Software Architecture Patterns

Chapter 5. Space-Based Architecture

Most web-based business applications follow the same general request flow: a request from a browser hits the web server, then an application server, then finally the database server. While this pattern works great for a small set of users, bottlenecks start appearing as the user load increases, first at the web-server layer, then at the application-server layer, and finally at the database-server layer. The usual response to bottlenecks based on an increase in user load is to scale out the web servers. This is relatively easy and inexpensive, and sometimes works to address the bottleneck issues. However, in most cases of high user load, scaling out the web-server layer just moves the bottleneck down to the application server. 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). 

In any high-volume application with an extremely large concurrent user load, the database will usually be the final limiting factor in how many transactions you can process concurrently. While various caching technologies and database scaling products help to address these issues, the fact ...