Blockchain Interoperability Study: 3 Key Approaches Revealed

The first major category identified is Cryptocurrency-directed interoperability approaches. These strategies were initially developed for interoperability between public blockchains, primarily those implementing cryptocurrencies, at a time when private blockchains were not yet prevalent. The classification within this category follows a well-known framework, breaking down into sidechains, notary schemes, and hashed time-locks.

Sidechains, or relay chains, are mechanisms where one blockchain considers another as an extension of itself, enabling interoperability and scaling. Notary schemes involve a trusted entity that monitors multiple chains and triggers transactions on one chain based on events occurring on another, with centralized cryptocurrency exchanges being a typical example. Hashed time-lock contracts (HTLCs) offer an alternative to these centralized exchanges by using cryptographic hashlocks and timelocks to enforce the atomicity of cross-chain operations between two parties. The category also includes combined solutions that do not fit neatly into the other sub-categories, representing early alternative approaches to the interoperability problem.

Moving beyond cryptocurrency-specific use cases, the second category is Blockchain Engines. These are frameworks designed for general use-cases and heterogeneous systems. Unlike the first category, Blockchain Engines provide reusable layers—for data, network, consensus, incentives, and contracts—that allow developers to create customized blockchains. These custom blockchains are built to interoperate with each other from the ground up.

The most prominent examples in this category are the Cosmos Network and Polkadot. These projects represent a significant evolution in interoperability thinking, shifting from connecting existing, often monolithic chains to providing ecosystems where new, inherently interoperable blockchains can be spawned. This engine-based approach aims to solve scalability and sovereignty issues by allowing application-specific blockchains to communicate within a shared security or communication framework.

The third and most diverse category is Blockchain Connectors, encompassing solutions that are neither purely cryptocurrency-directed nor full-stack engines. The research notes that most available studies on Blockchain Connectors—and recent updates from Blockchain Engines—are very recent, with the majority dated from 2019. This category is subdivided into four types: Trusted Relays, Blockchain-Agnostic Protocols, Blockchain of Blockchains, and Blockchain Migrators.

Trusted Relays are often used in permissioned blockchain environments, where a trusted registry facilitates the discovery of target blockchains and cross-chain transactions are routed by escrow parties. Blockchain-agnostic protocols provide technology-agnostic standards for interoperation but require changes to the source code of existing blockchains, lacking backward compatibility. Blockchain of blockchains solutions provide mechanisms for developers to build cross-chain decentralized applications (dApps). Finally, Blockchain Migrators perform data migration across chains, resembling the notary schemes from the first category.

The study specifically highlights Hyperledger Cactus as a very promising open-source project in this space. Launched in May 2020, Cactus is the newest Hyperledger project and offers a variety of use-cases, including blockchain migration. Although currently implemented as a trusted relay, its architecture allows the source of trust to be modified, for instance, by depositing trust into a blockchain-based application running on Ethereum. This flexibility points to the innovative and rapidly evolving nature of solutions within the Blockchain Connector category.