FA-456 / Consensus / Open access

Serve a linearizable read at a confirmed index: A read uses a response set lacking a voting majority · case 01

The read index operation is admitted even though a read uses a response set lacking a voting majority.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The admission path omits the quorum confirmation invariant while validating the other operation preconditions.

THE FAILURE

The admission path omits the quorum confirmation invariant while validating the other operation preconditions.

Unsuccessful approach: Adding the quorum confirmation check repairs the reported defect, but replacing the adjacent applied floor check loses that independent invariant.

Case contract

Return a Boolean admission decision for serve a linearizable read at a confirmed index. The record r must satisfy all of: r['current_term_commit'][0] == r['current_term_commit'][1]; len(set(r['quorum_confirmation'][0])) * 2 > r['quorum_confirmation'][1]; r['applied_floor'][0] >= r['applied_floor'][1]; r['context_binding'][0] == r['context_binding'][1]; r['term_stability'][0] == r['term_stability'][1]. Extra tracing fields are ignored; validation does not mutate the record.

Why this case matters

A deterministic local contract for consensus. Each negative fixture violates exactly one invariant. No transport timing, persistence, cryptographic verification, or full protocol implementation is claimed.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(r):
    return (r['current_term_commit'][0] == r['current_term_commit'][1]) and (r['applied_floor'][0] >= r['applied_floor'][1]) and (r['context_binding'][0] == r['context_binding'][1]) and (r['term_stability'][0] == r['term_stability'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'current_term_commit': [5, 5], 'quorum_confirmation': [[1, 2], 3], 'applied_floor': [9, 9], 'context_binding': ['r2', 'r2'], 'term_stability': [5, 5]}
check('valid operation', solve(r), True)
check('A leader reads before committing an entry in its term', solve(dict(r, **{'current_term_commit': [4, 5]})), False)
check('A read uses a response set lacking a voting majority', solve(dict(r, **{'quorum_confirmation': [[1], 3]})), False)
check('The state machine has not applied the confirmed read index', solve(dict(r, **{'applied_floor': [8, 9]})), False)
check('A delayed response confirms another read request', solve(dict(r, **{'context_binding': ['r1', 'r2']})), False)
check('A read completes after the leader observed a higher term', solve(dict(r, **{'term_stability': [5, 6]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'current_term_commit': [4, 5], 'quorum_confirmation': [[1], 3], 'applied_floor': [8, 9], 'context_binding': ['r1', 'r2'], 'term_stability': [5, 6]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
valid operation	`True`	`True`	Passed
A leader reads before committing an entry in its term	`False`	`False`	Passed
A read uses a response set lacking a voting majority	`True`	`False`	Failed
The state machine has not applied the confirmed read index	`False`	`False`	Passed
A delayed response confirms another read request	`False`	`False`	Passed
A read completes after the leader observed a higher term	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / 1193f1f7ebf330625052aeddd663d875fa75a92419973c500a17d629fce4c0af

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(r):
    return (r['current_term_commit'][0] == r['current_term_commit'][1]) and (len(set(r['quorum_confirmation'][0])) * 2 > r['quorum_confirmation'][1]) and (r['context_binding'][0] == r['context_binding'][1]) and (r['term_stability'][0] == r['term_stability'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'current_term_commit': [5, 5], 'quorum_confirmation': [[1, 2], 3], 'applied_floor': [9, 9], 'context_binding': ['r2', 'r2'], 'term_stability': [5, 5]}
check('valid operation', solve(r), True)
check('A leader reads before committing an entry in its term', solve(dict(r, **{'current_term_commit': [4, 5]})), False)
check('A read uses a response set lacking a voting majority', solve(dict(r, **{'quorum_confirmation': [[1], 3]})), False)
check('The state machine has not applied the confirmed read index', solve(dict(r, **{'applied_floor': [8, 9]})), False)
check('A delayed response confirms another read request', solve(dict(r, **{'context_binding': ['r1', 'r2']})), False)
check('A read completes after the leader observed a higher term', solve(dict(r, **{'term_stability': [5, 6]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'current_term_commit': [4, 5], 'quorum_confirmation': [[1], 3], 'applied_floor': [8, 9], 'context_binding': ['r1', 'r2'], 'term_stability': [5, 6]}
keys = list(invalid)
pair = {keys[N % len(keys)]: invalid[keys[N % len(keys)]], keys[(N+1) % len(keys)]: invalid[keys[(N+1) % len(keys)]]}
check('two independent violations in variant', solve(dict(r, **pair)), False)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
valid operation	`True`	`True`	Passed
A leader reads before committing an entry in its term	`False`	`False`	Passed
A read uses a response set lacking a voting majority	`False`	`False`	Passed
The state machine has not applied the confirmed read index	`True`	`False`	Failed
A delayed response confirms another read request	`False`	`False`	Passed
A read completes after the leader observed a higher term	`False`	`False`	Passed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / 6a5f4b551c63d66b89f695c8a49d5fb602c329438d2fb90782c68894ebe6aa9d

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

Verification & scope

This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:36:53.186742+00:00.

Case digest / fcb1ae561b035fa0b6b101aa263050bae08556257046e6ffda7a4307b787fd1d