The Challenge of Fairness in Distributed Systems
For years, research in distributed systems has focused on two core principles: consistency and liveness. These ensure that every node in a network agrees on the order of transactions while keeping the system active and reliable.
However, public blockchains introduced a new challenge. Validators — the participants who confirm transactions — can manipulate transaction ordering for financial gain. This manipulation leads to what’s known as Maximal Extractable Value (MEV), where some users profit unfairly by controlling the sequence in which transactions are processed.
To counter this, researchers have been exploring a new concept known as transaction order-fairness. The idea is simple yet powerful: transactions should be processed based on objective factors, such as the time they’re received by the network, rather than on who can pay or manipulate the system the most.
From Receive-Order-Fairness to Block-Order-Fairness: The Path Forward
The strongest form of fairness, called Receive-Order-Fairness (ROF), ensures that the first transaction received by the network is also the first one executed. While this sounds ideal, it’s not always achievable. The Condorcet paradox — a problem from voting theory — shows that collective preferences can form cycles, making it impossible to maintain a perfectly consistent order among all transactions.
To solve this, researchers proposed a more practical model called Block-Order-Fairness (BOF). Instead of forcing every transaction into a strict sequence, BOF allows conflicting transactions to be grouped within the same block. This ensures fairness without requiring an impossible universal order.
Building on this concept, the Themis protocol offers an advanced solution. It improves communication efficiency and allows blockchains to process transactions continuously, even when conflicts occur. By doing so, it strengthens both fairness and performance across decentralized systems.
In essence, the evolution from ROF to BOF — and the development of protocols like Themis — marks a crucial step toward trustless, verifiable fairness in blockchain technology. As blockchains continue to evolve, cryptographic verification will play an increasingly vital role, ensuring not just trust, but provable integrity in every transaction.
