On Finality in Blockchains

TL;DR

This paper investigates the conditions for achieving deterministic eventual finality in blockchain systems, allowing for asynchronous operation and Byzantine failures, and shows limitations of classical longest-chain mechanisms.

Contribution

It provides necessary and sufficient conditions for deterministic eventual finality and demonstrates the impossibility of bounded displacement in stronger finality forms.

Findings

Deterministic eventual finality can be achieved under specific conditions.

Bounded displacement of revoked blocks is impossible in stronger finality models.

Longest chain selection mechanisms like Bitcoin's are incompatible with eventual finality solutions.

Abstract

There exist many forms of Blockchain finality conditions, from deterministic to probabilistic terminations. To favor availability against consistency in the face of partitions, most blockchains only offer probabilistic eventual finality: blocks may be revoked after being appended to the blockchain, yet with decreasing probability as they sink deeper into the chain. Other blockchains favor consistency by leveraging the immediate finality of Consensus-a block appended is never revoked-at the cost of additional synchronization. In this paper, we focus on necessary and sufficient conditions to implement a blockchain with deterministic eventual finality, which ensures that selected main chains at different processes share a common increasing prefix. This is a much weaker form of finality that allows us to provide a solution in an asynchronous system subject to unlimited number of byzantine failures. We study stronger forms of eventual finality as well and show that it is unfortunately impossible to provide a bounded displacement. By bounded displacement we mean that the (unknown) number of blocks that can be revoked from the current blockchain is bounded. This problem reduces to consensus or eventual consensus depending on whether the bound is known or not. We also show that the classical selection mechanism, such as in Bitcoin, that appends blocks at the longest chain is not compliant with a solution to eventual finality.