# apollo-response-cache

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Caching and invalidation mechanisms (plugins, directives) of Apollo GraphQL, now featuring Stale While Revalidate.

[`responseCachePlugin`](./src/plugins/responseCachePlugin.ts) is forked from [`apollo-server-plugin-response-cache`](https://github.com/apollographql/apollo-server/tree/main/packages/apollo-server-plugin-response-cache).

### How it works?

![Cache Mechanisms](./assets/cache-mechanisms.svg)

On each query request,

1. `responseCachePlugin` creates an empty key set, and injects it to the context.
2. `@logCache` collects nodes on its field, then add to the key set.
3. `responseCachePlugin` writes query response cache (`fqc`) and node-fqc key mapping to in-memory data store.

Once a mutation updates this node, `@purgeCache` will purge related `fqc`.

### Usage

Install package:

```bash
npm i @stewartmcgown/apollo-response-cache
```

Add plugin and directives to the constructor:

```ts
import {
  responseCachePlugin,
  LogCacheDirective,
  PurgeCacheDirective,
} from '@stewartmcgown/apollo-response-cache'

const server = new ApolloServer({
  plugins: [responseCachePlugin()],
})

const schema = makeExecutableSchema({
  schemaDirectives: {
    logCache: LogCacheDirective(),
    purgeCache: PurgeCacheDirective(),
  },
})
```

Add definitions to your schema:

```graphql
directive @logCache(
  type: String!
  identifier: String = "id"
) on FIELD_DEFINITION

directive @purgeCache(
  type: String!
  identifier: String = "id"
) on FIELD_DEFINITION
```

Use in the schema:

```graphql
type Query {
  article(id: ID!): Article! @logCache(type: "Article")
}

type Mutation {
  archiveArticle(id: ID!): Article! @purgeCache(type: "Article")
}
```

You can also purge cache in the resolver:

```ts
const schema = makeExecutableSchema({
  schemaDirectives: {
    purgeCache: PurgeCacheDirective({ extraNodesPath: '__invalid_nodes__' }),
  },
})

const resolvers = {
  Mutation: {
    archiveArticle: (parent, args, context) => {
      // ...
      article.__invalid_nodes__ = [
        {
          id: '2',
          type: 'Article',
        },
        {
          id: '3',
          type: 'Comment',
        },
      ]

      return article
    },
  },
}
```

#### Customize node type & id resolvers

You might want a custom function to resolve node's type and id since it may be a `union` or `interface` type.

```ts
const typeResolver = (type: string, result: any) => {
  if (['Node', 'Response'].indexOf(type) >= 0) {
    return result.__type
  }
  return type
}
const idResolver = (type: string, result: any) => {
  if (['Node', 'Response'].indexOf(type) >= 0) {
    return result.__unusual_id__
  }
  return result.id
}

const schema = makeExecutableSchema({
  schemaDirectives: {
    purgeCache: PurgeCacheDirective({ typeResolver, idResolver }),
  },
})
```

#### Mutations

Caching is disabled for Mutations by default.

### TODOs

- [x] responseCachePlugin
- [x] @logCache
- [x] @purgeCache
- [ ] Unit Test