Web Cluster Optimal Installation

Does anyone know or have recommendations on what is needed to setup web cluster installations? We are trying to get one off the group and having all sorts of issues. The main one is the connection speed between the web servers and database servers.

We have the web servers running apache on one set of servers and we have the database servers running mysql on a second set of servers.

There seems to be a constant speed bottle neck between the communication between the database servers and web servers even though they are on same network and I am seeing not tcp errors. 

Can anyone share, are you using some sort of special caching software to speed up web to database communication. 

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