TOWARDS CHARACTERIZATION AND OPTIMIZATION FOR BLOCKCHAIN SYSTEMS

The Hong Kong University of Science and Technology
Department of Computer Science and Engineering


PhD Thesis Defence


Title: "TOWARDS CHARACTERIZATION AND OPTIMIZATION FOR BLOCKCHAIN SYSTEMS"

By

Miss Yuechen TAO


Abstract

Blockchain technology has now been widely used in a variety of distributed 
applications such as cryptocurrencies, insurance, healthcare, and supply 
chains. In this thesis, we study two types of blockchain systems depending 
on the number of blockchain involved, and we focus on characterizing and 
optimizing their respective performances. First, we consider blockchain 
systems that utilize a single blockchain, whose performance usually 
suffers due to serialized transaction confirmations and long consensus 
latency. Sharding is commonly used to enhance the performance by 
parallelizing transaction confirmations. However, existing sharding 
solutions incur a significant number of cross-shard validations, which may 
offset the performance improvement. Payment channel networks (PCNs) 
utilizing off-chain for financial transactions, enable constant 
transaction confirmations. Yet its fundamental performance 
characterization is largely unknown. Second, we examine blockchain systems 
that involves multiple heterogeneous blockchains, where transactions are 
confirmed across different blockchains, referred as cross-chain 
transactions. A third-party blockchain such as a relay chain can help to 
facilitate cross-chain confirmations. However, existing solutions cannot 
ensure confidentiality and atomicity at the same time while also largely 
ignoring the existence of failures when reading or writing data across 
blockchains (i.e., r/w failures). Moreover, the third-party blockchain may 
well become a performance bottleneck during cross-chain confirmations. We 
collectively address the above research problems in this thesis.

The first work focuses on improving the degree of parallelism in sharded 
blockchain systems by eliminating cross-shard validations. This turns out 
to be challenging due to the transaction dependency and imbalanced 
sharding distribution in term of the number of transactions in each shard. 
Such transaction dependencies affect cross-shard validations and lead to 
imbalanced sharding distribution that reduces the confirmation 
parallelization. We derive a innovative sharding formation by capturing 
the inherent relationship between transactions and smart contracts. We 
propose an inter-shard merging algorithm and an intra-shard transaction 
selection mechanism to minimize the imbalanced sharding distribution.

The second work characterizes the fundamental performance limits in 
payment channel networks (PCNs). We develop a novel mathematical model 
capturing the PCN performance, and we study the impact from a number of 
key factors including channel capacity and type of transactions. Through 
rigorous mathematical analysis, we derive the gap between the 
theoretically optimal performance and the performance achievable in 
practice, which helps to characterize the design space in PCNs for 
scheduling transactions.

The third work addresses the atomicity and confidentiality for cross-chain 
confirmations under different failures. When failures occur during reading 
or writing data, we consider two scenarios depending on whether data is 
the latest or not. We design a four-phase-commit protocol (4pc) and a 
smart contract-based solution (SSC) respectively to ensure atomicity and 
confidentiality under different scenarios.

The fourth work aims to improve the cross-chain confirmation performance 
by optimizing the throughput of a relay chain through sharding techniques. 
To eliminate cross-shard validations, we capture transaction dependencies 
on the relay chain and place transactions with dependency into a single 
shard. To ensure a balanced sharding distribution, we mathematically 
formulate the transaction distribution problem into an integer 
optimization problem and design an efficient solution.


Date:			Wednesday, 1 June 2022

Time:			3:00pm - 5:00pm

Zoom Meeting: 
https://hkust.zoom.us/j/95407187831?pwd=dkVrNnlXQTQ1RkxHTHhnTTdOZUlvQT09

Chairperson:		Prof. Xun WU (PPOL)

Committee Members:	Prof. Bo LI (Supervisor)
 			Prof. Qiong LUO
 			Prof. Yangqiu SONG
 			Prof. Shuhuai YAO (MAE)
 			Prof. Song GUO (PolyU)


**** ALL are Welcome ****