Capacity Limitation Calculations of a Web Server Cluster

The purpose of this work was to find out what factors could be used to calculate the capacity of a web server cluster-system, and calculate the amount of load based on the found factors. The system in question is an internet cookie counter-system named TNS-counter, which gathers information about users of specific internet sites. The system is comprised of many different parts, and each part has different metrics that concern it. For example, the router has very different limitations than a web-server. In addition, it was important to find out which metrics for which parts actually are relevant in calculating the limitations of the system. Many limitations can be applied to several of the system components, but they are not relevant to all of them. Three major components of the system were extensively studied with the help of the system’s creator, using system statistics and other available information. The parts in question are the router, load balancers, and the front-end web servers. Studied metrics and limitations include bits per second transferred, connections per second processed and established connections, among others. In the end, it was found that the system is currently performing at roughly 20% load, and has load peaks of up to 50% concerning some parts. The most significant limitation was found to be the number of HTTPS-connections per second. The reasons being, that the system can handle less HTTPS-connections than normal HTTP-connections, as well as the fact that the system is receiving more HTTPS-connections than HTTP-connections. The cryptographic techniques used to secure HTTPS-connections cause a significant extra load on the system, and therefore these connections have a large impact on the system load.