Future Monitors in Optimistic Rollups

Blockchains are decentralized systems that provide trustable execution guarantees. Most modern blockchains offer smart contracts, programs governing the functionality and logic of blockchain transactions. The model of computation of smart contracts is typically transactional: either transactions immediately revert or immediately commit. When transactions commit, the effect of executing them becomes permanent and cannot be undone.

Future monitors were proposed as a defense mechanism that enables smart contracts to state properties across multiple transactions. By delaying the consolidation of a transaction, future monitors wait for future transactions to occur before committing or failing.

Optimistic Rollups provide the perfect environment to implement future monitors. These systems are layer-2 solutions proposed for scaling blockchains, and they delay transaction effects in order to allow observers to challenge and remove malicious block proposals.

In this paper, we describe an implementation of future monitors in Optimistic Rollups. We propose a succinct way to encode all potential future states in rollup blocks. We provide novel fraud-proof mechanisms designed to arbitrate the high-level algorithm that determines the effect of transactions using future monitors. Finally, we present winning strategies for honest players, proving that our fraud-proof mechanisms preserve the guarantees of Optimistic Rollups while enabling future monitor capabilities.