Apa iku masalah N+1 ing GraphQL lan carane DataLoader mbenerake?

Question

IT Interview Practice · Accepted Answer

**Masalah N+1** kedadeyan nalika sawijining field resolver mlaku sepisan kanggo saben item parent lan saben mlaku ngetokake query database dhewe. Njupuk daftar N post banjur author saben post ngeluarke **1 query kanggo post + N query kanggo author** — N+1 round trip sing mateni performa nalika beban dhuwur.

## Kenapa iki kedadeyan

Resolver mlaku saben field, saben item. `Post.author` dieksekusi kapisah kanggo saben post ing daftar, lan ora ana sing ngerti liyane, mula saben ngelakoni `findById` dhewe-dhewe.

```js
// BAD: one DB call per post -> N+1
const resolvers = {
  Post: {
    author: (post, _a, ctx) => ctx.db.users.findById(post.authorId),
  },
};
```

## DataLoader: batch + cache

**DataLoader** nundha panggilan `.load(id)` individu sing digawe ing tick sing padha, banjur nyerahake kunci sing wis diklumpukake menyang siji **fungsi batch** sing njupuk kabeh mau ing siji query. Iki uga **nyimpen cache** miturut kunci sajrone request, mula id sing dibaleni ora mbebani.

```js
import DataLoader from "dataloader";

// create ONE loader per request (put it on context so caching is request-scoped)
const userLoader = new DataLoader(async (ids) => {
  // ids arrives as [1, 2, 3, ...] -> a single batched query
  const users = await db.users.findByIds(ids);
  // MUST return values in the SAME ORDER as ids
  return ids.map((id) => users.find((u) => u.id === id));
});

const resolvers = {
  Post: {
    author: (post, _a, ctx) => ctx.userLoader.load(post.authorId), // batched
  },
};
// Now N author resolvers collapse into 1 query: total 2 queries, not N+1.
```

Rong aturan: gawe loader **saben request** (aja sacara global — kowe bakal mbocorake data lawas antar pangguna), lan fungsi batch kudu mbalekake asil **ing urutan kunci sing persis**.

## Kenapa iki penting

N+1 iku jebakan performa GraphQL sing paling umum lan meh mesthi dadi pitakon wawancara. Nuduhake yen kowe nyekel DataLoader — lan ngerti *kenapa* iku kudu request-scoped lan njaga urutan — nuduhake pengalaman produksi tenanan.

Kenapa iki kedadeyan

DataLoader: batch + cache

Kenapa iki penting

Pitakon sing gegandhengan