The SASHA architecture for network-clustered web servers

We present the Scalable, Application-Space, Highly-Available (SASHA) architecture for network-clustered web servers that demonstrates high performance and fault tolerance using application-space software and Commercial-Off-The-Shelf (COTS) hardware and operating systems. Our SASHA architecture consists of an application-space dispatcher, which performs OSI layer 4 switching using layer 2 or layer 3 address translation; application-space agents that execute on server nodes to provide the capability for any server node to operate as the dispatcher, a distributed state-reconstruction algorithm; and a token-based communications protocol that supports self-configuring, detecting and adapting to the addition or removal of servers. The SASHA architecture of clustering offers a flexible and cost-effective alternative to kernel-space or hardware-based network-clustered servers with performance comparable to kernel-space implementations.