chainselection

Chainselection is the set of rules used by a blockchain node to determine the canonical chain when multiple chains exist due to forks. In a distributed network, blocks may be found nearly simultaneously, producing competing branches. Each node maintains a local view of the blockchain and, when faced with forks, applies a chain selection rule to decide which branch to extend.

The most common form, in proof-of-work systems such as Bitcoin, is to choose the chain with the greatest accumulated work, often referred to as the heaviest or longest chain. This is approximated by the chain with the highest total difficulty. As blocks are added to one chain, other chains may become less favorable, and nodes gradually converge on a single chain. If a longer chain with more work becomes visible, nodes will reorganize (reorg) to switch to it, and blocks from the shorter chain may be discarded.

In other consensus models, such as proof-of-stake, chain selection may use different criteria, such as total stake weight or protocol-defined finality rules, but the underlying goal remains: identify a single canonical chain that all honest nodes agree upon for validation and future extension.

Key considerations include liveness (the ability to continue adding blocks) and finality (the point at which a block is unlikely to be reversed). Network latency, block production rate, and adversarial behavior can influence the frequency and impact of chain forks.