Why apache is a process based server

Apache has been around since 1995, a time when a process-based approach was sufficient. It has grown over the years and serves over 100 million websites so continues to perform well with this approach. Of course there are some scenarios where this process-based approach is insufficient and a real CPU hog. In those cases you'll want to use nginx. You'll need to be seeing a tremendous amount of traffic before you run into a scenario where the event-driven approach is necessary. If you are starting off and are sure you're going to see that kind of traffic, nginx can work easily for small sites so you should use it at the beginning.