Forks, Upgrades, and Chain Reorganization

What you'll learn

• Forks: When Blockchains Disagree
• Soft Forks: Backwards-Compatible Upgrades
• Hard Forks: When Chains Split
• Chain Reorganizations
• For Deeper Reading

Key terms

Soft fork

A backwards-compatible blockchain upgrade. New rules are stricter than old rules; non-upgraded nodes still see the new chain as valid. Bitcoin's Taproot (Nov 2021) and SegWit (Aug 2017) were soft forks.

Hard fork

A non-backwards-compatible blockchain upgrade. Old nodes see the new chain as invalid. If the community splits, two chains continue; if everyone upgrades, the old chain dies.

Chain reorganization (reorg)

When nodes temporarily disagree about which is the canonical chain, then reorganize when more information arrives. 1-2 block reorgs are normal; deeper reorgs are concerning.

Ethereum Classic (ETC)

The minority continuation of the Ethereum chain that resulted from the July 2016 hard fork response to the DAO hack. The chain that did NOT roll back the hack.

Bitcoin Cash (BCH)

A hard fork of Bitcoin (August 1, 2017) that increased block size to 8 MB. Subsequently split further into Bitcoin SV. Has lost most of its market relevance.

Segregated Witness (SegWit)

Bitcoin soft fork activated August 2017. Restructured signature storage, increased effective block capacity, and fixed transaction malleability. Enabled Lightning Network.

Taproot

Bitcoin soft fork activated November 14, 2021 at block 709,632. Added Schnorr signatures and improved smart contract privacy. Currently the most significant Bitcoin upgrade since SegWit.

BIP / EIP

Bitcoin Improvement Proposal / Ethereum Improvement Proposal. The formal process for proposing protocol changes. Anyone can submit; community must achieve rough consensus.

Speedy Trial

The activation method used for Bitcoin's Taproot upgrade. A 90% miner signaling threshold over a defined window. Designed as a compromise between user activation and miner activation approaches.

The DAO Hack

A reentrancy exploit on June 17, 2016 that drained 3.6M ETH (~$60M) from a major Ethereum smart contract. Resulted in Ethereum's contentious hard fork creating ETH (rolled back) and ETC (didn't).

Block height

The position of a block in the chain, counted from genesis. Bitcoin's genesis block is height 0. Block 840,000 was mined on April 19/20, 2024 (the 4th halving).

Activation

The process by which a soft or hard fork upgrade goes live on the network. Various mechanisms exist (miner signaling, fixed block height, user-activated soft fork) with different political dynamics.