To capacity plan for server machines that run Tomcat, you could study and plan for any of the following items (this isn’t meant to be a comprehensive list, but instead a list of some common items):

Server computer hardware

Which computer architecture(s)? How many computers will your site need? One big one? Many smaller ones? How many CPUs per computer? How much RAM? How much hard drive space and what speed I/O? What will the ongoing maintenance be like? How does switching to different JVM implementations affect the hardware requirements?

Network bandwidth

How much incoming and outgoing bandwidth will be needed at peak times? How might the web application be modified to lower these requirements?

Server operating system

Which operating system works best for the job of serving your site? Which JVM implementations are available for each operating system, and how well does each one take advantage of the operating system? For example, does the JVM support native multithreading? Symmetric multiprocessing (SMP)? If SMP is supported by the JVM, should you consider multiprocessor server computer hardware? Which serves your webapp faster, more reliably, and less expensively: multiple single-processor server computers or a single four-CPU server computer?

Here’s a general procedure for all types of capacity planning, and one that is particularly applicable to Tomcat:

1. Characterize the workload. If your site is already up and running, you can measure the requests per second, summarize the different kinds of possible requests, and measure the resource utilization per request type. If your site isn’t running yet, you can make some educated guesses at the request volume and run staging tests to determine the resource requirements.
   
2. Analyze performance trends. You need to know what requests generate the most load and how other requests are in comparison. Knowing which requests generate the most load or use the most resources, will help you know what to optimize to have the best overall impact on your server computers. For example, if a servlet that queries a database takes too long to send its response, maybe caching some of the data in RAM would safely improve response time.
   
3. Decide on minimum acceptable service requirements. For example, you may not want the end user to ever wait longer than 20 seconds for a web page response. That means that even during peak load, no request’s total time from the initial request to the completion of the response can take longer than 20 seconds. That may include any and all database queries and filesystem access needed to complete the heaviest resource-intensive request in your application. The minimum acceptable service requirements are up to each company and vary from company to company. Other kinds of service minimums include the number of requests per second the site must be able to serve and the minimum number of concurrent sessions and users.
   
4. Decide what infrastructure resources you will use, and test it in a staging environment. Infrastructure resources include computer hardware, bandwidth circuits, operating system software, and so on. Order, deploy, and test at least one server machine that mirrors what you’ll have for production and see if it meets your requirements. While testing Tomcat, make sure you try more than one JVM implementation, try different memory size settings, and request thread pool sizes (discussed earlier in this chapter).
   
5. If step 4 meets your service requirements, you can order and deploy more of the same thing to use as your production server computers. Otherwise, redo step 4 until service requirements are met.

Be sure to document your work because it tends to be a time-consuming process that must be repeated if someone needs to know how your company arrived at the answers. Also, because the testing is an iterative process, it’s important to document all of the test results on each iteration and the configuration settings that produced the results so you know when your tuning is no longer yielding noticeable positive results.

Once you’ve finished with your capacity planning, your site will be much better tuned for performance, mainly due to the rigorous testing of a variety of options. You should have gained a noticeable amount of performance just by having the right hardware, operating system, and JVM combination for your particular use of Tomcat.