Forks and Finality
Two miners on opposite sides of the world find a valid block at almost the same moment. Half the network sees one first, and half sees the other. For a short while the chain has two tips: a fork. The same word also describes something much bigger, when a community changes the rules. This lesson separates the two.
The rule
Accidental forks heal on their own; hard forks split unless everyone upgrades.
Accidental forks. Two valid blocks appear at the same height. Each miner builds on whichever it saw first. When the next block arrives on one branch, that branch has more accumulated work, and every node switches to it. The losing block becomes stale (often called orphaned). Its transactions go back to the mempool and usually land in a later block.
This is why people wait for confirmations. On Bitcoin, a payment 6 blocks deep, about an hour after inclusion, is treated as very unlikely to be reversed.
Finality is the point after which a transaction cannot be reversed. On proof-of-work chains finality is probabilistic: a reversal grows ever less likely and never becomes strictly impossible. BFT-style chains offer deterministic finality: once finalized, never undone.
Rule changes. When a chain's rules change, the new software either tightens or loosens what counts as a valid block:
| Soft fork | Hard fork | |
|---|---|---|
| The rules become | Stricter | Looser or different |
| Old nodes | Still accept the new blocks | Reject the new blocks |
| Who must upgrade | Mainly the block producers | Every node |
| Main risk | Low, the chain stays whole | A permanent split if some refuse |
| Example | Bitcoin's SegWit upgrade (2017) | Ethereum and Ethereum Classic (2016) |
In 2016 most of the Ethereum community adopted a hard fork to reverse the effects of a large contract exploit. A minority kept running the old rules, and that chain continues today as Ethereum Classic.
The shortcut
Classify any fork with two questions. Was it planned? If not, it is temporary: wait for confirmations. If it was planned: would an old node accept the new blocks? Yes means a soft fork; no means a hard fork.
Two blocks at height 40
Asha's and Kabir's nodes each produce a block 40 at the same moment. Nobody planned it: a temporary fork.
The trap is treating proof-of-work finality as absolute. Deeper is safer, but a buried transaction is never mathematically impossible to reverse.
Drills
Answer fast — trust the rule.
Two valid blocks appear at the same height. How is the tie settled?
What happens to the transactions in a stale block?
A new rule makes some previously invalid blocks valid, and old nodes do not upgrade. What kind of fork is this?
Which kind of chain offers deterministic finality?
Roughly how long do 6 confirmations take on Bitcoin?
Remember
- Accidental forks heal on their own; hard forks split unless everyone upgrades.
- Shortcut: planned? If not, wait for confirmations. If so, would old nodes accept the new blocks? Yes is soft, no is hard.
- Trap: proof-of-work finality is probabilistic. Deeper is safer, never absolute.