Finality

In classical blockchain consensus, it is common to employ the "longest fork wins" rule, meaning that if conflicting blocks (called "forks") appear at a given height, this will eventually resolve itself by one fork outpacing the other. In traditional distributed systems parlance, this could be described as "eventual consistency".

On Bitcoin, a rule of thumb is that a transaction is practically impossible to evict from the chain once six blocks have been added on top of it. Given Bitcoin's 10 minute block interval, this means that a transaction can be said to have reached finality after an hour. The six-block rule is based on the exponentially increasing cost of rewriting the chain for each added block. Some other chains simply impose a certain depth beyond which they do not accept any forks being introduced.

Since an hour is much too long to wait for many transactions ("paying for coffee" is often given as an example), users often choose to settle for much less—again in the case of Bitcoin, perhaps even peeking into the "mempool" (list of pending transactions) and spotting the transaction there. Accepting a transaction that hasn't even reached the chain yet is called a "zero-confirmation" trade.

Several competing systems claim finality within seconds. Given knowledge of the CAP Theorem (see The Blockchain Trilemma), one wonders if their models cover all potential situations, especially in a fully decentralized setting.