- The Emergence of Web Applications
- Basic Definitions
- The Nature of the Web and Its Challenges
- Performance and Scalability
- The Internet Medium
- Wide Audience
- Always On
- Lack of Complete Control
- Measuring Performance and Scalability
- Measuring Performance
- Beyond Benchmarking
- Measuring Scalability
- Throughput and Price/Performance
- Scalability and Performance Hints
- Think End-to-End
- Scalability Doesn't Equal Performance
The Nature of the Web and Its Challenges
Although Web applications have rapidly made the Internet a productive medium, the nature of the Internet poses many engineering puzzles. Even the most basic of challengesengineering how a provider can quickly and reliably deliver information to all who want itis neither simple nor well understood. Like other challenges, this problem's complexity has to do with the nature of the medium. The Internet is different from the information-sharing paradigms of radio, television, and newspapers for several reasons. Perhaps two of the most important reasons are its incredibly wide audience (unpredictable number of customers) and the potential at any time for that wide audience to request information from any given provider (unpredictable work demands).
Unlike in other media, Internet information providers simply do not have the ability to know their audience in advance. Newspapers, for example, know their circulation before they print each edition. They also have the advantage of being able to control their growth, making sure they have enough employees to deliver the paper daily, and have the resources and time to go from deadline on the previous night to delivery on the next morning. Furthermore, newspapers do not have to deal with sudden jumps in circulation. Compared to the Internet, the growth of big newspapers in metropolitan areas seems far more gradual. For example, when the Washington Post was founded in 1877, it had a circulation of 10,000. By 1998, that circulation had reached nearly 800,000 for its daily edition and more than that for its Sunday edition.1 That's an average growth rate of just over 6,500 subscribers per year, or 17 per day.
Deployers of Web applications have a love/hate relationship with their growth rates. In one sense, they would love the gradual growth of 17 new users per day. How nice life would be if you had to worry about scaling at that rate! You could finally go home at 5 P.M., not 9:30 P.M. At the same time, such growth rates are the reason that people are so excited about Web applicationsbecause you can potentially reach the whole world in a matter of seconds. Your growth rate out of the gate could be hundreds of thousands of users. Although this bodes well for the business side of the things, it creates a tremendous challenge in terms of dealing with such demands.
On the Internet, circulation is akin to page hits, that is, the number of requests for a given document. Page hits can jump wildly overnight. A favorite example in the Web-caching community is the popularity of the online distribution of the Starr report. As most Americans know, this report was put together by the Office of the Independent Counsel during the Clinton administration. Let us just say that, while it was not flattering by any means, it was eagerly awaited by both the American public and the international press corps.
When the Starr report was released online in the summer of 1998 at government Web sites, tens of thousands of people tried to download it. A representative for Sprint, Inc., one of the Internet's backbone providers, reported a surge in bandwidth demand that ranged between 10 and 20 percent above normal; a representative of AOL reported an "immediate 30 percent spike"; and NetRatings, a Nielsen-like Internet content popularity company, estimated that at one point, more than one in five Web users was requesting the report or news about it. CNET.COM ran a number of stories about the event and its ramifications for Internet scalability in the Fall of 1998.2 The conclusion among network administrators and engineers was universal. There were simply too many requests to be handled at once, and the distribution mechanisms were unable to scale to demand. It was a real test of the scalability of the Internet itself. Not only were the Web servers that provided this information overloaded, but the networking infrastructure connecting consumers to providers became heavily congested and severely inefficient. The effect was much like that of a traffic jam on a freeway.
This phenomenon was unique because it demonstrated the effects of sudden popularity as well as the short-lived nature of that popularity. For example, it is unlikely that you or anyone else remembers the URL(s) where the report was first available. And it is unlikely that you have it bookmarked. Thus, even had those sites been able to accommodate the demands of the time by buying bigger and faster machines, it would likely have been money wasted because the need for those resources dropped dramatically after the public lost interest in the report.
Other media, such as radio and television, are broadcast and do not need to worry about the size of their audience affecting their ability to deliver information. Consider television or radio programs, such as the national and local news. Their programmers know in advance when they are scheduled to broadcast. They have the luxury of being able to prepare ahead of time. Even when live radio or television beckons, the fact that both media are broadcast means that there is only one audience to address. Cable companies and good old TV antennae are already in place to facilitate the transport of that information. If we all watch or listen to the same channel, we all see or hear the same program. This is not the case with Internet audiences, where it is usually impossible to prepare for every request, where every consumer of information requires a unique response, and where there is a continual need for new virtual links (HTTP connections) between consumer and provider to be both created and then destroyed.