Archived article · Original source date: 2024-08-17
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Distributed systems need a way to agree on ordering even though participants do not share a perfectly synchronized clock. Solana's Proof of History provides a cryptographically verifiable sequence that helps establish when events occurred relative to one another.
The sequential hash chain
Proof of History repeatedly applies a hash function so each output becomes the next input. Producing the sequence is inherently sequential, while checking recorded checkpoints can be parallelized. Data can be mixed into the sequence to show that it existed before a later checkpoint.
Proof of History is not consensus
Solana uses proof-of-stake validators and Tower BFT to vote on forks and reach consensus. Proof of History supplies a shared ordering and timing source that the consensus process uses; it does not independently decide which fork is canonical.
Relationship to leaders and slots
A stake-weighted leader schedule assigns validators opportunities to produce blocks during slots. The Proof of History sequence helps leaders timestamp entries and helps other validators verify ordering as they replay the ledger.
What it does not guarantee
- It does not make every transaction instant or guarantee inclusion.
- It does not eliminate proof-of-stake, validator voting, network communication, or fork choice.
- It does not by itself guarantee decentralization or application security.
- It is not the source of Solana's parallel execution; that depends on declared account access and the runtime scheduler.
Read the Solana whitepaper together with the current documentation for the original construction and the network's present implementation.