  Blockchain technology has been revolutionizing many fields since last decade.
Its true potential is not practically utilized yet. In a very short period of
time, it has evolved twice - Smart contracts and Directed Acyclic Graph (DAG).
DAG based blockchains currently referred to as Blockchain 3.0 solves many
issues in the current conventional blockchain technologies including
transaction fees, transaction approval times and scalability. In this paper, we
present a comparative analysis of blockchain implementations based on DAG
including IOTA, NxT, Byteball, Nano, DAGCoin, Fantom, XDAG and Caixapay. We
discuss limitations of both conventional and DAG based blockchains and suggest
when to prefer DAG based blockchains.
