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# Cache Store
A Cache Store is responsible for storing and retrieving cached responses.
It is also responsible for deciding which specific response to use based off of
a response's `Vary` header (if present). It is expected to be compliant with
[RFC-9111](https://www.rfc-editor.org/rfc/rfc9111.html).
## Pre-built Cache Stores
### `MemoryCacheStore`
The `MemoryCacheStore` stores the responses in-memory.
**Options**
- `maxSize` - The maximum total size in bytes of all stored responses. Default `104857600` (100MB).
- `maxCount` - The maximum amount of responses to store. Default `1024`.
- `maxEntrySize` - The maximum size in bytes that a response's body can be. If a response's body is greater than or equal to this, the response will not be cached. Default `5242880` (5MB).
### Getters
#### `MemoryCacheStore.size`
Returns the current total size in bytes of all stored responses.
### Methods
#### `MemoryCacheStore.isFull()`
Returns a boolean indicating whether the cache has reached its maximum size or count.
### Events
#### `'maxSizeExceeded'`
Emitted when the cache exceeds its maximum size or count limits. The event payload contains `size`, `maxSize`, `count`, and `maxCount` properties.
### `SqliteCacheStore`
The `SqliteCacheStore` stores the responses in a SQLite database.
Under the hood, it uses Node.js' [`node:sqlite`](https://nodejs.org/api/sqlite.html) api.
The `SqliteCacheStore` is only exposed if the `node:sqlite` api is present.
**Options**
- `location` - The location of the SQLite database to use. Default `:memory:`.
- `maxCount` - The maximum number of entries to store in the database. Default `Infinity`.
- `maxEntrySize` - The maximum size in bytes that a response's body can be. If a response's body is greater than or equal to this, the response will not be cached. Default `Infinity`.
## Defining a Custom Cache Store
The store must implement the following functions:
### Getter: `isFull`
Optional. This tells the cache interceptor if the store is full or not. If this is true,
the cache interceptor will not attempt to cache the response.
### Function: `get`
Parameters:
* **req** `Dispatcher.RequestOptions` - Incoming request
Returns: `GetResult | Promise<GetResult | undefined> | undefined` - If the request is cached, the cached response is returned. If the request's method is anything other than HEAD, the response is also returned.
If the request isn't cached, `undefined` is returned.
Response properties:
* **response** `CacheValue` - The cached response data.
* **body** `Readable | undefined` - The response's body.
### Function: `createWriteStream`
Parameters:
* **req** `Dispatcher.RequestOptions` - Incoming request
* **value** `CacheValue` - Response to store
Returns: `Writable | undefined` - If the store is full, return `undefined`. Otherwise, return a writable so that the cache interceptor can stream the body and trailers to the store.
## `CacheValue`
This is an interface containing the majority of a response's data (minus the body).
### Property `statusCode`
`number` - The response's HTTP status code.
### Property `statusMessage`
`string` - The response's HTTP status message.
### Property `rawHeaders`
`Buffer[]` - The response's headers.
### Property `vary`
`Record<string, string | string[]> | undefined` - The headers defined by the response's `Vary` header
and their respective values for later comparison
For example, for a response like
```
Vary: content-encoding, accepts
content-encoding: utf8
accepts: application/json
```
This would be
```js
{
'content-encoding': 'utf8',
accepts: 'application/json'
}
```
### Property `cachedAt`
`number` - Time in millis that this value was cached.
### Property `staleAt`
`number` - Time in millis that this value is considered stale.
### Property `deleteAt`
`number` - Time in millis that this value is to be deleted from the cache. This
is either the same sa staleAt or the `max-stale` caching directive.
The store must not return a response after the time defined in this property.
## `CacheStoreReadable`
This extends Node's [`Readable`](https://nodejs.org/api/stream.html#class-streamreadable)
and defines extra properties relevant to the cache interceptor.
### Getter: `value`
The response's [`CacheStoreValue`](/docs/docs/api/CacheStore.md#cachestorevalue)
## `CacheStoreWriteable`
This extends Node's [`Writable`](https://nodejs.org/api/stream.html#class-streamwritable)
and defines extra properties relevant to the cache interceptor.
### Setter: `rawTrailers`
If the response has trailers, the cache interceptor will pass them to the cache
interceptor through this method.