受欢迎的博客标签

caching of data, partial pages and full pages at the server and client level in ASP.NET Core

Published
Created: 2017年8月21日星期一 下午2:16:16 Latest updated:2020年11月30日星期一 上午7:32:05 views(2860)

A guide to caching in ASP.NET Core

This post looks at the various techniques available in ASP.NET Core for caching. We'll look at caching of data, partial pages and full pages at the server and client level and explain when to use each.

Why cache?

Caching adds some complexity to an application and as developers we must always be cautious when making our applications more complex so why bother with caching at all? A few reasons spring to mind:

Just to emphasise how much of a speed benefit you can achieve through caching, if your page requires database calls to render, then caching the page can sometimes be an order of magnitude faster. Even completely static MVC views, without any external calls are faster when cached.

Given all these benefits and the fact that it can be very easy to add caching to your sites, it makes it a very attractive proposition. You do have to be careful though and later in the article, we will discuss some of the problems that caching can introduce.

What to cache?

Choosing what to cache is highly dependent on application. Generally speaking, to maximize performance, we want to cache at the highest level we can get away with.

1.For assets such as CSS, JS and images, we should be aggressively caching at the browser level and a cache duration of a year or more is fairly standard.

2.For relatively static, non-personalised pages, we can cache the entire page at both client and server level.

3.If this is not possible, caching parts of a page can avoid unnecessary calls and help decrease response times.

4.At an even lower level, caching data to reduce calls to databases can be useful - particularly if the cached data is the result of multiple queries.

Typically, most applications will use a combinations of the above techniques for different areas.

Where to cache?

It makes sense to instruct the web browser to cache pages that rarely change but it is important to be aware of the limitations of caching at the client.

Unlike server side caching, it is not possible to instruct the browser to invalidate cache entries. This means that we need to be very careful with cache durations. The current best practice advice is to use very high cache durations for static assets and simply change the filename if an update is necessary. Obviously this is not an option for web pages. We can hardly change the URL of published pages every time an update is required. Instead, for web pages, we need to set a much lower cache duration at the client and rely on the server to efficiently handle requests.

Using a CDN is beyond the scope of this article, but is always worth considering, especially given how easy it is to set up today.

We also need to think about how server-side caching works once we move beyond a single server instance. When you have a single server, in-memory caching is easy to set up and generally works perfectly well. Most commercial sites however run on multiple load-balanced servers and dealing with caching becomes significantly more complex.

You have two high-level choices to consider. Do you:

  • Maintain a discrete cache on every server
  • Use a centralised cache that each server accesses

The first option, although slightly faster, has too many negatives to be recommended as a general solution:

  • Discrepancies between caches can cause major headaches
  • It is difficult to invalidate cache entries
  • Wasted RAM filled with duplicated data

Using sticky sessions (server affinity) can help avoid some issues but if you have multiple web servers, I strongly recommend you use a distributed cache solution, for all but the simplest of cases (i.e. caching static lookup data).

Caching data with IMemoryCache and IDistributedCache

The lowest level of caching in ASP.NET Core that we are going to discuss is the caching of data using IMemoryCache and IDistributedCache. These interfaces are the standard, in-built mechanisms for caching data in .NET Core. All other techniques that we discuss later in the article rely on IMemoryCache or IDistributedCache internally.

IMemoryCache

IMemoryCache is very similar to the System.Runtime.Caching.MemoryCache cache from .NET 4.

The interface itself is rather minimal:

public interface IMemoryCache : IDisposable
{
    bool TryGetValue(object key, out object value);
    ICacheEntry CreateEntry(object key);
    void Remove(object key);
}

This only tells half the story though because there are multiple extension methods available which make the API much richer and easier to use:

public static class CacheExtensions
{
    public static TItem Get<TItem>(this IMemoryCache cache, object key);
    
    public static TItem Set<TItem>(this IMemoryCache cache, object key, TItem value, MemoryCacheEntryOptions options);

    public static bool TryGetValue<TItem>(this IMemoryCache cache, object key, out TItem value);
    ...
}

You can register IMemoryCache in ConfigureServices using:

services.AddMemoryCache();

If you are using MVC though then it will be automatically registered.

