Smart Listeners: a Hybrid-Optimistic Inter-Blockchain Communication Protocol

In recent years, blockchain technology has seen significant practical growth, yet it has not seen the same advancement from a theoretical perspective. This has led to the creation of numerous blockchains that are very different from each other and behave like isolated worlds. The research and development of theoretical frameworks, which define the fundamental properties of blockchains and define standards to follow for a more homogeneous implementation approach, have become extremely important. A theoretical model can help not only to design blockchains in the future but also to define a set of minimum requirements to be met for the creation of interoperability protocols between existing blockchains. In this work, we propose a theoretical model of blockchain that describes its most significant properties. Starting from our theoretical model, we present Smart listeners, a blockchain interoperability protocol that finalises an inter-chain transaction with just two transactions: one on the source blockchain and one on the destination blockchain. The protocol is optimistic since changes on the source blockchain occur as if inter-chain transactions were successful. It is a hybrid since it combines some properties of watchtowers and oracles in the off-chain components. We provide a benchmark on the performance of the proposed protocol in terms of latency and transactions per second. Finally, we define the minimum requirements that a blockchain should satisfy to allow the application of general-purpose interoperability protocols.