Solana’s Alpenglow Upgrade: How Blockchain Finality Went from Seconds to Milliseconds
za LCX Team ·
Solana has switched to a new consensus system called Alpenglow. Alpenglow targets about 150 milliseconds to finality, down from the roughly 12.8 seconds Anza cited for the old system.
This guide explains what finality is, how Solana reached it before, how Alpenglow reaches it now, and what the change means for people who use the network.
What is finality?
Finality is the point after which a transaction can no longer be reversed or reordered. Until a transaction is final, the network could still replace the block it sits in with a competing block.
Think of a bank transfer. “Pending” means the bank has your request. “Completed” means the money has moved and the transfer cannot be undone. Finality is the blockchain’s “completed” moment.
Blockchains reach finality in one of two ways:
- Probabilistic finality: each new block built on top makes a reversal less likely, but never impossible. Bitcoin works this way, so exchanges often wait for several confirmations.
- Deterministic finality: once enough validators formally agree on a block, it is final and the protocol guarantees it stays. Alpenglow works this way.
Finality matters because anything built on a transaction has to wait for it. A merchant handing over goods, an exchange crediting a deposit, or a lending app releasing collateral all depend on knowing the payment will stick.
How Solana worked before: Proof of History and TowerBFT
Before Alpenglow, Solana combined two systems to reach agreement:
- Proof of History (PoH): a continuous chain of hashes that acted as a cryptographic clock. It let validators agree on the order and timing of events without constantly messaging each other.
- TowerBFT: the voting system on top of PoH. Each validator vote added a “lockout” period during which the validator could not switch to a competing fork. Lockouts doubled with each confirmation, and a block was finalized after 32 of them.
This design had three costs:
- Slow finality. A transaction showed as “confirmed” in about 500ms, but “finalized” took about 12.8 seconds.
- Votes clogged blocks. Every validator vote was an on-chain transaction. The week before the switch, votes made up about 62% of all processed transactions, roughly 263 million a day.
- Voting was expensive. Validators paid a fee on every vote, about 1 SOL a day, which made it hard for small validators to break even.
How Alpenglow works: Votor and Rotor
Alpenglow replaces TowerBFT and Proof of History with two new parts. Votor is live now. Rotor comes in a later release.
Votor: a faster way to vote
Votor handles voting and finality. Validators no longer post votes as on-chain transactions. They send small messages to each other directly, and those votes are combined into a single certificate.
The combining uses BLS signature aggregation, a cryptographic method that merges many signatures into one. Each block’s vote data shrinks from roughly 500KB to a single proof of about 1,000 bytes.
Votor finalizes a block in one of two ways:
- Fast path: if validators holding at least 80% of stake vote for a block in the first round, it is final in one round, about 100ms.
- Slow path: if fewer validators respond, a second round runs. The block is final once two separate certificates each reach 60% of stake, about 150ms in total.
In practice the two paths run in parallel. The fast path wins when almost every validator is online and responsive.
Both paths run at the same time, and whichever finishes first wins. Because 80% plus 60% is more than 100%, two conflicting blocks cannot both be approved.
Alpenglow also drops the Proof of History clock. Each block gets a timer instead: a block that arrives in time can be voted on, and one that misses its window is skipped.
Rotor: faster block delivery (coming later)
Rotor will change how new blocks spread across the network, replacing the current system, Turbine. The Solana Foundation says it is planned for a later release with no date set.
What faster finality changes for users
For most users, nothing about how they use Solana changes. The Solana Foundation says transactions, programs, fees and the virtual machine all stay the same, so wallets and apps keep working. What changes is how quickly a transaction becomes certain.
Payments
At about 150ms, a payment is final fast enough to feel instant at checkout. A shop or payment app can treat a Solana payment as settled almost at once, without waiting several seconds or taking on the risk of accepting an unfinalized payment.
Trading
Traders get certainty sooner that an order has landed and cannot be reversed. Exchanges and bridges that wait for finality before crediting a deposit can, in principle, credit funds much sooner. Each platform still sets its own confirmation rules.
DeFi
DeFi apps chain actions together, such as a swap, then a deposit, then a loan. When each step is final in a fraction of a second, those chains run faster. Liquidations and price updates also land on a settled state sooner.
Validators and the network
With votes off-chain, block space that went to votes is now free for user transactions. Validators no longer pay a fee per vote. Alchemy estimates the minimum stake needed to run a profitable validator falls from about 4,850 SOL to about 450 SOL, which could let smaller operators join.
The bottom line
Alpenglow changes how Solana reaches agreement, not how people use it. Wallets, apps and fees work as before. What changes is how long it takes for a transaction to become permanent: from about 12.8 seconds to a target of about 150 milliseconds.
The idea behind it reaches beyond Solana. Finality has long been the hidden wait behind every blockchain payment, trade and DeFi action. Cutting it to a fraction of a second brings blockchain settlement closer to what people expect from everyday apps, where “done” means done right away.
There are still open questions: how the new security model holds up, how quickly validator clients catch up, and whether real-world speed matches the target. Rotor, the second half of the upgrade, is still to come. For anyone learning how blockchains work, Alpenglow is a clear example of how consensus design shapes what a network can do.
