  Data marketplaces (DMs) promote the benefits of the Internet of Things (IoT)
in smart cities. To facilitate the easy exchanges of real-time IoT data streams
between device owners and third-party applications, it is required to provide
scalable, interoperable, and secured services for large numbers of distributed
IoT devices operated by different application vendors. Thanks to
decentralization, immutability, and auditability, Blockchain is promising to
enable a tamper-proof and trust-free framework to enhance performance and
security issues in centralized DMs. However, directly integrating blockchains
into large-scale IoT-based DMs still faces many limitations, such as high
resource and energy demands, low transaction throughput, poor scalability, and
challenges in privacy preservation. This paper introduces a novel Federated
Ledgers-based Framework for Hierarchical Decentralized Data Marketplaces
(Fed-DDM). In Fed-DDM, participants are divided into multiple permissioned
domains given their registrations. Each domain leverages an efficient Byzantine
Fault Tolerance (BFT) consensus protocol to commit transactions of a domain on
a private intra-ledger. A public inter-ledger network adopts a scalable
Proof-of-Work (PoW) consensus protocol to federate multiple private
intra-ledger networks. We design a smart contract-enabled inter-ledger protocol
to guarantee the security of the cross-domain operations on a public federated
ledger without exposing sensitive privacy information from private ledgers. A
proof-of-concept prototype is implemented, and the experimental results verify
the feasibility of the proposed Fed-DDM solution with performance and security
guarantees.
