  Blockchain has come a long way: a system that was initially proposed
specifically for cryptocurrencies is now being adapted and adopted as a
general-purpose transactional system. As blockchain evolves into another data
management system, the natural question is how it compares against distributed
database systems. Existing works on this comparison focus on high-level
properties, such as security and throughput. They stop short of showing how the
underlying design choices contribute to the overall differences. Our work fills
this important gap and provides a principled framework for analyzing the
emerging trend of blockchain-database fusion.
  We perform a twin study of blockchains and distributed database systems as
two types of transactional systems. We propose a taxonomy that illustrates the
dichotomy across four dimensions, namely replication, concurrency, storage, and
sharding. Within each dimension, we discuss how the design choices are driven
by two goals: security for blockchains, and performance for distributed
databases. To expose the impact of different design choices on the overall
performance, we conduct an in-depth performance analysis of two blockchains,
namely Quorum and Hyperledger Fabric, and two distributed databases, namely
TiDB, and etcd. Lastly, we propose a framework for back-of-the-envelope
performance forecast of blockchain-database hybrids.
