  A Coordination Blockchain is a blockchain with the task of coordinating
activities of multiple private blockchains. This paper discusses the pros and
cons of using Ethereum MainNet, the public Ethereum blockchain, as a
Coordination Blockchain. The requirements Ethereum MainNet needs to fulfil to
perform this role are discussed within the context of Ethereum Private
Sidechains, a private blockchain technology which allows many blockchains to be
operated in parallel, and allows atomic crosschain transactions to execute
across blockchains. Ethereum MainNet is a permissionless network which aims to
offer strong authenticity, integrity, and non-repudiation properties, that
incentivises good behaviour using crypto economics. This paper demonstrates
that Ethereum MainNet does deliver these properties. It then provides a
comprehensive review of the features of Ethereum Private Sidechains, with a
focus on the potential usage of Coordination Blockchains for these features.
Finally, the merits of using Ethereum MainNet as a Coordination Blockchain are
assessed. For Ethereum Private Sidechains, we found that Ethereum MainNet is
best suited to storing long term static data that needs to be widely available,
such as the Ethereum Registration Authority information. However, due to
Ethereum MainNet's probabilistic finality, it is not well suited to information
that needs to be available and acted upon immediately, such as the Sidechain
Public Keys and Atomic Crosschain Transaction state information that need to be
accessible prior to the first atomic crosschain transaction being issued on a
sidechain. Although this paper examined the use of Ethereum MainNet as a
Coordination Blockchain within reference to Ethereum Private Sidechains, the
discussions and observations of the typical tasks a Coordination blockchain may
be expected to perform are applicable more widely to any multi-blockchain
system.