In either case, if you specify IMemoryCache in a component's constructor then it will get resolved along with the rest of the dependency graph.

The following code shows a naive example of using IMemoryCache to avoid hitting the database. The example returns a cached version of the data if available, else it queries the database, caches the data and returns the result:

public class BlahService
{
    private const string BlahCacheKey = "blah-cache-key";
	
    private readonly IMemoryCache _cache;
    private readonly IDatabase _db;

    public BlahService(IMemoryCache cache, IDatabase db)
    {
        _cache = cache;
        _db = db;
    }
	
    public async Task<IEnumerable<Blah>> GetBlahs()
    {
        if (_cache.TryGet(BlahCacheKey, out IEnumerable<Blah> blahs))
        {
            return blahs;
        }
		
        blahs = await _db.getAll<Blah>(...);
		
        _cache.Set(BlahCacheKey, blahs, ...);
		
        return blahs;
    }
}

When saving to IMemoryCache, MemoryCacheEntryOptions provides you with many ways to expire cache content. Options include absolute expiry (a fixed time), sliding expiry (time since last accessed) and expiry based on a token which is a powerful technique for creating dependencies between cache items. There are also overloads of Set which allow you to choose an expiration time directly. Here are a few examples:

//absolute expiration using TimeSpan
_cache.Set("key", item, TimeSpan.FromDays(1));

//absolute expiration using DateTime
_cache.Set("key", item, new DateTime(2020, 1, 1));

//sliding expiration (evict if not accessed for 7 days)
_cache.Set("key", item, new MemoryCacheEntryOptions
{
    SlidingExpiration = TimeSpan.FromDays(7)
});

//use both absolute and sliding expiration
_cache.Set("key", item, new MemoryCacheEntryOptions
{
    AbsoluteExpirationRelativeToNow = TimeSpan.FromDays(30),
    SlidingExpiration = TimeSpan.FromDays(7)
});

// use a cancellation token
var tokenSource = new CancellationTokenSource();

var token = new CancellationChangeToken(tokenSource.Token);

_cache.Set("key", item, new MemoryCacheEntryOptions().AddExpirationToken(token));

When using cancellation tokens, you can store the CancellationTokenSource itself in the cache and access it when you need to. To evict all cache entries using tokens from a particular CancellationTokenSource, you can just call the Cancel method:

tokenSource.Cancel();

The documentation provides full details for all the options you can use with IMemoryCache.

IDistributedCache

For web farm scenarios, you will want to make use of IDistributedCache instead of IMemoryCache:

public interface IDistributedCache
{
    byte[] Get(string key);
    Task<byte[]> GetAsync(string key);

    void Set(string key, byte[] value, DistributedCacheEntryOptions options);
    Task SetAsync(string key, byte[] value, DistributedCacheEntryOptions options);

    void Refresh(string key);
    Task RefreshAsync(string key);

    void Remove(string key);
    Task RemoveAsync(string key);
}

The interface provides similar functionality to IMemoryCache but there are some notable differences:

  • Additional async methods
  • Refresh methods (which just reset sliding expirations without retrieving data as far as I can tell)
  • Byte based rather than object based (though extension methods add the ability to use string values)

This last change means that you will need to serialize any objects being stored yourself. The example below uses Json.NET for this:

public async Task SaveToCache<T>(string key, T item, int expirationInHours)
{
    var json = JsonConvert.SerializeObject(item);

    await _cache.SetStringAsync(key, json, new DistributedCacheEntryOptions
    {
        AbsoluteExpirationRelativeToNow = TimeSpan.FromHours(expirationInHours)
    });
}

public async Task<T> RetrieveFromCache<T>(string key)
{
    var json = await _cache.GetStringAsync(key);

    return JsonConvert.DeserializeObject<T>(json);
}

DistributedCacheEntryOptions offers absolute and sliding expiration much like MemoryCacheEntryOptions but token based expiration is absent. This makes adding cache dependencies much more of a challenge and you will need to roll your own implementation if you need this functionality.

