Open and permissionless blockchains are distributed systems with thousands to tens of thousands of nodes, establishing novel platforms for decentralized applications. When realizing such an application, data might be stored and retrieved from one or more blockchains by distributed network nodes without relying on centralized coordination and trusted third parties. Data access could be provided through a query language such as SQL at the application level, establishing a unified view on application-level data that is verifiably stored. However, when accessing multiple blockchains through their node software and APIs, interoperability cannot be assumed today, resulting in challenges of inhomogeneous data access. In addition, different feature sets and trade-offs exist, e.g., regarding smart contract functionality, availability, distribution, scalability, and security. For increasing interoperability, the paper at hand suggests pursuing the development of a cross-chain query language at the application level. The language abstracts from implementation by providing a standardized syntax, an integrated data model, and a processing architecture for data queries. This research is an extended and updated paper of a prior publication demonstrating the language syntax, data model, and architecture with an evaluation of compatibility against the largest open and permissionless blockchains today.

翻译：开放和无许可区块链是由数千至数万个节点组成的分布式系统，为去中心化应用构建了新型平台。在实现此类应用时，分布式网络节点可在无需依赖中心化协调和可信第三方的情况下，从一条或多条区块链中存储和检索数据。可通过SQL等查询语言在应用层提供数据访问，建立对可验证存储的应用层数据的统一视图。然而，当前通过节点软件及API访问多条区块链时，尚无法实现互操作性，导致数据访问存在异构性挑战。此外，不同区块链在智能合约功能、可用性、分布性、可扩展性及安全性等方面存在差异化特征集与权衡。为增强互操作性，本文建议在应用层开发跨链查询语言。该语言通过提供标准化语法、集成数据模型及数据查询处理架构，对具体实现进行抽象。本研究是对先前论文的扩展与更新，通过评估与当前最大开放及无许可区块链的兼容性，展示了该语言的语法、数据模型及架构。