Http Cache Headers Message Handler for Http Client

Delegating message handler for Http Client that caches responses using a distributed cache. This handler will cache responses in the following way:

1. Cheks the response for valid Etag and Cache-Control headers, and caches the response.

2. The next time the same path is requested, it checks if there is a cache entry and appends Conditional headers to the request. Read here

3. If the server responds with NotModified status code, the response content is replaced with the cached entry.

This message handler is responsible for caching ONLY on the client side. Please check HttpCacheHeaders, an easy to use middleware for supporting cache headers on the server side. The sample in this repo uses this middleware and the tests reference some of the code from there.

Installation (NuGet)

Install-Package Http.Cache.Headers.MessageHandler

Usage

First, register the services with your dependency injection container

services.AddHttpCacheHeadersMessageHandler(options =>
{
    options.IgnoreNonGetRequest = true;
    options.CacheEntryPrefix = "EtagCachingMessageHandler";
    options.CacheKeyHeaderNameExclusions = new[] { "some-auto-generated-header" };
})

You will also need to register a distrubuted cache

services.AddDistributedMemoryCache();

Add the message handler to your http client as follows

services.AddHttpClient<MyHttpClient>()
    .AddHttpMessageHandler<HttpCacheHeadersDelegatingHandler>();

Note: the back-end needs to already support http cache headers for this handler to work. Also make sure to header the next section

Header Exclusions

The cache key is generated using a combination of the request path plus the headers.

Headers with auto-generated values must be excluded when creating the cache key. If your header gets a new value for every request then it should be excluded

Not excluding these header means the handler will never find your cache entries, and will never append the Conditional headers to the request.

You can exclude these types of headers when registering the services

options.CacheKeyHeaderNameExclusions = new[] { "some-auto-generated-header" };