Capacity planning is another important part of tuning the performance of your Tomcat server in production. Regardless of how much configuration file-tuning and testing you do, it won’t really help if you don’t have the hardware and bandwidth your site needs to serve the volume of traffic you are expecting.

Here’s a loose definition of capacity planning as it fits into the context of this section: capacity planning is the activity of estimating the necessary computer hardware, operating system, and bandwidth necessary for a web site by studying and/or estimating the total network traffic a site will have to handle, deciding on acceptable service characteristics, and finding appropriate hardware and operating systems that meet or exceed the server software’s requirements to meet the service requirements. In this case, the server software includes Tomcat, as well as any third-party web servers and load balancers that you are using “in front” of Tomcat.

If you don’t do any capacity planning before you buy and deploy your production servers, you won’t know if the server hardware can handle your web site’s traffic load. Or, worse still, you won’t realize the error until you’ve already ordered, paid for, and deployed applications on the hardware—usually too late to change direction very much. You can usually add a larger hard drive or even order more server computers, but sometimes it’s less expensive overall to buy and/or maintain fewer server computers in the first place.

The higher the volume of traffic on your web site, or the larger the load that is generated per client request, the more important capacity planning becomes. Some sites get so much traffic that only a cluster of server computers can handle it all within reasonable response time limits. Conversely, sites with less traffic have less of a problem finding hardware that meets all their requirements. It’s true that throwing more or bigger hardware at the problem usually fixes things, but, especially in the high traffic cases, that may be prohibitively costly. For most companies, the lower the hardware costs are (including ongoing maintenance costs after the initial purchase), the higher profits can be. Another factor to consider is employee productivity. If having faster hardware would make the developers 20 percent more effective in getting their work done quickly, for example, then depending on the size of the team, it may be worth the hardware cost difference to order bigger/faster hardware up front.

Capacity planning is usually done at upgrade points as well. Before ordering replacement hardware for existing mission-critical server computers, it’s probably a good idea to gather information about what your company needs, based on updated requirements, common traffic load, software footprints, etc.

There are at least a couple of common methods of arriving at decisions when conducted capacity planning. In practice, we’ve seen two main types: anecdotal approaches and academic approaches, such as enterprise capacity planning.

Anecdotal Capacity Planning

Anecdotal capacity planning is a sort of light capacity planning that isn’t meant to be exact, but close enough to keep a company out of situations that would be caused by doing no capacity planning at all. This method follows capacity and performance trends that are obtained from previous industry experience. For example, you could make your best educated guess at how much outgoing network traffic your site will have at its peak usage (hopefully from some other real-world site), and double that figure. That figure is your site’s new outgoing bandwidth requirement for which you will make sure to buy and deploy hardware that can handle it. Most people will do capacity planning this way because it’s quick and requires little effort and time.

Enterprise Capacity Planning

Enterprise capacity planning is meant to be more exact and takes much longer. This method is necessary for sites with a very high volume of traffic, often combined with a high load per request. Detailed capacity planning like this is necessary to keep hardware and bandwidth costs as low as they can be, while still providing the quality of service that the company guarantees or is contractually obligated to live up to. Usually, this involves the use of commercial capacity planning analysis software in addition to iterative testing and modeling. Few companies do this kind of capacity planning, but the few that do are very large enterprises that have a budget large enough to afford doing it (mainly because this sort of thorough planning ends up paying for itself).

The biggest difference between anecdotal and enterprise capacity planning is depth. Anecdotal capacity planning is governed by rules of thumb and is more of an educated guess, whereas enterprise capacity planning is an in-depth requirements-and-performance study whose goal is to arrive at numbers that are as exact as possible.