CacheCow

An implementation of HTTP Caching in .NET Core and 4.52+ for HttpClient, ASP.NET Web API, ASP.NET MVC Core and Carter.

This document covers topics below:

• Breaking changes in Version 2.0.0
• Concepts and definitions
• Getting started:
  • Getting Started - Client
  • Getting Started - ASP.NET Core MVC
  • Getting Started - ASP.NET Web API
• CacheCow Header
  • CacheCow.Client Header
  • CacheCow.Server Header
• Running Samples
  • CacheCow.Samples.MvcCore
  • CacheCow.Samples.WebApi
  • CacheCow.Samples.WebApi.WithQueryAndIoc
  • CacheCow.Samples.Carter
• CacheCow.Server Advanced Options
• Dependency Injection scenarios in ASP.NET MVC Core
• Configuration options with ASP.NET Core
• Dependency Injection scenarios in ASP.NET Web API
• Migrating projects using older CacheCow.Server

NOTE: Breaking changes in Version 2.0.0

There are quite a few changes in the server-side implementation of CacheCow due to shortcomings of previous approach while the client-side mostly remained the same. Hence if you have used CacheCow.Server in the past and you would like to migrate, read the guide on migration further down. CacheCow.Client has some changes which may or may not break your code (depending whether you had used .NET 4.5 packages) which includes:

• All interfaces are fully async now
• Many cache storages no longer maintained and available including: MongoDb, Memcached, File and Elasticsearch. Currently only Redis, SQL and InMemory are supported although I accept PR if you would like to add your storage.
• ICacheStore implementations optionally supported a cache quota management interface which is no longer supported.
• Renaming CacheCow client header from "x-cachecow" to "x-cachecow-client"
• CacheCow.Client now copies any headers starting with x-cachecow it receives from the server to the response it delivers to your HttpClient.

Concepts and definitions

It is useful to start with concepts but feel free to use it as a reference. If you get bored reading this 😀, jump straight to Getting Started sections or simply browse and run a couple of samples.

• CacheCow.Client: Client constructs for HTTP Caching when making HTTP calls with .NET's HttpClient.
• CacheCow.Server: Server constructs for HTTP Caching when serving HTTP APIs in ASP.NET Web API or MVC Core.
• Resource: A RESTful resource - generally identified by its URI.
• Representation: A particular embodiment of a resource according to its format, language, etc. For example /api/car/1 is a resource and it might have two representations as JSON or XML. Also the same resource could have two representations in Chinese or Spanish. Headers by which representations will vary is defined by the Vary header from the server.
• HTTP Caching: Caching as described in HTTP 1.1 Specification (RFC 7230 or 2616). Despite the common notion, the representations get cached on the client (not server) and the server is responsible for providing cache directives and validate conditional requests on resources.
• ICacheStore: CacheCow interface responsible for storing cacheable representation on the client. There are choice of storages, at this point InMemory and Redis are available. CacheCow itself is responsible for storing representations separately.
• Cache directives: These are various cache-related headers that the server provides to guide the client on how best cache and validate the resources. These headers include Cache-Control, Vary, ETag and in HTTP 1.0 would include Expires, Last-Modified and Pragma (for the purpose of this article, we include ETag and Last-Modified as part of directives although purely speaking they are not). CacheCow has ICacheDirectiveProvider interface responsible for controlling these headers.
• Expires header: Defines expiry of a resource in absolute time. Cache-Control header provides richer semantic and supersedes it. CacheCow sets both to support both HTTP 1.0 and 1.1.
• Expiry: Expiry of a cached representation on the client is defined by the directives sent by the server. It is crucial to note that the client normally does not eject the cached representation right after the expiry - the client can carry on using the cached representation provided it checks with the server if the representation is still valid (see Consistency below).
• Last-Modified header: Since its precision is at seconds, its use is not recommended for valiation and instead, it is recommended to use ETag.
• ETag header: ETag or EntityTag is an opaque string that identifies a version of resource (not representation). If you have a high-precision Last-Modified date, it is better to turn it to ETag by binarising the date (example).
• TimedETag: A CacheCow construct that combines Last-Modifed and ETag (can have either of them but not both) and represents a version of the resource and implemented as TimedEntityTagHeaderValue. It is recommended to construct it with an ETag (due to low precision of DateTime in HTTP Spec's Last-Modified header).
• ITimedETagExtractor: A CacheCow interface responsible for extracting TimedETag from view models sent back by the API. By default, it checks to see if the ViewModel has implemented ICacheResource, if so it enquires the TimedETag directly. If not, it will resort to serialising the ViewModel to JSON and use its hash as ETag. This can be expensive hence it is suggested to either your ViewModels implementing ICacheResource interface or implement ITimedETagExtractor - plenty of examples in the samples.
• Consistency: Resources underlying the cached representationa could change without any notice making them invalid - this problem has been half-joking known as one of the two most difficult problems in computer science, the other being naming. For many resources this could be acceptable while for some simply not tolerable. HTTP solves this problem with a single round-trip call that would either approve the use of the version client has cached (by returing status 304) or return the latest version of the resource in the representation requested. This is known as conditional GET. HTTP also provides means for optimistic concurrency and only update (or delete) of the resource if it matches the ETag or has not changed since the Last-Modified date supplied. This is known as conditional PUT/PATCH/DELETE. CacheCow supports these scenarios OOB.
• Zero-Expiry resources (consistent resources): These resources require a high consistency and the clients are allowed to cache the representations and re-use only if they make a conditional GET to confirm the version they have is valid. Essentially, the representation is expired as soon as it leaves the server but can be safely re-used if the client makes a conditional GET. It might seem that caching in these scenarios is not really beneficial, but in fact it helps to reduce network traffic, client and server resource usage and even can protect back-end systems. The trick on the server is to just find out whether a resource has changed without loading it all the way from back-end systems. For example, to know whether a record has been changed, we can check whether its LastModifiedDate (or Timestamp) has been modified against the ETag or a date. Or for a list of records, the most recent LastModifiedDate of all records along with the count can be used which can be executed in a single query (For example in SQL you would use SELECT COUNT(1), MAX(LastModifiedDate) FROM MyTable). Such queries will be fast and cheap. CacheCow provides ITimedETagQueryProvider interface to preemptively query the backend stores for conditional HTTP calls without load the resources.
• ITimedETagQueryProvider: This interface allows server implementations to query their back-end and carry out validation against it. This is the best way to have APIs support consistency and the most efficient level of caching.

Getting started - Client

Client scenario is perhaps the most common use case of CacheCow. Most of the concepts discussed above relate to the server-side. Client-side CacheCow has been implemented as a DelegatingHandler and has very few concept counts - most of the complexity of HTTP Caching has been hidden away from you. For the purpose of this guide, we choose an In-Memory storage which is the default.

1) Install the nuget package

> install-package CacheCow.Client

2) Use ClientExtensions to create an HttpClient (piped to a `C