Block Reorganization (Reorgs)
A reorg occurs when a node switches from its currently head block to a new canonical chain, discarding previously seen blocks.
Block re-orgs are a fundamental aspect of decentralized consensus. In asynchronous networks, latency or simultaneous block production can lead to temporary forks. The consensus protocol provides a 'fork choice rule' to determine the canonical chain. In Proof-of-Work, this is usually the 'longest chain' rule. In Proof-of-Stake (like Ethereum Post-Merge), it involves LMD GHOST for short-term fork choice and Casper FFG for long-term finality. A re-org occurs when the fork choice rule points to a different branch than the one a node was previously following. While shallow re-orgs (1-2 blocks) are common due to network jitters, deep re-orgs can indicate systemic issues or malicious attacks. Transaction finality ensures a block cannot be re-orged out; in many modern PoS systems, this is a deterministic economic guarantee.
graph LR
Center["Block Reorganization (Reorgs)"]:::main
Rel_double_spending["double-spending"]:::related -.-> Center
click Rel_double_spending "/terms/double-spending"
Rel_51_percent_attack["51-percent-attack"]:::related -.-> Center
click Rel_51_percent_attack "/terms/51-percent-attack"
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🧠 Knowledge Check
🧒 Explain Like I'm 5
Imagine two groups of people are writing the same history book. Suddenly, one group realizes the other group has written more and has more supporters. They throw away their last few pages and start copying the other group's version instead. That's a [block](/en/terms/block) re-org.
🤓 Expert Deep Dive
Deep re-orgs in Ethereum's Gasper consensus involve the interaction between attestations and block proposals. Validators use LMD GHOST to find the head of the chain by counting the latest weights of attestations. A re-org can be triggered if a previously 'unseen' branch suddenly gains a supermajority of attestations. MEV-boosted re-orgs (Reorg-as-a-Service) have emerged as a significant risk, where actors induce a 1-block re-org to capture MEV from the previous proposer. Deterministic finality (Casper FFG) provides a backstop; once an epoch is 'finalized', a re-org would require a minimum of 1/3 of the total stake to be slashed, providing trillions of dollars in economic security.