CDN Recommendations

How it works

The purpose of a CDN is to ensure high availability when serving content to visitors. A CDN consists of a globally distributed network of proxy servers deployed in multiple data centers. CDNs deliver content from the fastest and closest server available.

The CDN has its own URL pointing to the original URL. A visitor goes to something like www.customer.com. A request goes to the CDN, which in turn goes to the original URL for non-cached data. The request returns to the CDN, which caches the content.

By reducing the number of server requests to geographically dispersed locations, you can provide and manage more requests from edge servers closest to the visitor location, resulting in a faster browsing experience.

The cache hit ratio describes the percentage of requests the CDN could answer from its own cache.

The CDN stores a copy of your objects and, upon request, serves objects from these caches. The CDN servers are designed to cache content according to the cache rules you define in the HTTP cache headers for your web application. It is critical that you set these caching rules correctly for your solution to scale properly.

CDNs and web applications

Setting up a CDN is fairly easy and, even with minor configuration work, you can get better performance. Develop your application with the CDN in mind from the start, and add proper cache headers based on what you want to achieve. This way, when you connect the CDN, it works automatically.

You should control the cache settings in the web application rather than writing overriding rules in the CDN, because this can quickly get quite complex. However, there may be situations where you need  rules, such as when you need special configurations, or if you do not want to deploy with these changes.

General recommendations

Evaluate whether you can cache frequently-requested objects. You can store Staticobjects indefinitely in the caches. For cloud-based solutions, you should  cache resources (by the intermediary proxies , such as those provided by a CDN) such as static and resized images, JavaScript and CSS files, and scripts.

In most cases, you can include a version identifierin the path to the resources. If an object changes, the version identifier updates the URL to it, and immediately reflects the change. The best way to control the cache for assets is to use unique file names for every deployment. For example: use site-1.0.css in the first deployment, and site-1.1.css in the second deployment. This ensures that the CDN uses the latest CSS file right after deployment.

If you cannot use version identifiers for frequently-requested objects, you can set a maximum lifespan for an object. This is one of the values that are typically set in the cache-control. The ordered priority should be:

1. Version identifier plus indefinite caching
2. No version identifier plus maximum lifespan

Configuring cache headers

Using the correct cache headers in your application ensures that you get the most out of the CDN service, whether you want to just cache static content, or text/HTML. You can use Last-Modified, cache-control settings, ETags, or a combination of these, to determine whether content has changed.

Cache-control

There are many HTTP header settings, but cache-control is one of the most important ones because this setting determines how long a cached object should remain cached.

A CDN monitors the cache headers in the response settings:

• cache-control: no-cache. The CDN will notcache the content.
• cache-control: public.  The CDN caches the content.

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Important

You must set the cache-control to public  (not private where only the browser is allowed to cache the content).

Typical values:

• max-age=$seconds. Determines the lifespan for an object.
• must-revalidate. Tells the cache to strictly follow maximum lifespan information.
• public. Can be cached by intermediaries.
• private. Can be cached, but only by the browser.

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Caution

Do not set cache to public on page requests if the page has private information, such as a cart page in e-commerce.

Example: Caching an object for a maximum of 20 minutes (1200 seconds).

Cache-Control: public, max-age=1200, must-revalidate

Caching of static content

To minimize the trips to the web server, use as long max-age as possible, at least for static content. In StaticFileOptions,there are specific sections for setting cache headers for static content. You can see an example here:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    ...

    const string cacheMaxAge = "604800";
    app.UseStaticFiles(new StaticFileOptions
    {
        OnPrepareResponse = ctx =>
        {
            // using Microsoft.AspNetCore.Http;
            ctx.Context.Response.Headers.Append(
                 "Cache-Control", $"public, max-age={cacheMaxAge}");
        }
    });
    
    ...
}

Overriding SetCachePolicy

If you use MaxAge, this sets cache-control to public. The following example sets the max-age using a TimeSpan of one hour, which is converted automatically to 3600 seconds.

public class StartPageController : PageControllerBase
{
    public ActionResult Index(StartPage currentPage)
    {
        Response.GetTypedHeaders().CacheControl.Public = true;
        Response.GetTypedHeaders().CacheControl.MaxAge = TimeSpan.FromHours(1);

        return View(currentPage);
    }
}

If you use Expires, this sets cache-control to public, and Expires to your selected date and time.

public class StartPageController : PageControllerBase
{
    public ActionResult Index(StartPage currentPage)
    {
        Response.Cache.SetCacheability(HttpCacheability.Public);
        Response.GetTypedHeaders().Expires = DateTime.Now.AddHours(1);

        return View(currentPage);
    }
}

Using ETags

The ETag(entity tag) is a part of the HTTP protocol determining cache validation, and is best used for staticcontent, such as files, uploaded assets, and pages without output caching. You should not use ETags for requests to resources composed from dynamic content objects such as a landing page.

You can combine ETags with other settings. You can use both Cache-Control max-age and ETags to refine cache management and optimize performance. You can also use ETags only, if you have specific caching control requirements.

Recommendations

• Static deployed files such as CSS
  Set the expire date time interval for as long as possible, then update the URL to include a version number that changes when new static files are uploaded.
• Uploaded assets such as images
  ETags are good for this. However, do not set expire times that are longer than the editors can wait for a file to be updated because editors might get confused if they upload a new image with the same name, and the cache still shows the old image.
• Custom API controllers and "normal" HTML
  Normally, you should not cache dynamic content, but it may be useful in some cases. Only cache if you do not have personalized content. If an API is cached, make sure all variables are pulled from the URI rather than headers to avoid displaying incorrectly cached information, and test thoroughly before going live. Do not cache administrative or confidential information.

Caching levels for query strings

The default cache layer setting in DXP for query strings is Standard, meaning that a different resource is delivered each time the query string changes.

SSL requirements

Proper encryption practices are necessary to prevent bad actors from accessing data. The CDN used in DXP includes web encryption based on TLS (Transport Layer Security)/SSL (Secure Sockets Layer) protocols, for communication with other services over HTTP (HTTPS).

The service includes a default certificate provided by Optimizely, but you can replace this with your own. If you replace the certificate, the following requirements apply:

• The certificate must be valid (not expired), issued by a trusted Certificate Authority (not self-signed), and the DNS must be correct.
• The certificate files (certificate and key) should be PEM-encoded.

Versioned URLs for assets

When the application is deployed to a DXP environment, there can be lag-time for assets to update when replacing them in the CMS, because the asset is still cached in CDN or browsers. You can overcome this by versioning URLs to assets. See https://github.com/bjuris/EPiServer.CdnSupport for an (unsupported) example of how to accomplish this.

Related topics

• Content Delivery Network (CDN) Configuration
• Cache-Control headers (IETF Tools)