fix(epoxy): make quorum sizes match paper

[MasterPtato]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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[MasterPtato]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• fix: remove useless config params #4319
• fix: add serverless leak fix oneoff #4318
• chore: add log for runners with no alloc idx #4316
• fix(ups): upgrade nats, fix chunking, add chunking tests #4315
• chore: fix tests #4314
• fix: various serverless bug fixes #4313
• fix(guard, runner): enforce http body size and ws msg size #4252
• fix(pegboard): add threshold parameters to runner protocol metadata #4247
• chore: get docker compose working again #4246
• fix(epoxy): make quorum sizes match paper #4234 👈 (View in Graphite)
• fix(rivetkit): stall stop handler until start completes #4192
• main

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[claude]

Code Review: fix(epoxy): make quorum sizes match paper

The core idea is solid — separating the "sender-excluded" fanout quorum calculation out of the HTTP client and into a dedicated calculate_fanout_quorum function is a clear improvement in separation of concerns. The old code's blanket quorum_size - 1 subtraction in the Fast | Slow arm was both ad-hoc and only correct for n >= 2.

Issues

utils.rs still has a glob import (CLAUDE.md violation)

The PR correctly replaces use anyhow::*; in http_client.rs, but utils.rs still has it at line 1:

// utils.rs line 1 — unchanged
use anyhow::*;

This should be updated to explicit imports (e.g., use anyhow::{bail, Result};) to match project style.

---

Missing unit tests for the quorum calculation logic

The PR fixes a correctness bug in consensus quorum sizing — this is exactly the kind of logic that should have explicit unit tests. Without them, it's very hard to verify the edge-case behavior or catch regressions. At a minimum, the following should be tested:

• calculate_quorum for n = 0, 1, 2, 3, 5 for each QuorumType
• calculate_fanout_quorum for the same inputs
• That calculate_fanout_quorum(n, q) == calculate_quorum(n, q) - 1 holds for n >= 3 (at least for non-Any types)

The relationship between calculate_quorum and calculate_fanout_quorum is also currently implicit. A test explicitly asserting fanout_quorum == quorum - 1 for the general case would make the intent self-documenting.

---

calculate_fanout_quorum n=2 ignores QuorumType

The n=2 arm hardcodes to 1 regardless of q:

2 => 1,

For QuorumType::Any with n=2, calculate_quorum(2, Any) = 1, so fanout = 0 (sender alone satisfies quorum). Returning 1 instead means we'll wait for the other node unnecessarily. Whether this matters in practice depends on whether Any is ever used with n=2, but a comment explaining why q is ignored for this case would help.

---

warn\! for zero fanout target will fire on every operation in single-node clusters

if target_responses == 0 {
    tracing::warn\!("no fanout, target is 0");
    return Ok(Vec::new());
}

calculate_fanout_quorum(1, _) always returns 0, so any single-node cluster will emit a warn on every fanout call. If single-node deployments are a supported configuration, this will be very noisy. Consider tracing::debug\! for this path, or better yet, prevent callers from invoking fanout at all when n=1 and document that as a precondition.

---

Quorum formula for even N — worth a comment or test

The comment explains that even N gives no additional fault tolerance (N=4 acts like N=3), but the resulting quorum sizes are noticeably smaller than a traditional majority. For example:

N	f	Slow quorum (code)	Traditional majority
4	1	2	3
6	2	3	4

If EPaxos section 4.3 justifies this for even N clusters, a brief inline comment with that citation (beyond just "See EPaxos 4.3") would make the reasoning more accessible. A table-driven test would also make the expected values auditable.

Minor

• The new debug log tracing::debug\!(?target_responses, len=?responses.len(), "fanout target") drops quorum_type from the call site. It's still captured in the parent span's fields so this isn't a loss in practice, but the spacing around len=? doesn't match the project's tracing conventions elsewhere (compare to len = ?responses.len() in the old code).

Summary

The directional change is correct and the logic refactoring is an improvement. Main asks before merging:

1. Fix the glob import in utils.rs
2. Add unit tests for both quorum functions covering edge cases
3. Address the warn\! log level for single-node fanout

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commit: 5973cf6