Unafanyaje pagination katika GraphQL: offset dhidi ya cursor?

Question

IT Interview Practice · Accepted Answer

Mikakati miwili: **inayotegemea offset** (`limit`/`offset`) na **inayotegemea cursor**. GraphQL haina pagination iliyojengwa ndani, lakini kiwango cha jamii ni maelezo ya **Relay Connection**, ambayo hurasimisha cursor pagination.

## Offset dhidi ya cursor

- **Offset** (`items(limit: 10, offset: 20)`) ni rahisi lakini huvunjika kwenye data hai: ikiwa row imeingizwa mtumiaji akiwa anapitia kurasa, vitu husogea na unaona nakala au mapengo. Pia huwa polepole kwenye offsets za kina (DB bado huchambua rows zilizorukwa).
- **Cursor** huelekeza kwenye nafasi thabiti ("nipe 10 baada ya kitu hiki"). Ni salama dhidi ya kuingiza (insertion-safe) na hukua hadi kurasa za kina, kwa gharama ya kutokuwa na "ruka hadi ukurasa 50" wa bahati nasibu.

## Relay connections

Connection hufunga list ndani ya `edges` (kila moja ikiwa na `cursor` na `node`) pamoja na `pageInfo`:

```graphql
type Query {
  posts(first: Int!, after: String): PostConnection!   # first = page size, after = cursor
}

type PostConnection {
  edges: [PostEdge!]!
  pageInfo: PageInfo!
}
type PostEdge {
  cursor: String!        # opaque token for THIS node's position
  node: Post!            # the actual item
}
type PageInfo {
  endCursor: String      # feed this back as 'after' for the next page
  hasNextPage: Boolean!  # so the client knows when to stop
}
```

```js
// resolver: fetch ONE extra row to compute hasNextPage cheaply
const rows = await db.posts.find({ after: args.after, limit: args.first + 1 });
const hasNextPage = rows.length > args.first;
const page = rows.slice(0, args.first);
return {
  edges: page.map((p) => ({ node: p, cursor: encodeCursor(p.id) })),
  pageInfo: { endCursor: page.at(-1) && encodeCursor(page.at(-1).id), hasNextPage },
};
```

Cursor ni **opaque** — kwa kawaida base64 ya sort key — hivyo clients huitendea kama token, si namba.

## Kwa nini ni muhimu

Kuchagua offset dhidi ya cursor kunaonyesha unaelewa **uthabiti chini ya maandishi ya wakati mmoja (concurrent writes)** na **utendaji wa ukurasa wa kina**. Kujua muundo wa Relay connection ni muhimu kwa sababu clients za Apollo/Relay zina ushughulikiaji wa cache uliojengwa ndani kwa ajili yake.

Offset dhidi ya cursor

Relay connections

Kwa nini ni muhimu

Maswali yanayohusiana