Fetch Dedupe

A fetch-like HTTP library that implements request deduplication and response caching.

Motivation

Making a single HTTP request is not difficult to do in JavaScript. However, complex web applications often make many requests as the user navigates through the app.

Features such as request deduplication and response caching can often save the developer of apps like these from headache and bugs.

fetch-dedupe is a library with a familiar, fetch-like API that includes request deduplication and response caching for you, and it's a delight to use.

Installation

Install using npm:

npm install fetch-dedupe

or yarn:

yarn add fetch-dedupe

Getting Started

Because fetch-dedupe is such a lightweight wrapper around fetch, you'll benefit from having knowledge of that API. If you're new to fetch, I recommend reading the Using Fetch guide on MDN. It's a great introduction.

The following example demonstrates using Fetch Dedupe with the ES2015 module syntax.

import { fetchDedupe } from 'fetch-dedupe';

const fetchOptions = {
  method: 'PATCH',
  body: JSON.stringify({ a: 12 })
};

fetchDedupe('/test/2', fetchOptions)
  .then(res => {
    console.log('Got some data', res.data);
  });

// Additional identical requests are deduped. Nifty.
fetchDedupe('/test/2', fetchOptions)
  .then(res => {
    console.log('Got some data', res.data);
  });

API

This library exports the following:

• fetchDedupe()
• getRequestKey()
• responseCache
  • .get()
  • .set()
  • .has()
  • .delete()
  • .clear()
  • .useCachedResponse()
• activeRequests
  • isRequestInFlight()
  • clear()
• CacheMissError

fetchDedupe( [url] [, options] )

Creates an HTTP request. You may pass a URL as the first argument to make a basic GET request:

fetch('/api/books/2')

If you need to configure the request with more details, you can pass a second argument, options:

fetch('/api/books/2', {
  method: 'POST'
});

You may also only pass options, including the URL as part of the options, if you'd prefer:

fetch({
  url: '/api/books/2',
  method: 'POST'
});

In addition to all of the options supported by fetch's init, this library supports a few more options:

• responseType (String|Function): Any of the methods from the Body mixin. The default is "json", unless the response status code is "204", in which case "text" will be used.

  If a function is passed, then it will be passed the response object. This lets you dynamically determine the response type based on information about the response, such as the status code.

• requestKey (String): A string that is used to determine if two requests are identical. You may pass this to configure how the request key is generated. A default key will be generated for you if this is omitted.

• dedupe (Boolean): Whether or not to dedupe the request. Pass false and it will be as if this library was not even being used. Defaults to true.

• cachePolicy (String): Determines interactions with the cache. Valid options are "cache-first", "cache-only", and "network-only". For more, refer to the section on Caching.

Let's run through valid calls to fetchDedupe:

import { fetchDedupe } from 'fetch-dedupe';

fetchDedupe('/test/2');

fetchDedupe('/test/2', {
  method: 'DELETE'
});

fetchDedupe('/test/2', {
  method: 'PATCH',
  body: JSON.stringify({value: true}),
  credentials: 'include',
  responseType: 'json',
  requestKey: generateCustomKey(opts),
  dedupe: false,
});

getRequestKey({ url, method, responseType, body })

Returns a unique request key based on the passed-in values. All of the values, including body, must be strings.

Every value is optional, but the deduplication logic is improved by adding the most information that you can.

Note: The method option is case-insensitive.

Note: You do not need to use this method to generate a request key. You can generate the key in whatever way that you want. This should work for most use cases, though.

import { getRequestKey } from 'fetch-dedupe';

const keyOne = getRequestKey({
  url: '/books/2',
  method: 'get'
});

const keyTwo = getRequestKey({
  url: '/books/2',
  method: 'patch',
  body: JSON.stringify({
    title: 'My Name is Red'
  })
});

keyOne === keyTwo;
// => false

responseCache.get( requestKey )

Returns the cached response for requestKey. If the response does not exist, then undefined will be returned instead.

responseCache.set( requestKey, res )

Call this to manually update the cached value of requestKey with res. Returns the responseCache.