There are three Microsoft implementations of the IDistributedCache interface currently available. These include a local, in-memory version ideal for development, plus Redis and SQL Server versions. Given that SQL Server is disk based rather than in-memory, I can't imagine many people opting for this version.

The local in-memory version of IDistributedCache is part of Microsoft.Extensions.Caching.Memory so is already brought in by the MVC package. If you should need to manually add it, you can use:

services.AddDistributedMemoryCache();

The Redis implementation is available in the Microsoft.Extensions.Caching.Redis NuGet package. After referencing the NuGet, simply add the following to ConfigureServices:

services.AddDistributedRedisCache (option =>
{
    option.Configuration = "your connection string";
    option.InstanceName = "your instance name";
});

Caching partial pages with Tag Helpers

There is no doubt that caching of data can significantly speed up server responses but we can often go one step further and cache rendered page output rather than (or in addition to) raw data. Partial page caching is available using the built-in Caching Tag Helpers.

The cache tag helper

At it's simplest, you can wrap part of a view in cache tags to enable caching:

<cache>...</cache>

This results in that part of the page being cached for a default duration of 20 minutes.

Obviously caching an arbitrary part of the page is next to useless unless that part of the page is expensive to render. One obvious candidate for caching is a view component call. If you are not familiar with view components then you can think of them as more capable successors to child actions. They are very useful for secondary parts of the page such as sidebars. If your sidebar is dynamically generated from a database then caching the result can be extremely beneficial.

<cache expires-on="@TimeSpan.FromSeconds(600)">
    @await Component.InvokeAsync("BlogPosts", new { tag = "popular" })
</cache>

The above example caches the blog posts view component for 10 minutes.

The cache tag helper allows you to vary the cache (i.e. create multiple separate copies) by many different criteria including headers, queries, routes, cookies and even users.

<cache expires-sliding="@TimeSpan.FromHours(1)" vary-by-user="true">
    ...user specific content...
</cache>

I am not sure how well some of these options will scale on busy sites, but the cache tag helper is certainly very feature rich. See the docs for full details.

The distributed-cache tag helper

As with IMemoryCache, the Cache tag helper has a sibling for use in web farm situations where a distributed solution is required.

<distributed-cache name="key">
    @await Component.InvokeAsync("BlogPosts", new { tag = "popular" })
</Cache>

In use, the distributed cache tag helper is very similar to the in memory version. Other than the tag name change from cache to distributed-cache, the only notable difference is the requirement of a name attribute for the distributed version. This value is used to generate a key for the cache entry.

Internally, the tag helper uses the IDistributedCache outlined in the previous section. If you do not configure an IDistributedCache implementation in ConfigureServices then the in-memory version is used without any configuration necessary.

Caching full pages with response caching

The highest level of caching that we can make use of is caching of the entire page. Caching full pages in the browser results in the very minimum of server load and caching fully rendered pages on the server can also hugely reduce load and response times.

In .NET core, these two techniques are closely related. We have a ResponseCache attribute which is used to set cache headers and we have a ResponseCaching piece of middleware which can optionally be used to enable server side caching.

Caching in the browser

You can set cache headers manually using Response.Headers but the preferred approach for caching actions is to make use of the ResponseCache attribute which can be applied to both actions and controllers.

[ResponseCache(Duration = 3600)]

Duration is measures in seconds so the above attribute would cache the page in the browser for 3600 seconds or one hour.

