HQ Replication

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“HQ Replication” by James Cowling. Masters thesis, MIT, May 2007.


There are currently two approaches to providing Byzantine-fault-tolerant state machine replication: an agreement-based approach, e.g., BFT, that uses communication between replicas to agree on a proposed ordering of requests, and a quorum-based approach, such as Q/U, in which clients contact replicas directly to optimistically execute operations. Both approaches have shortcomings: the quadratic message cost of inter-replica communication is unnecessary when there is no contention, and Q/U requires a large number of replicas and performs poorly under contention.

This thesis present HQ, a hybrid Byzantine-fault-tolerant state machine replication protocol that overcomes these problems. HQ employs a lightweight quorum-based protocol when there is no contention, but uses BFT to resolve contention when it arises. Furthermore, HQ uses only 3f+1 replicas to tolerate f faults, providing optimal resilience to node failures.

We implemented a prototype of HQ, and we compare its performance to BFT and Q/U analytically and experimentally. Additionally, in this work we use a new implementation of BFT designed to scale as the number of faults increases. Our results show that both HQ and our new implementation of BFT scale as f increases; additionally our hybrid approach of using BFT to handle contention works well.

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BibTeX entry:

   author = {James Cowling},
   title = {HQ Replication},
   school = {MIT},
   month = may,
   year = {2007}

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Programming Methodology Group