  The interoperability across multiple blockchains would play a critical role
in future blockchain-based data management paradigm. Existing techniques either
work only for two blockchains or requires a centralized component to govern the
cross-blockchain transaction execution, neither of which would meet the
scalability requirement. This paper proposes a new distributed commit protocol,
namely \textit{cross-blockchain transaction} (CBT), for conducting transactions
across an arbitrary number of blockchains without any centralized component.
The key idea of CBT is to extend the two-phase commit protocol with a heartbeat
mechanism to ensure the liveness of CBT without introducing additional nodes or
blockchains. We have implemented CBT and compared it to the state-of-the-art
protocols, demonstrating CBT's low overhead (3.6\% between two blockchains,
less than $1\%$ among 32 or more blockchains) and high scalability (linear
scalability on up to 64-blockchain transactions). In addition, we developed a
graphic user interface for users to virtually monitor the status of the
cross-blockchain transactions.
