  There appears to be an insatiable desire for spawning new bespoke blockchains
to harness the functionality provided by blockchain technologies, resulting in
a constant stream of blockchain start-up companies entering the market with
their own unique vision and mission. Some target a particular niche market such
as supply chain and financial services, while others strive to differentiate
themselves from the increasingly saturated market by offering new
functionality. This dynamic and constantly changing blockchain ecosystem makes
it very challenging to keep abreast of all the latest breakthroughs and
research. It is evident that there is also a growing desire to collaborate with
others developing blockchain solutions, which brings new impetus to blockchain
interoperability research. We review the strategies that some key players in
the blockchain ecosystem have implemented, or are proposing to develop, to
satisfy this increasing demand for cross-chain communication and transactions
between sidechains. Interoperability presents a complex and challenging
stumbling block to the wider uptake of blockchain technology. We find that
although there is a plethora of blockchains and interoperability
implementations, or proposals, at a higher level of abstraction there is only a
handful of approaches. However, the way they are implemented can differ quite
substantially. We present a summary of the reviews we conducted in a table for
ease of comparing and contrasting.
