Chain Reorganization

A chain reorganization, commonly called a reorg, is an event where a blockchain’s history is revised because the network discovers that what it thought was the correct chain of blocks was actually not the longest or most valid one. In networks like Bitcoin, the rule is that the chain with the most cumulative computational work behind it is considered the truth. When a competing chain grows longer than the current one, all nodes switch over to it, effectively “undoing” some recent transactions.

Reorgs happen because blockchains are built by many independent computers spread across the world, and sometimes two miners find valid blocks at nearly the same time. Short reorgs — one or two blocks deep — happen naturally and are handled automatically. Longer reorgs are much rarer and can be signs of a serious attack — specifically a 51% attack, where a malicious actor who controls the majority of the network’s computing power secretly builds an alternative chain and then releases it all at once, making it longer than the honest chain.

Example: Imagine a shared Google Doc where ten people are editing different sections at the same time with no internet connection. When they reconnect, there are conflicting versions. The group agrees to keep the version that has the most edits made in good faith. A chain reorg is what happens when the blockchain network goes through that same process of resolving conflicting histories.