Finalité et forks

Adding a block is not the same as it being permanent. Finality is the guarantee that a block can no longer be reverted.

Understanding finality and forks is key to knowing when a transaction is truly settled.

A fork occurs when the chain temporarily splits into two competing branches — for example, when two valid blocks appear at nearly the same time.

The network must decide which branch becomes the official history.

A fork choice rule tells nodes which branch to follow. PoW chains often use the longest (heaviest) chain rule.

Whichever branch accumulates the most work or attestations becomes canonical.

The losing branch's blocks become orphaned (or stale). Their transactions return to the mempool to be included in a future block.

This is why a single confirmation is not yet fully safe.

In PoW, finality is probabilistic: the more blocks built on top of yours, the less likely it can be reversed.

Each additional confirmation makes a reversal exponentially harder. Bitcoin users often wait for 6 confirmations.

Some PoS systems offer absolute (deterministic) finality: once a block is finalized by validator votes, it can never be reverted without massive slashing.

This gives a clear, hard cutoff instead of waiting for probabilities to improve.

Ethereum groups slots into epochs. After two epochs of supermajority attestations, blocks become finalized.

Reverting a finalized block would require burning at least one-third of all staked ETH.

A soft fork is a backward-compatible rule change: blocks valid under the new rules are still valid under the old ones.

Non-upgraded nodes can still follow the chain, so soft forks are less disruptive.

A hard fork is a backward-incompatible change. Nodes must upgrade, or they will reject the new blocks.

If the community disagrees, a hard fork can permanently split one chain into two (for example, Ethereum and Ethereum Classic).

A reorg happens when nodes switch from one branch to a longer competing branch, replacing recently accepted blocks.

Small reorgs are normal near the chain tip; deep reorgs are a serious security concern.

Forks are temporary disagreements resolved by a fork choice rule. Finality tells us when a block is safe: probabilistic in PoW, often absolute in PoS.

Soft and hard forks describe how rules themselves change over time.

Test your understanding of finality and forks.

You learned that:

• A fork is a temporary split resolved by a fork choice rule
• Losing blocks become orphaned
• PoW gives probabilistic finality; PoS can give absolute finality
• Soft forks are compatible; hard forks are not

Next course: gas and transactions.