When to start clustering of J2EE Servers
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I'm running an application on JBoss 7.1.1 using J2EE6, EJB 3.1, CDI, JSF 2 and Hibernate. 

How do I know whether one standalone instance of J2EE AS is enough to process all the requests or whether multiple instances of AS are necessary. 

How many requests per second should be JBoss 7.1.1 able to cover (in general case)? 

One time in a year the number of requests increase significantly and the server (that time it was JBoss 4.2.3) was failing. And I am not able to say whether it was due to poor design of the application or just due to big amount of requests.

Thanks,
Lukas