  Decentralization, which has backed the hyper growth of many blockchains,
comes at the cost of scalability. To understand this fundamental limitation,
this paper proposes a quantitative measure of blockchain decentralization, and
discusses its implications to various trust models and consensus algorithms.
Further, we identify the major challenges in blockchain decentralization. Our
key findings are that true decentralization is hard to achieve due to the
skewed mining power and that a fully decentralized blockchain inherently limits
scalability as it incurs a throughput upper bound and prevents scaling smart
contract execution. To address these challenges, we outline three research
directions to explore the trade-offs between decentralization and scalability.
