Solana's Alpenglow Upgrade Replaces TowerBFT With Votor

Solana's "Alpenglow" upgrade has substantially altered the network's consensus mechanism, replacing the TowerBFT system with a new voting engine called Votor. This change has dramatically reduced finality times from 12.8 seconds to roughly 100-150 milliseconds, improving the blockchain's responsiveness for applications. The upgrade also tightens the Byzantine fault tolerance threshold from 33% to 20%, meaning the network can now tolerate a smaller percentage of malicious or faulty nodes while still maintaining safety guarantees.

Votor functions differently from TowerBFT in several key ways. Firstly, it no longer requires on-chain votes, which were previously a significant portion of transactions on the Solana network. Instead, validators broadcast lightweight vote messages directly to each other, and a BLS12-381 certificate is created once a threshold of stake weight has signed off. This certificate is the only on-chain data required for finalization.

Secondly, Votor runs two finalization paths simultaneously. The fast path finalizes a block after 80% of stake has voted, while the slower path requires two rounds, with 60% stake needed for a notarization certificate followed by another 60% for a finalization certificate. The protocol then selects the path that resolves first, allowing network conditions to dictate the speed of finality.

These changes have resulted in finality times of approximately 100 to 150 milliseconds, significantly faster than TowerBFT's 12.8 seconds. However, the upgrade also removes the fee revenue and burn mechanism that was generated by TowerBFT's on-chain votes. To compensate for this, validators now pay a flat fee of approximately 1.6 SOL per epoch to be admitted into the active validator set, which is then burned.