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 demonstrating the language syntax, data model, and architecture with an evaluation of compatibility against the largest open and permissionless blockchains today.

翻译：开放无许可区块链是由数千至数万个节点构成的分布式系统，为去中心化应用创造了新型平台。在实现此类应用时，分布式网络节点可在不依赖集中式协调和可信第三方的情况下，从一个或多个区块链中存储和检索数据。通过应用层的SQL等查询语言提供数据访问，可建立可验证存储的应用层数据统一视图。然而，当通过其节点软件和API访问多个区块链时，当前无法假设互操作性，导致异构数据访问的挑战。此外，不同区块链在智能合约功能、可用性、分布性、可扩展性和安全性等方面存在差异化的功能集与权衡。为增强互操作性，本文提出在应用层开发跨链查询语言的研究方向。该语言通过提供标准化语法、统一数据模型和数据查询处理架构，实现对底层实现细节的抽象。作为扩展更新的研究论文，本文展示了语言语法、数据模型和架构，并针对当前最大的开放无许可区块链进行了兼容性评估。