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Rule 4—Reduce DNS Lookups

As we've seen so far in this chapter, reducing is the name of the game for performance improvements and increased scalability. A lot of rules are focused on the architecture of the Software as a Service (SaaS) solution, but for this rule let's consider your customer's browser. If you use any of the browser level debugging tools such as Mozilla Firefox's plug-in Firebug,5 you'll see some interesting results when you load a page from your application. One of the things you will most likely notice is that similarly sized objects on your page take different amounts of time to download. As you look closer you'll see some of these objects have an additional step at the beginning of their download. This additional step is the DNS lookup.

The Domain Name System (DNS) is one of the most important parts of the infrastructure of the Internet or any other network that utilizes the Internet Protocol Suite (TCP/IP). It allows the translation from domain name (www.akfpartners.com) to an IP address (184.72.236.173) and is often analogized to a phone book. DNS is maintained by a distributed database system, the nodes of which are the name servers. The top of the hierarchy consists of the root name servers. Each domain has at least one authoritative DNS server that publishes information about that domain.

This process of translating domains into IP addresses is made quicker by caching on many levels, including the browser, computer operating system, Internet service provider, and so on. However, in our world where pages can have hundreds or thousands of objects, many from different domains, small milliseconds of time can add up to something noticeable to the customer.

Before we go any deeper into our discussion of reducing the DNS lookups we need to understand at a high level how most browsers download pages. This isn't meant to be an in-depth study of browsers, but understanding the basics will help you optimize your application's performance and scalability. Browsers take advantage of the fact that almost all Web pages are comprised of many different objects (images, JavaScript files, css files, and so on) by having the ability to download multiple objects through simultaneous connections. Browsers limit the maximum number of simultaneous persistent connections per server or proxy. According to the HTTP/1.1 RFC6 this maximum should be set to 2; however, many browsers now ignore this RFC and have maximums of 6 or more. We'll talk about how to optimize your page download time based on this functionality in the next rule. For now let's focus on our Web page broken up into many objects and able to be downloaded through multiple connections.

Every distinct domain that serves one or more objects for a Web page requires a DNS lookup either from cache or out to a DNS name server. For example, let's assume we have a simple Web page that has four objects: 1) the HTML page itself that contains text and directives for other objects, 2) a CSS file for the layout, 3) a JavaScript file for a menu item, and 4) a JPG image. The HTML comes from our domain (akfpartners.com), but the CSS and JPG are served from a subdomain (static.akfpartners.com), and the JavaScript we've linked to from Google (ajax.googleapis.com). In this scenario our browser first receives the request to go to page www.akfpartners.com, which requires a DNS lookup of the akfpartners.com domain. Once the HTML is downloaded the browser parses it and finds that it needs to download the CSS and JPG both from static.akfpartners.com, which requires another DNS lookup. Finally, the parsing reveals the need for an external JavaScript file from yet another domain. Depending on the freshness of DNS cache in our browser, operating system, and so on, this lookup can take essentially no time up to hundreds of milliseconds. Figure 1.1 shows a graphical representation of this.

Figure 1.1

Figure 1.1 Object download time

As a general rule, the fewer DNS lookups on your pages the better your page download performance will be. There is a downside to combining all your objects into a single domain, and we've hinted at the reason in the previous discussion about maximum simultaneous connects. We explore this topic in more detail in the next rule.

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