What is the Fork Choice Rule?
- A fork choice rule is a function, evaluated by the client, that takes as input the set of blocks and other messages that have been seen, and outputs to the client what the “canonical chain” is.
- Fork choice rules are required because there may be multiple valid chains to choose from (eg. if two competing blocks with the same parent get published at the same time).
In some fork choice rules (eg. Tendermint), reorgs cannot happen; the fork choice rule simply extends the existing chain by appending any blocks that have been finalized through BFT consensus. In other fork choice rules, reorgs are very frequent.
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