33 lines
1001 B
Markdown
33 lines
1001 B
Markdown
---
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title: Parallel Nested Data Fetching
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impact: CRITICAL
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impactDescription: eliminates server-side waterfalls
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tags: server, rsc, parallel-fetching, promise-chaining
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---
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## Parallel Nested Data Fetching
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When fetching nested data in parallel, chain dependent fetches within each item's promise so a slow item doesn't block the rest.
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**Incorrect (a single slow item blocks all nested fetches):**
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```tsx
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const chats = await Promise.all(chatIds.map((id) => getChat(id)));
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const chatAuthors = await Promise.all(
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chats.map((chat) => getUser(chat.author)),
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);
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```
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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.
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**Correct (each item chains its own nested fetch):**
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```tsx
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const chatAuthors = await Promise.all(
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chatIds.map((id) => getChat(id).then((chat) => getUser(chat.author))),
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);
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```
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Each item independently chains `getChat` → `getUser`, so a slow chat doesn't block author fetches for the others.
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