In most blockchain-based application scenarios, a complete application logic consists of multiple continuous transactions, in which the initiation of one transaction depends on the confirmation result of the previous one. This mandates that continuous transactions must be processed in the correct order. Unfortunately, existing chain-based blockchains fail to effectively support continuous transaction processing due to considerable latency in confirming continuous transactions. Recent studies shifted from chain-based blockchains toDirected Acyclic Graph(DAG) based blockchains, which reduced transaction confirmation latencies. However, DAG-based blockchains store transactions in an out-of-order manner that leads to unordered transaction processing. To address this challenge, we propose FLUID, a new DAG-based blockchain that supports continuous transaction processing while delivering high performance. The fundamental idea of FLUID is to design a transaction dependency tracking structure to ensure that continuous transactions can be processed in the correct order. FLUID utilizes a conflict resolution mechanism to provide instant confirmation and to support concurrent transaction processing with lower latencies. In addition, FLUID builds a checkpoint-based verification mechanism to achieve deterministic consensus on transaction processing results in the DAG. Extensive experiments demonstrate that our proposed FLUID can improve the throughput over state-of-the-art OHIE by 66% with two orders of magnitude lower latencies.

With customization of demands and functionalities, diverse blockchain systems are integrated with IoT in different application scenarios, forming a multichain environment. Multichain interoperability has become a crucial emerging issue, i.e., different blockchain systems are difficult to interact with each other credibly and efficiently. It further leads to isolated data islands across multichain. Therefore, it is of significant importance to facilitate data sharing among multichain systems. Toward achieving this, there are two main challenges. First, data across multiple blockchains must be shared reliably to meet the tamper-proof merits of blockchain technology. Second, we must control the data access process in a fine-grained way to protect sensitive data and user privacy. This article proposes SynergyChain, a multichain framework to enable reliable data sharing with controllable data access. By aggregating the data from multiple blockchains and reorganizing it in SynergyChain, we can achieve data reliability with the verification. Meanwhile, SynergyChain provides hierarchical access control based on smart contracts, making access control automated and credible. Experiments show that SynergyChain can support data sharing reliably and efficiently and reduce data query latency compared with multichain data requesting sequentially.