Fundamentals of Software Architecture

Scaling application servers can be more complex and expensive than web servers and usually just moves the bottleneck down to the database server, which is even more difficult and expensive to scale. Even if you can scale the database, what you eventually end up with is a triangle-shaped topology, with the widest part of the triangle being the web servers (easiest to scale) and the smallest part being the database (hardest to scale), as illustrated in Figure 15-1. In any high-volume application with a large concurrent user load, the database will usually be the final limiting factor in how many transactions you can process concurrently.