The Fail-Heterogeneous Architectural Model

Fault tolerant distributed protocols typically utilize a homogeneous fault model, either fail-crash or fail-Byzantine, where all processors are assumed to fail in the same manner. In practice, due to complexity and evolvability reasons, only a subset of the nodes can actually be designed to have a restricted, fail-crash failure mode, provided that they are free of design faults. Based on this consideration, we propose a fail-heterogeneous architectural model for distributed systems which considers two classes of nodes: (a) full-fledged execution nodes, which can be fail-Byzantine, and (b) lightweight, validated coordination nodes, which can only be fail-crash. To illustrate the model we introduce HeterTrust as a practical trustworthy service replication protocol. It has a low latency overhead, requires few execution nodes with diversified design, and prevents intruded servers from disclosing confidential data. We also discuss applications of the model to DoS attacks mitigation and to group membership.