I know this may be a recurring topic, but I have read a lot of articles and I still have doubts. Also, I would like to hear more recent opinions about this.
The main requirements of my application server are: flexible configuration, support for a extremely high number of concurrent users. It will be a system for the mobile communications industry, so it must have high availability as well.
I am going to develop a Java EE application and Open Source Applications Servers are my only option. I have used GlassFish for a very small project and I really liked it.