Note: this is an advanced method, and you generally do not need to manually update the store.

responseCache.has( requestKey )

Pass in a requestKey to see if there is a cache entry for the request. This can be used to determine if a call to fetchDedupe will hit the cache or not.

responseCache.delete( requestKey )

Deletes the cached value associated with requestKey. Returns false if the value did not exist in the cache, or true if it existed and has been deleted.

responseCache.clear()

Remove all responses from the cache.

responseCache.useCachedResponse( fn )

By default, fetch-dedupe caches responses indefinitely. You can customize this behavior using this method.

This method accepts a single argument, fn, which is a function. fn will be called any time that a request is made that has a cached response. It's called with a single argument, cacheObject, an object with the following properties:

• res: The cached response object.
• createdAt: A timestamp (in milliseconds) when the value was added to the cache.
• lastAccessedAt: A timestamp (in milliseconds) when the value was last read from the cache.
• accessCount: An integer representing the number of times that the value has been read from the cache.

Return true to use the cached response, or false to remove the value from the cache and make a network request instead.

For instance, to invalidate cached responses that are more than 10 minutes old:

import { responseCache } from 'fetch-dedupe';

// 1000 = 1 second in milliseconds
// * 60 = 1 minute
// * 10 = 10 minutes
const TEN_MINUTES = 1000 * 60 * 10;

responseCache.useCachedResponse(({ createdAt }) => {
  const currentTimestamp = Date.now();
  return currentTimestamp - createdAt <= TEN_MINUTES;
});

activeRequests.isRequestInFlight( requestKey )

Pass in a requestKey to see if there's already a request in flight for it. This can be used to determine if a call to fetchDedupe() will actually hit the network or not.

import { activeRequests, getRequestKey } from 'fetch-dedupe';

const key = getRequestKey({
  url: '/books/2',
  method: 'get'
});

// Is there already a request in flight for this?
const readingBooksAlready = activeRequests(key);

activeRequests.clear()

Removes all of the tracked in-flight requests. In-flight requests are not cancelled: calling this method only ensures that subsequent identical requests are not deduped.

Note: you typically should not need to use this method.

CacheMissError

A call to fetch-dedupe will reject to this value if you specify a cachePolicy of cache-only, and there is no cached response in the store.

The Promise only rejects to this whith a cache-only cache policy, because any other policy would make a network request if the value isn't found.

import { CacheMissError } from 'fetch-dedupe';

fetch('/api/books/23', {
  cachePolicy: 'cache-only'
}).then(
  () => console.log('Succeeded'),
  (err) => {
    if (typeof err === CacheMissError) {
      console.log('This request did not having an associated response in the store');
    }
  } 
)

Guides

Caching

Any time tbat a response from the server is received, it will be cached using the request's request key. Subsequent requests are matched with existing cached server responses using their request key.

Interactions with the cache can be controlled with the cachePolicy option. There are three possible values:

cache-first

This is the default behavior.

Requests will first look at the cache to see if a response for the same request key exists. If a response is found, then it will be returned, and no network request will be made.

If no response exists in the cache, then a network request will be made.

network-only

The cache is ignored, and a network request is always made.

cache-only

If a response exists in the cache, then it will be returned. If no response exists in the cache, then an error will be passed into the render prop function.

FAQ & Troubleshooting

Why is response.data set to null sometimes?

If the response cannot be parsed as the responseType, then it will be set as null.

There are two common situations for this:

• The response body is an empty string when you specify responseType: 'json'

• The response body is a raw text string when you specify responseType: 'json' (i.e.; invalid JSON)

You can use the responseType option to have fine-grained control over the parsing of the response body from the server.

Why is responseType even an option?

The argument that is returned to you in the .then callback of a call to fetch() is a Response object. The body of a Response object can only be read a single time, because it is a ReadableStream.

For Fetch Dedupe to work, it must pass the result of a single request to many "consumers." The only way for this to work is if the library reads it for you, which requires that the library know what its content type is.

What request body types are supported?

Just strings for now, which should work for the majority of APIs. Support for other body types is in the works.

Is the data duplicated?

Although you receive a new Response object with every call to fetch-dedupe, the body will be read, so the response's body stream will be empty. In addition, the data property between every response is shared. Accordingly, the data returned by the server is never duplicated.

This is an optimization that allows fetch-dedupe to be used in applications that fetch large payloads.

Requirements

• a global fetch() method. If your browser does not support it, then we recommend GitHub's fetch polyfill.

Note: Node users can try and use node-fetch, although we aren't currently targeting Node support with this library.

Acknowledgements

Apollo inspired me to write this library.