  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.
