What is Lachesis (aBFT)?

Lachesis is an asynchronous Byzantine Fault Tolerant (aBFT) consensus mechanism developed for the Fantom network. It operates by having each validator maintain a local DAG (Directed Acyclic Graph) of event blocks, where each event block contains transactions and references to previous events from both the same validator and other validators. Validators continuously create event blocks and share them through gossip protocol — when a validator receives events from peers, it incorporates them into its local DAG. Consensus is determined through virtual voting on the DAG structure: validators can independently determine what other validators would have voted based on the DAG topology, without actually exchanging vote messages. This eliminates the need for explicit voting rounds and their associated network overhead.

As an aBFT protocol, Lachesis provides one of the strongest possible guarantees in distributed systems theory — it can reach consensus even under asynchronous network conditions where messages can be delayed by arbitrary amounts. The protocol achieves finality in approximately 1-2 seconds under normal conditions. It tolerates up to one-third of validators being Byzantine (malicious or faulty). The DAG-based approach allows for high throughput because multiple validators can produce events concurrently without blocking each other, and the virtual voting mechanism eliminates explicit communication rounds. Lachesis separates the consensus layer from transaction ordering, allowing the consensus mechanism to be used independently of any specific blockchain execution environment.

Lachesis was developed by the Fantom Foundation and has been running on the Fantom Opera mainnet since December 2019. Fantom is an EVM-compatible blockchain that has hosted a significant DeFi ecosystem, gaining particular prominence in 2021-2022. The protocol was designed with the goal of achieving near-instant finality while maintaining decentralization through a permissionless validator set secured by staking. The Fantom team, led by Michael Kong and with contributions from distributed systems researchers, designed Lachesis to be modular, and the consensus engine has been proposed for use in other blockchain contexts beyond Fantom's main network.