Lessons from HotStuff

This article will take you on a journey to the core of blockchains, their Byzantine consensus engine, where HotStuff emerged as a new algorithmic foundation for the classical Byzantine generals consensus problem. The first part of the article underscores the theoretical advances HotStuff enabled, including several models in which HotStuff-based solutions closed problems which were opened for decades. The second part focuses on HotStuff performance in real life setting, where its simplicity drove adoption of HotStuff as the golden standard for blockchain design, and many variants and improvements built on top of it. Both parts of this document are meant to describe lessons drawn from HotStuff as well as dispel certain myths

BBCA-CHAIN: One-Message, Low Latency BFT Consensus on a DAG

This paper presents a partially synchronous BFT consensus protocol powered by BBCA, a lightly modified Byzantine Consistent Broadcast (CBC) primitive. BBCA provides a Complete-Adopt semantic through an added probing interface to allow either aborting the broadcast by correct nodes or exclusively, adopting the message consistently in case of a potential delivery. It does not introduce any extra type of messages or communication cost to CBC. BBCA is harnessed into BBCA-CHAIN to make direct commits on a chained backbone of a causally ordered graph of blocks, without any additional voting blocks or artificial layering. With the help of Complete-Adopt, the additional knowledge gained from the underlying CBC completely removes the voting latency in popular DAG-based protocols. At the same time, causal ordering allows nodes to propose blocks in parallel and achieve high throughput. BBCA-CHAIN thus closes up the gap between protocols built by consistent broadcasts (e.g., Bullshark) to those without such an abstraction (e.g., PBFT/HotStuff), emphasizing their shared fundamental principles. Using a Bracha-style CBC as an example, we fully specify BBCA-CHAIN with simplicity, serving as a solid basis for high-performance replication systems (and blockchains)

Sync HotStuff: Simple and Practical Synchronous State Machine Replication

Synchronous solutions for Byzantine Fault Tolerance (BFT) can tolerate up to minority faults. In this work, we present Sync HotStuff, a surprisingly simple and intuitive synchronous BFT solution that achieves consensus with a latency of $2\Delta$ in the steady state (where $\Delta$ is a synchronous message delay upper bound). In addition, Sync HotStuff ensures safety in a weaker synchronous model in which the synchrony assumption does not have to hold for all replicas all the time. Moreover, Sync HotStuff has optimistic responsiveness, i.e., it advances at network speed when less than one-quarter of the replicas are not responding. Borrowing from practical partially synchronous BFT solutions, Sync HotStuff has a two-phase leader-based structure, and has been fully prototyped under the standard synchrony assumption. When tolerating a single fault, Sync HotStuff achieves a throughput of over 280 Kops/sec under typical network performance, which is comparable to the best known partially synchronous solution

Scaling the Infrastructure of Practical Blockchain Systems

196 pagesThe infrastructure of a blockchain system consists of a replication service that tolerates limited adversarial behavior among participants. It requires both a Byzantine fault tolerant (BFT) replication protocol to defend against the adversaries and an underlying storage system to preserve states. This dissertation explores two designs for BFT replication and one design for the persistent storage. We first present HotStuff, a leader-based BFT replication protocol for the partially synchronous system model. Once network communication becomes synchronous, HotStuff enables a correct leader to drive the protocol to consensus at the pace of actual (vs. maximum) network delay—a property called responsiveness—and with communication complexity that is linear in the number of replicas. To the best of our knowledge, HotStuff is the first partially synchronous BFT replication protocol exhibiting these combined properties. HotStuff is built around a novel framework that forms a bridge between classical BFT foundations and blockchains. It allows the expression of other known protocols (DLS, PBFT, Tendermint, Casper), and ours, in a common framework. Our deployment of HotStuff over a network with over 100 replicas achieves throughput and latency comparable to that of BFT-SMaRt, while enjoying a linear communication footprint during leader failover (vs. cubic with BFT-SMaRt). Then, we introduce a family of leaderless BFT protocols, exploiting metastable properties of network subsampling. These protocols provide a strong probabilistic safety guarantee in the presence of Byzantine adversaries while their concurrent and leaderless nature enables them to achieve high throughput and scalability. Unlike blockchains that rely on Proof-of-Work, blockchains built on our protocols are quiescent and green. Unlike traditional consensus protocols where typically one or more nodes must process a linear number of bits in the number of total nodes per decision, no node processes more than a logarithmic number of bits. It does not require accurate knowledge of all participants and exposes new possible tradeoffs and improvements in safety and liveness for building consensus protocols. We describe the Snow protocol family, and how it can be used to construct the core of an internet-scale electronic payment system, Avalanche, which is evaluated in a large scale deployment. Experiments demonstrate that the system can achieve high throughput, provide low confirmation latency and scale well compared to existing systems that deliver similar functionality. For our implementation and setup, the bottleneck of the system is in transaction verification. Finally we propose a new in-memory index that is also storage-friendly. A “lazy-trie” is a variant of the hash-trie data structure that achieves near-optimal height, has practical storage overhead, and can be maintained on-disk with standard write-ahead logging. We present CedrusDB, a persistent key-value store based on a lazy-trie. The lazy-trie is kept on disk while made available in memory using standard memory-mapping. The lazy-trie organization in virtual memory allows CedrusDB to better leverage concurrent processing than other on-disk index schemes (LSMs, B+-trees). CedrusDB achieves comparable or superior performance to recent log-based in-memory key-value stores in mixed workloads while being able to recover quickly from failures

Research data supporting "Multi-Task Joint-Learning of Deep Neural Network for Robust Speech Recognition"

This is speech recognition output data and detailed results. Please see the README for details.This work was supported by the EPSRC [Natural Speech Technology programme grant http://www.natural-speech-technology.org]

Cooling Induced Surface Reconstruction during Synthesis of High-Ni Layered Oxides

Cooling Induced Surface Reconstruction during Synthesis of High-Ni Layered Oxide