Altair hard fork
Beacon Chain · Mainnet 2021-10-27
The first upgrade of the Beacon Chain. It supported light clients through sync committees and reworked the inactivity penalty.
What changed
- Validators
- The inactivity penalty calculation changed. The penalty applied to validators that miss their duties while the chain is not finalizing was changed to accumulate according to each validator's participation record, so a validator that was briefly offline and came back sees its penalty shrink quickly, while one that keeps missing duties loses more. The slashing penalty, which had been set low at launch, was also raised one step. Validators selected for the sync committee take on the duty of additionally signing block headers for a set period and receive rewards for it.
- Node operators
- Beacon chain clients had to be upgraded to a version that supports Altair. Nodes that did not update before the fork epoch could not follow the chain afterward. The structure of the beacon state also changed: instead of storing the full list of attestations for each epoch, each validator's participation is recorded as small flags. This was a change that client teams had to work on in their state handling. Operators rarely needed to change settings themselves.
- Wallet & app developers
- The introduction of the sync committee laid the groundwork for building light clients. The structure lets a client follow the chain's latest header by checking only the signatures of a small set of validators selected for each period, without downloading full blocks and every validator signature. This opened a path for browser wallets and mobile apps to verify the chain directly without relying on external node services. However, only the groundwork was added at the time of Altair, and wallets or apps that use it did not appear right away.
- Users
- Nothing changed directly for transactions or wallet use. Ordinary transactions at the time were still processed on the proof-of-work (PoW) chain, and Altair changed only the internal rules of the beacon chain, which handles staking. Staking reward and penalty calculations were adjusted, but users who were not staking noticed no change.