Content delivery networks

1. What’s a CDN?

A CDN is a large number of web servers that are distributed across the world. These web servers usually sit close to the internet backbone and can quickly serve up large files. When a user makes a request to the CDN for a file, the CDN figures out which data center in the CDN is closest to the user’s location and serves up the content from that data center.

Figure 1 shows that if you’re based in Edinburgh, your files are served from the Dublin data center, rather than from one in Hong Kong.

Figure 1. A CDN delivers files using the server closest to you.

In a CDN, a user makes a request via the nearest edge server and the origin server answers the request. Let’s look at these servers in more detail.

Edge Servers

Figure 1 shows that the CDN network has the following data centers

• Los Angeles

• New York

• Dublin

• London

• Dubai

• Hong Kong

• Tokyo

These data centers, which are represented by circles in figure 1, are known as edge servers. An edge server is the name used for one of the geographically dispersed web servers that are responsible for serving your content.

Origin Server

The origin server is the web server that contains the original version of the content being distributed. In figure 1, the origin server is represented by the triangle and resides in Washington State. For Windows Azure, the origin server could be either your web role or a public container in your BLOB storage account. The content held on your origin server would never be accessed directly by your end user; the origin server only serves content to the edge servers.

When a request is made for a file held in a CDN, the request is redirected to the nearest edge server. If the edge server doesn’t have a local copy of that file, it requests the file from the origin server and caches it locally.

2. CDN performance advantages

Using a CDN network is a simple method of improving performance on your web servers without significantly changing your architecture or code. How so, you ask? Well, read on.

Reduced Load from Your Web Application Servers

If your static content is offloaded from your web application servers, your web servers will have more capacity to handle incoming requests. The reduced load will reduce active connections, CPU use, and network traffic on the server.

Increased Client-Side Performance

The largest bottleneck on web applications tends to be the time it takes to download static content from the web server, not the time it takes to serve the HTML page. Figure 2 shows the number of requests made to www.manning.com and the length of time it has taken for the content to be served.

Figure 2. Comparison of the number of seconds it takes to serve HTML pages versus the number of seconds it takes to download the static content at www.mannning.com

If you look carefully at figure 2, you’ll see that it takes only 1.72 seconds to serve up the HTML page, but an additional 3.5 seconds to serve the static content. This extra time is used because most internet browsers (including Firefox and Internet Explorer) can download only two files simultaneously from the same domain. If more than two files are requested at the same time, all other requests from the same domain are queued. The graph in figure 2 shows that the .gif files from manning.com are being downloaded only two files at a time, but the files being served from google-analytics.com can be downloaded at the same time as the manning.com gifs.

A CDN moves static content to other subdomains, which allows the browser to simultaneously download more files and reduces the time it takes to render content on the client browser.

With the reduced network latency, faster internet connections, and the increased number of distributed web servers, any static content served from the CDN is delivered more quickly than it would be from a standard web server.

OK, you’ve got the message that you need to use CDNs, so pay close attention while we tell you how to use them with Windows Azure.

3. Using the Windows Azure CDN

So far, we’ve looked at CDNs from a generic standpoint; in this section, we’ll take a look at what options are available in Windows Azure. In particular, we’ll look at how you can use Windows Azure as an origin server and what options you have regarding edge servers.

Although in this section we’re showing you how to use the Windows Azure CDN (c’mon, it’s a Windows Azure book, after all), you can use other CDN providers, such as Akamai and Amazon Cloud Front. As you’re about to discover, using the Windows Azure CDN is probably the easiest option (we have no idea whether it’s the cheapest; you’ll need to use our JavaScript calculator to work that one out).

Using BLOB Storage as an Origin Server

Although web roles can be used as origin servers with non-Windows Azure CDNs, using them is a more expensive option than using BLOB storage. Because edge servers can serve only static content, there’s no need for them to be able to generate dynamic server-side content. An efficient cost-effective solution is to use your storage account as an origin server rather than use a web role.

To use our movie player example, the original version of the Big Buck Bunny movie, or even the original presplit chunk files, would be stored in BLOB storage, and BLOB storage would act as the origin server.

Enabling the Windows Azure CDN

To enable the Windows Azure CDN, open the storage account in the Windows Azure Developer portal and click Enable CDN, as shown in figure 3.

After you click this button, any BLOBs stored in public containers will be available on the CDN after about an hour. Figure 4 shows the Developer portal after your CDN account has been enabled.

A new custom domain is assigned to your CDN-enabled BLOBs. In figure 4, the CDN domain is http://az1903.vo.msecdn.net/. All you need to do now is replace where we used http://chrishayuk.blob.core.windows.net/ with http://az1903.vo.msecdn.net/, and the files will start being fed from the Windows Azure CDN instead of from our BLOB storage account.

If you don’t like the assigned subdomain, you can assign your own custom domain to the CDN domain.