What Is a “Fork Choice Rule” and How Does It Prevent the Nothing-at-Stake Problem?
A fork choice rule is an algorithm that nodes use to determine which chain is the canonical (correct) one when a fork occurs. Ethereum PoS uses the LMD-GHOST rule, which favors the chain with the heaviest weight of attestations (i.e. the chain supported by the most staked ETH).
By explicitly defining the "correct" chain and penalizing validators who attest to any other, the rule forces rational validators to commit to one chain, thus solving the nothing-at-stake problem.
Glossar
Penalizing Validators
Slashing ⎊ Penalizing validators is primarily achieved through the mechanism known as slashing, which involves the programmatic destruction of a portion of the validator's staked cryptocurrency collateral.
Objective Fork Choice
Rule ⎊ Objective Fork Choice refers to the deterministic protocol rule that dictates how a node must select the single, canonical chain when confronted with multiple competing forks.
Fork Choice Rule
Genesis ⎊ The Fork Choice Rule, fundamentally, dictates consensus in blockchain networks, particularly those employing Proof-of-Work mechanisms, by establishing a deterministic method for selecting the valid chain when multiple forks arise.
Fork Choice
Divergence ⎊ Fork choice, within cryptocurrency, represents the probabilistic selection among competing chains following a hard fork, impacting network consensus and asset valuation.