  Recently, decentralised (on-blockchain) platforms have emerged to complement
centralised (off-blockchain) platforms for the implementation of automated,
digital (smart) contracts. However, neither alternative can individually
satisfy the requirements of a large class of applications. On-blockchain
platforms suffer from scalability, performance, transaction costs and other
limitations. Off-blockchain platforms are afflicted by drawbacks due to their
dependence on single trusted third parties. We argue that in several
application areas, hybrid platforms composed from the integration of on- and
off-blockchain platforms are more able to support smart contracts that deliver
the desired quality of service (QoS). Hybrid architectures are largely
unexplored. To help cover the gap, in this paper we discuss the implementation
of smart contracts on hybrid architectures. As a proof of concept, we show how
a smart contract can be split and executed partially on an off-blockchain
contract compliance checker and partially on the Rinkeby Ethereum network. To
test the solution, we expose it to sequences of contractual operations
generated mechanically by a contract validator tool.
