  With the increasing adoption of private blockchain platforms, consortia
operating in various sectors such as trade, finance, logistics, etc., are
becoming common. Despite having the benefits of a completely decentralized
architecture which supports transparency and distributed control, existing
private blockchains limit the data, assets, and processes within its closed
boundary, which restricts secure and verifiable service provisioning to the
end-consumers. Thus, platforms such as e-commerce with multiple sellers or
cloud federation with a collection of cloud service providers cannot be
decentralized with the existing blockchain platforms. This paper proposes a
decentralized gateway architecture interfacing private blockchain with
end-users by leveraging the unique combination of public and private blockchain
platforms through interoperation. Through the use case of decentralized cloud
federations, we have demonstrated the viability of the solution. Our testbed
implementation with Ethereum and Hyperledger Fabric, with three service
providers, shows that such consortium can operate within an acceptable response
latency while scaling up to 64 parallel requests per second for cloud
infrastructure provisioning. Further analysis over the Mininet emulation
platform indicates that the platform can scale well with minimal impact over
the latency as the number of participating service providers increases.
