`, so a slow chat doesn't block author fetches for the others.
35