Solana replaced TowerBFT with Alpenglow on its mainnet on October 21, in its largest consensus change since its launch. The new protocol aims for a finality close to 150 milliseconds, compared to around 12.8 seconds previously, but part of the planned architecture still remains to be deployed.
In brief
- Solana activated Alpenglow on October 21, replacing TowerBFT on its mainnet.
- The new consensus aims for a finality of 150 milliseconds, compared to 12.8 seconds previously.
- The transition took place without general network disruption, with the participation of 654 validators.
- The Votor mechanism accelerates block validation thanks to new voting procedures and BLS signatures.
- Alpenglow promises faster finalized transactions and greater resistance to failures.
Alpenglow replaces TowerBFT on mainnet
Activation began at the launch of epoch 1064. The network, however, retained TowerBFT for the first 5000 slots of this epoch, or approximately 18 minutes, in order to prepare for the transfer to the new system.
The transition was completed at slot 459653000. The validators then formed the cryptographic certificate which marks the birth of the Alpenglow consensus. Anza, the lead developer of the Agave client, has qualified this operation of “biggest upgrade in Solana history”.
The main elements of activation make it possible to measure the scope of change:
- Alpenglow replaces the TowerBFT consensus used since the beginning of Solana;
- The targeted finality increases from 12.8 seconds to approximately 150 milliseconds;
- 654 validators participated in the transition;
- The transition from one consensus to the other took approximately 900 milliseconds;
- Validator votes leave transactions recorded on-chain;
- BLS signatures allow multiple votes to be grouped into a single certificate;
- Each admitted validator must pay a ticket of 0.8 SOL per epoch.
The transfer took place without a general network shutdown. During the critical phase, blocks temporarily stopped integrating user transactions and only carried the necessary votes. This precaution was to allow a rollback without canceling an operation signed by a user.
Once the starting certificate was formed, the nodes handed over consensus to the new Votor algorithm. Transactions then resumed under the Alpenglow regime.
Votor greatly reduces the final confirmation time
Consensus allows validators to agree on the order of transactions and when a block can no longer be rolled back. With TowerBFT, Solana produced its blocks quickly, but they took about 12.8 seconds to complete.
Votor uses two voting procedures in parallel. The first can finalize a block in one round when 80% of the staked tokens respond. The second can conclude after two rounds with a participation of 60%. The protocol retains the path that finishes first.
Votes are no longer recorded as ordinary transactions. Validators directly exchange signed messages with the BLS system. Multiple signatures can then be packaged into a compact certificate, freeing up block space for user operations.
Alpenglow also aims for better resistance to failure. Its design plans to simultaneously tolerate 20% of tokens placed with offline validators and 20% with hostile actors. TowerBFT relied on a lower limit of one-third of malicious participation.
For users, the expected effect mainly concerns payments, decentralized exchanges and applications that require rapid certainty. A finality close to 150 milliseconds would reduce the wait before an operation is considered irreversible. This gain does not mean that all applications will respond as quickly, however, because their interfaces, servers and controls add their own delays.
Part of the upgrade still to come
The activation on October 21 mainly concerns Votor, the new consensus mechanism. It does not yet deploy all the components described in the Alpenglow project.
Rotor, the future block distribution system, is to replace Turbine in a later phase. It plans to distribute data fragments through a single relay and mobilize the total bandwidth of the network. Its activation schedule has not yet been set.
The economic functioning of validators is also changing. Off-chain votes remove the old fees paid for each vote transaction. In their place, the protocol takes a validation ticket of 0.8 SOL per epoch from the voting account. An operator who does not have the necessary amount can neither vote nor produce a block during the following epoch.
The comparison with “The Merge” of Ethereum above all illustrates the technical importance of the change. Ethereum had abandoned proof of work in favor of proof of stake in 2022. Solana retains its general model, but directly replaces the engine which organizes the agreement between its validators.
The coming weeks will allow us to verify whether the observed finality remains close to 150 milliseconds during periods of high activity. Network stability, validator participation, and future Rotor deployment will determine whether Alpenglow delivers on all its promises.
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