HACC: An Architecture for Cluster-Based Web Servers

This paper presents the design, implementation, and performance of the Harvard Array of Clustered Computers (HACC), a cluster-based design for scalable, cost-effective web servers. HACC is designed for locality enhancement. Requests that arrive at the cluster are distributed among the nodes so as to enhance the locality of reference that occurs on individual nodes in the cluster. By improving locality on individual cluster nodes, we can reduce their working set sizes and achieve superior performance for less cost than conventional approaches. We implemented HACC on Windows NT 4.0 and evaluated its performance for both static documents and workloads of dynamically generated documents adapted from logs of commercial web servers. Our performance results show that HACC's locality enhancement can improve performance by up to 121% for our stochastically generated static file case, by up to 40% for our trace-based static file case, and by up to 52% for our trace-based dynamic document case, compared to an IP- Sprayer approach to building cluster-based web servers.