system_assignation/node_modules/cacheable-request/README.md

# cacheable-request

> Wrap native HTTP requests with RFC compliant cache support

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[RFC 7234](http://httpwg.org/specs/rfc7234.html) compliant HTTP caching for native Node.js HTTP/HTTPS requests. Caching works out of the box in memory or is easily pluggable with a wide range of storage adapters.

**Note:** This is a low level wrapper around the core HTTP modules, it's not a high level request library.

## Features

- Only stores cacheable responses as defined by RFC 7234
- Fresh cache entries are served directly from cache
- Stale cache entries are revalidated with `If-None-Match`/`If-Modified-Since` headers
- 304 responses from revalidation requests use cached body
- Updates `Age` header on cached responses
- Can completely bypass cache on a per request basis
- In memory cache by default
- Official support for Redis, MongoDB, SQLite, PostgreSQL and MySQL storage adapters
- Easily plug in your own or third-party storage adapters
- If DB connection fails, cache is automatically bypassed ([disabled by default](#optsautomaticfailover))
- Adds cache support to any existing HTTP code with minimal changes
- Uses [http-cache-semantics](https://github.com/pornel/http-cache-semantics) internally for HTTP RFC 7234 compliance

## Install

```shell
npm install cacheable-request
```

## Usage

```js
const http = require('http');
const CacheableRequest = require('cacheable-request');

// Then instead of
const req = http.request('http://example.com', cb);
req.end();

// You can do
const cacheableRequest = new CacheableRequest(http.request);
const cacheReq = cacheableRequest('http://example.com', cb);
cacheReq.on('request', req => req.end());
// Future requests to 'example.com' will be returned from cache if still valid

// You pass in any other http.request API compatible method to be wrapped with cache support:
const cacheableRequest = new CacheableRequest(https.request);
const cacheableRequest = new CacheableRequest(electron.net);
```

## Storage Adapters

`cacheable-request` uses [Keyv](https://github.com/lukechilds/keyv) to support a wide range of storage adapters.

For example, to use Redis as a cache backend, you just need to install the official Redis Keyv storage adapter:

```
npm install @keyv/redis
```

And then you can pass `CacheableRequest` your connection string:

```js
const cacheableRequest = new CacheableRequest(http.request, 'redis://user:pass@localhost:6379');
```

[View all official Keyv storage adapters.](https://github.com/lukechilds/keyv#official-storage-adapters)

Keyv also supports anything that follows the Map API so it's easy to write your own storage adapter or use a third-party solution.

e.g The following are all valid storage adapters

```js
const storageAdapter = new Map();
// or
const storageAdapter = require('./my-storage-adapter');
// or
const QuickLRU = require('quick-lru');
const storageAdapter = new QuickLRU({ maxSize: 1000 });

const cacheableRequest = new CacheableRequest(http.request, storageAdapter);
```

View the [Keyv docs](https://github.com/lukechilds/keyv) for more information on how to use storage adapters.

## API

### new cacheableRequest(request, [storageAdapter])

Returns the provided request function wrapped with cache support.

#### request

Type: `function`

Request function to wrap with cache support. Should be [`http.request`](https://nodejs.org/api/http.html#http_http_request_options_callback) or a similar API compatible request function.

#### storageAdapter

Type: `Keyv storage adapter`<br>
Default: `new Map()`

A [Keyv](https://github.com/lukechilds/keyv) storage adapter instance, or connection string if using with an official Keyv storage adapter.

### Instance

#### cacheableRequest(opts, [cb])

Returns an event emitter.

##### opts

Type: `object`, `string`

- Any of the default request functions options.
- Any [`http-cache-semantics`](https://github.com/kornelski/http-cache-semantics#constructor-options) options.
- Any of the following:

###### opts.cache

Type: `boolean`<br>
Default: `true`

If the cache should be used. Setting this to false will completely bypass the cache for the current request.

###### opts.strictTtl

Type: `boolean`<br>
Default: `false`

If set to `true` once a cached resource has expired it is deleted and will have to be re-requested.

If set to `false` (default), after a cached resource's TTL expires it is kept in the cache and will be revalidated on the next request with `If-None-Match`/`If-Modified-Since` headers.

###### opts.maxTtl

Type: `number`<br>
Default: `undefined`

Limits TTL. The `number` represents milliseconds.

###### opts.automaticFailover

Type: `boolean`<br>
Default: `false`

When set to `true`, if the DB connection fails we will automatically fallback to a network request. DB errors will still be emitted to notify you of the problem even though the request callback may succeed.

###### opts.forceRefresh

Type: `boolean`<br>
Default: `false`

Forces refreshing the cache. If the response could be retrieved from the cache, it will perform a new request and override the cache instead.

##### cb

Type: `function`

The callback function which will receive the response as an argument.

The response can be either a [Node.js HTTP response stream](https://nodejs.org/api/http.html#http_class_http_incomingmessage) or a [responselike object](https://github.com/lukechilds/responselike). The response will also have a `fromCache` property set with a boolean value.

##### .on('request', request)

`request` event to get the request object of the request.

**Note:** This event will only fire if an HTTP request is actually made, not when a response is retrieved from cache. However, you should always handle the `request` event to end the request and handle any potential request errors.

##### .on('response', response)

`response` event to get the response object from the HTTP request or cache.

##### .on('error', error)

`error` event emitted in case of an error with the cache.

Errors emitted here will be an instance of `CacheableRequest.RequestError` or `CacheableRequest.CacheError`. You will only ever receive a `RequestError` if the request function throws (normally caused by invalid user input). Normal request errors should be handled inside the `request` event.

To properly handle all error scenarios you should use the following pattern:

```js
cacheableRequest('example.com', cb)
  .on('error', err => {
    if (err instanceof CacheableRequest.CacheError) {
      handleCacheError(err); // Cache error
    } else if (err instanceof CacheableRequest.RequestError) {
      handleRequestError(err); // Request function thrown
    }
  })
  .on('request', req => {
    req.on('error', handleRequestError); // Request error emitted
    req.end();
  });
```

**Note:** Database connection errors are emitted here, however `cacheable-request` will attempt to re-request the resource and bypass the cache on a connection error. Therefore a database connection error doesn't necessarily mean the request won't be fulfilled.

## License

MIT © Luke Childs