Fork Choice Rule

A finality gadget is a mechanism layered on top of a longest-chain rule to provide probabilistic or absolute finality. It allows nodes to confidently consider blocks irreversible after a certain threshold, preventing deep chain reorganizations. Examples include:

• Casper FFG (Friendly Finality Gadget): Used in Ethereum's transition to proof-of-stake, it finalizes checkpoints.
• GRANDPA (GHOST-based Recursive ANcestor Deriving Prefix Agreement): Provides deterministic finality for nominated proof-of-stake chains like Polkadot.

The Longest Chain Rule, also known as Nakamoto Consensus, is the foundational fork choice rule used by Bitcoin. The canonical chain is simply the one with the greatest cumulative proof-of-work. This provides probabilistic security—attacks become exponentially harder as honest blocks are added. Its key properties are:

• Simplicity: Easy for nodes to validate.
• Liveness: The chain with the most work always progresses.
• Temporary Forks: Natural forks are resolved as miners extend one branch.

The Greedy Heaviest Observed SubTree (GHOST) rule addresses the security weaknesses of the longest-chain rule in high-latency or high-throughput networks. Instead of only counting the longest chain, it considers the heaviest subtree, incorporating the weight of orphaned blocks (uncles) in the consensus calculation. This:

• Improves security: Reduces the incentive for selfish mining.
• Increases throughput: Allows for faster block times without compromising security.
• Is implemented in Ethereum's pre-Merge proof-of-work consensus.

Latest Message Driven Greedy Heaviest Observed SubTree (LMD-GHOST) is the fork choice rule at the heart of Ethereum's proof-of-stake consensus. It combines GHOST with a validator vote-based weighting system. The "heaviest" branch is determined by the cumulative votes (attestations) of validators, weighted by their stake. Key aspects:

• Vote-Driven: Validators attest to the head of the chain they believe is correct.
• Dynamic Weighting: The rule follows the subtree with the greatest sum of validator votes.
• Enables Finality: Works in tandem with Casper FFG to achieve finality.

Fork choice rules navigate the fundamental trade-off between safety (never agreeing on conflicting blocks) and liveness (the ability to produce new blocks). A rule must be asynchronous to guarantee one property at the expense of the other. For example:

• Longest Chain favors liveness, as it always allows chain growth, but permits temporary forks (safety violation).
• Classic BFT protocols favor safety, halting progress if consensus cannot be reached.
• Modern hybrid rules (e.g., Ethereum's) use accountable safety and plausible liveness to optimize both.

The fork choice rule must be identically implemented across all node client software (e.g., Geth, Erigon, Besu for Ethereum). Any divergence is a critical consensus bug that can cause a network split. Key considerations for client developers include:

• Determinism: Identical inputs must produce identical chain head selections.
• Performance: The algorithm must efficiently handle thousands of competing blocks and attestations.
• Specification Compliance: Rigorous adherence to the canonical protocol specification is non-negotiable for network health.

A fork choice rule designed to improve security and reduce confirmation times in fast-blockchains, notably adopted by Ethereum's execution layer during its Proof-of-Work era. Instead of just the longest chain, it selects the chain with the heaviest subtree, counting uncle blocks (stale blocks) in the weight calculation.

• Purpose: Mitigates the high orphan rate in high-throughput networks.
• Impact: Increases chain security by rewarding miners for referenced uncles.

Used by Cosmos and other BFT-style chains. This is a proposer-based rule where a round-robin leader proposes a block, and validators pre-vote and pre-commit in rounds. The canonical chain is determined by the first block that achieves a supermajority (2/3+) of pre-commits in a round.

• Key Feature: Offers instant finality; once a block is committed, it cannot be reverted without slashing >1/3 of the stake.
• Fork Prevention: Designed to avoid forks under normal, synchronous network conditions.

Employed by the Avalanche Primary Network, this uses a metastable mechanism based on repeated sub-sampled voting. Validators repeatedly poll a random subset of peers, steering the network toward consensus on one branch.

• Mechanism: A probabilistic, leaderless protocol where nodes converge on the chain preferred by an overwhelming majority.
• Property: Achieves high throughput and finality in 1-3 seconds without all-to-all communication.

The foundational algorithm for many permissioned blockchains (e.g., early Hyperledger Fabric). The fork choice is implicit in the three-phase commit protocol (pre-prepare, prepare, commit). The chain advances when a node collects 2f+1 matching commit messages for a proposed block, where f is the number of faulty nodes.

• Assumption: Operates in a partially synchronous network with known validator set.
• Output: Provides deterministic finality after the commit phase completes.

The foundational fork choice rule used in Nakamoto Consensus (e.g., Bitcoin). It selects the chain with the greatest cumulative proof-of-work difficulty. This rule probabilistically ensures security, as attacking the chain requires out-computing the honest majority of the network's hash power. It leads to probabilistic finality, where transactions become more secure as more blocks are built on top of them.

A fork choice rule designed to improve throughput and security in high-latency networks. Instead of discarding orphaned blocks (uncles), GHOST incorporates them into the weight calculation of the main chain. This reduces the incentive for selfish mining and improves chain resilience. Variants of GHOST are used in Ethereum's pre-Merge consensus to reward uncle blocks.

A mechanism layered on top of a fork choice rule to provide economic finality. Casper the Friendly Finality Gadget (FFG) works in tandem with LMD-GHOST. Validators cast votes to periodically 'finalize' checkpoints. Once a block is finalized, it cannot be reverted without slashing a significant portion of the validator stake, providing a much stronger security guarantee than probabilistic finality alone.

The specific fork choice rule used in Ethereum's proof-of-stake consensus. It selects the chain with the greatest weight of attestations from validators, where each validator's vote is their 'latest message'. This rule is executed at every slot to determine the canonical head of the chain. It is designed to be resilient to certain attacks, like balancing attacks, when combined with the finality gadget.

The process by which nodes in a Byzantine Fault Tolerant (BFT) consensus protocol agree on the current state of the protocol (the 'view') and the leader. The fork choice in such systems is often straightforward: follow the chain certified by a supermajority of validators in the current view. This is crucial for protocols like Tendermint or HotStuff, where the fork choice is implicit in the voting mechanism.

The event that occurs when the fork choice rule selects a different canonical chain than the one a node was previously following. This causes the node to reorganize its local blockchain, potentially discarding (orphaning) blocks it had previously considered valid. Reorgs are a normal part of network latency resolution but can be exploited in chain reorganization attacks if the fork choice rule has vulnerabilities.