Instead of specifying values for each instance of the attribute, we can also configure one or more cache profiles. Cache profiles are configured in ConfiguresServices when you add the MVC middleware:

services.AddMvc(options =>
{
    options.CacheProfiles.Add("Hourly", new CacheProfile()
    {
        Duration = 60 * 60 // 1 hour
    });
    options.CacheProfiles.Add("Weekly", new CacheProfile()
    {
        Duration = 60 * 60 * 24 * 7 // 7 days
    });
});

You can then reference the cache profile names in the ResponseCache attributes

[ResponseCache(CacheProfileName = "Weekly")]

You can also explicitly disable any caching with the following:

[ResponseCache(Location = ResponseCacheLocation.None, NoStore = true)]

Note that both Location and NoStore are required. NoStore returns the standard "cache-control: no-store" header but some older proxies do not understand this so Location = ResponseCacheLocation.None adds "no-cache" values to cache-control and pragma headers.

Caching at the server

As mentioned above, caching at the server uses a piece of middleware which reads the values set by the ResponseCache attribute and caches the page appropriately.

The response caching middleware is in a separate NuGet package so you will need to add a package reference to Microsoft.AspNetCore.ResponseCaching in order to use it. Once installed, it can be added to the pipeline by adding the following to ConfigureServices:

services.AddResponseCaching();

The middleware takes the cache duration from the cache-control header set by the Response Cache attribute. It also respects the VaryByHeader option allowing you to cache multiple versions of the page. One common use for this would be to vary by Accept-encoding header so you can cache both gzipped and non-gzipped responses, plus any other compression algorithms you are using. This of course assumes that you are compressing within your application rather than at the reverse proxy level.

As well as the standard VaryByHeader option, you can use the ResponseCache attribute to specify a VaryByQuery value which is used exclusively by the server side response caching middleware. As you would expect, this causes the middleware to store additional response copies based on the query string values specified.

Limitations

Server side response caching can provide astonishing gains in speed but it is important to be aware of the limitations of such an approach.

If you are caching fairly static content for anonymous users and the pages have no personalisation or forms then full-page caching is ideal. Unfortunately, this is rarely true and you will run into issues if you try to cache pages in these other situations.

In fact the built-in response caching middleware will not cache the page if any of the following are true:

  • The response code is not 200
  • The request method is not GET or HEAD
  • An Authorization header is present
  • A Set-Cookie header is present

In addition, using the Anti-CSRF features of MVC will override any explicit cache-control header you have set and replace it with "no-cache, no-store" resulting in your page not being cached. This is essential because this feature works by setting a cookie and embedding a form value so you do not want to cache these values.

Cache invalidation

One common approach to full page caching is to cache on the client for a short period of time and on the server for a much longer period. This technique relies on the fact that we can typically remove cache entries early on the server if necessary. This way we can effectively cache frequently accessed pages indefinitely and only invalidate the cache and store a new version if the page changes.

Unfortunately, the built-in response caching middleware makes this very difficult. Firstly, the same cache duration is used for both client and server caches. Secondly, currently there is no easy way to invalidate cache entries. This is a real shame and I hope it is something that will change. For now I ended up writing my own basic implementation which we will look at next time.

Caching JS, CSS, Images etc.

Images and other static files are served by adding the static files middleware. A typical registration in the Configure method of startup.cs looks like this:

app.UseStaticFiles();

This code will enable the serving of static files but not in the most efficient way. By default, no cache headers are used so the browser will request these files again and again, slowing your sites and putting more load on your server.

The good news is that it is very easy to change the static files registration code to enable browser caching. Here we set caching to a year:

app.UseStaticFiles(new StaticFileOptions
{
    OnPrepareResponse = (context) =>
    {
        var headers = context.Context.Response.GetTypedHeaders();

        headers.CacheControl = new CacheControlHeaderValue
        {
            Public = true,
            MaxAge = TimeSpan.FromDays(365)
        };
    }
});

Summary

Caching can drastically reduce your costs and also provide a more responsive site for your customers. We looked at a number of different techniques from low level caching of data through to entire page caching at both the client and server. We discussed some gotchas to be aware of when adding caching to your applications and also explained about a few limitations when using the built-in response caching.

Next time, we'll take a look at writing our own response caching solution.

Useful or Interesting?

If you liked the article, I would really appreciate it if you could share it with your Twitter followers.