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Monthly Archives: November 2018

HotStuff: BFT Consensus in the Lens of Blockchain

Blockchain is a Byzantine Fault Tolerant (BFT) replicated state machine, in which each state-update is by itself a Turing machine with bounded resources. The core algorithm for achieving BFT in a Blockchain appears completely different from classical BFT algorithms:

  • Classical solutions like DLS, PBFT solve BFT among a small-to-medium group of known participants. Such algorithms consist of multiple rounds of message exchanges carrying votes and safety-proofs. They are evidently quite intimidating to the non-expert.
  • In contrast, Bitcoin solves BFT among a very large group of unknown users. In each time-period, one user broadcasts a single message carrying a Proof-of-Work (PoW). No other messages or information is exchanged.

What a difference between the two worlds!

Recent advances in blockchain technology blur these boundaries. Namely, hybrid solutions such as Byzcoin, Bitcoin-NG, Hybrid Consensus, Casper and Solida, anchor off-chain BFT decisions inside a PoW chain or the other way around.

Moreover, innovative solutions in the age of blockchains, such as Honeybadger, ALGORAND, Tendermint, and SBFT, revisit the BFT setting with greater scalability and simplicity.

Confused?  The VMware’s HotStuff framework can help!

HotStuff describes Blockchain in the lens of BFT and BFT in the lens of Blockchain, and provide common algorithmic foundations for both. It operates over graphs of blocks (aka blockchains), such that a block becomes committed when votes embedded in other blocks refers to it. A feature of HotStuff is its economy of mechanism, involving only a single message type for proposing, a single message type for voting, and simple Commit and Preferred Branch Rules. The proposer protocol in HotStuff is the same regardless of whether the proposer is being replaced or not, thus relieving of the main source complexity in BFT protocols The Saddest Moment.

At the same time, the reasoning for its safety is subtle and deceptively simple—not unlike that for other designs that have subsequently been identified as problematic (see e.g., Revisit Fast BFT).  This led us to explore model checking to confirm the safety of HotStuff.

Interested in reading more? check the HotStuff whitepaper (“HotStuff: BFT Consensus in the Lens of Blockchain’’, https://arxiv.org/abs/1803.05069).

For more information on VMware Blockchain, go here.