Your question is not clear to me. Do you want failover Apache (HA) cluster or compute cluster (parallel computing)? Apache mostly comes under HA failover cluster. It will be behind load balancer (LB). LB can be crated using reverse proxy server such as nginx or any other supported Open Source software. LB need to share and create VIP (virtual IP) using pf+CARP or any other open source software.
Parallel computing is used for simulations, financial modeling, large data set mining and crunching.