LibraryConsensus2016Design paperCorpus record
Flexible Paxos: Quorum Intersection Revisited
Flexible Paxos. Heidi Howard, Dahlia Malkhi and Alexander Spiegelman.
Only quorums from different phases have to intersect. A phase-1 quorum and a phase-2 quorum must overlap. Two phase-2 quorums need not.
A reading of the public document. Not a copy of it, and not a claim about a later network that reused the name.
A system that boasts a fast quorum should say which phase got smaller and which intersection it still guarantees.
The five-minute read
The defect
Classic Paxos uses the same majority for every phase. That is sufficient, not necessary.
The rule
Only quorums from different phases have to intersect. A phase-1 quorum and a phase-2 quorum must overlap. Two phase-2 quorums need not.
How it is put together
You can make the common case, acceptance, a smaller quorum. You pay for that with a larger prepare quorum. Safety is still the intersection, not the word majority.
Where the claim stops
Flexible Paxos does not remove the need for a known set.
One action, walked through
- Name the prepare quorum and the accept quorum separately.
- Check that every prepare quorum hits every accept quorum.
- Run the common path on the smaller one.
- Do phase-1 and phase-2 quorums intersect?
The argument, unpacked
Why it is still on the desk
A system that boasts a fast quorum should say which phase got smaller and which intersection it still guarantees.
After the text
Later consensus systems treat quorum choice as a parameter. The 2016 note is the reason that parameter is not free.
What has to be true
- Flexible Paxos does not remove the need for a known set.
- It is not a licence to pick quorums that miss each other.
- It does not add Byzantine faults by itself.
What happened after the paper
Later consensus systems treat quorum choice as a parameter. The 2016 note is the reason that parameter is not free.
What to check before you use the idea
- Do phase-1 and phase-2 quorums intersect?
- Which phase is on the hot path?
- What fails if an operator shrinks both?
Terms
- Phase 1
- The prepare that discovers a value that might already be chosen.
- Phase 2
- The accept that tries to choose a value.
The problem the paper names
Classic Paxos uses the same majority for every phase. That is sufficient, not necessary.
What the design proposes
- You can make the common case, acceptance, a smaller quorum.
- You pay for that with a larger prepare quorum.
- Safety is still the intersection, not the word majority.
How the mechanism is specified
- Name the prepare quorum and the accept quorum separately.
- Check that every prepare quorum hits every accept quorum.
- Run the common path on the smaller one.
What this page does not treat as proven
- Flexible Paxos does not remove the need for a known set.
- It is not a licence to pick quorums that miss each other.
- It does not add Byzantine faults by itself.
Why a venture studio still reads it
A system that boasts a fast quorum should say which phase got smaller and which intersection it still guarantees.
This is Blockchain Lab's reading of a public design paper. It is not the paper, not a copy of it, and not an offer of tokens, equity, custody or a partnership. Later network behaviour can diverge from the text. Nothing here is investment, legal or technical advice.
Research status: Design paper. Last reviewed: 1 October 2026. This is a reading of a public paper, not investment, legal or security advice.