FA-716 / Replication / Open access

Acknowledge a replicated write concern: Replica acknowledgements refer to another write · case 01

The write concern operation is admitted even though replica acknowledgements refer to another write.

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

ROOT CAUSE

The admission path omits the write binding invariant while validating the other operation preconditions.

VERIFIED REPAIR

Require all(x == r['write_binding'][0] for x in r['write_binding'][1]) together with every other stated precondition before accepting the operation.

Unsuccessful approach: Adding the write binding check repairs the reported defect, but replacing the adjacent ack deadline check loses that independent invariant.

Case contract

Return a Boolean admission decision for acknowledge a replicated write concern. The record r must satisfy all of: len(set(r['distinct_replicas'][0])) >= r['distinct_replicas'][1]; sum(r['durable_acks'][0]) >= r['durable_acks'][1]; len(set(r['failure_domains'][0])) >= r['failure_domains'][1]; all(x == r['write_binding'][0] for x in r['write_binding'][1]); r['ack_deadline'][0] < r['ack_deadline'][1]. Extra tracing fields are ignored; validation does not mutate the record.

Why this case matters

A deterministic local contract for replication. 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 (len(set(r['distinct_replicas'][0])) >= r['distinct_replicas'][1]) and (sum(r['durable_acks'][0]) >= r['durable_acks'][1]) and (len(set(r['failure_domains'][0])) >= r['failure_domains'][1]) and (r['ack_deadline'][0] < r['ack_deadline'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'distinct_replicas': [['a', 'b'], 2], 'durable_acks': [[True, True, False], 2], 'failure_domains': [['z1', 'z2'], 2], 'write_binding': ['w2', ['w2', 'w2']], 'ack_deadline': [9, 10]}
check('valid operation', solve(r), True)
check('Repeated acknowledgements count as separate replicas', solve(dict(r, **{'distinct_replicas': [['a', 'a'], 2]})), False)
check('Volatile receipt acknowledgements satisfy durable concern', solve(dict(r, **{'durable_acks': [[True, False, False], 2]})), False)
check('Multiple copies in one failure domain satisfy zone concern', solve(dict(r, **{'failure_domains': [['z1', 'z1'], 2]})), False)
check('Replica acknowledgements refer to another write', solve(dict(r, **{'write_binding': ['w2', ['w1', 'w2']]})), False)
check('A write is reported successful after its response deadline', solve(dict(r, **{'ack_deadline': [10, 10]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'distinct_replicas': [['a', 'a'], 2], 'durable_acks': [[True, False, False], 2], 'failure_domains': [['z1', 'z1'], 2], 'write_binding': ['w2', ['w1', 'w2']], 'ack_deadline': [10, 10]}
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
Repeated acknowledgements count as separate replicas	`False`	`False`	Passed
Volatile receipt acknowledgements satisfy durable concern	`False`	`False`	Passed
Multiple copies in one failure domain satisfy zone concern	`False`	`False`	Passed
Replica acknowledgements refer to another write	`True`	`False`	Failed
A write is reported successful after its response deadline	`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 / ff86455c1df88840192766cd1648cf63cdf30527b0a32292a5b26bb8e962aad8

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(r):
    return (len(set(r['distinct_replicas'][0])) >= r['distinct_replicas'][1]) and (sum(r['durable_acks'][0]) >= r['durable_acks'][1]) and (len(set(r['failure_domains'][0])) >= r['failure_domains'][1]) and (all(x == r['write_binding'][0] for x in r['write_binding'][1]))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'distinct_replicas': [['a', 'b'], 2], 'durable_acks': [[True, True, False], 2], 'failure_domains': [['z1', 'z2'], 2], 'write_binding': ['w2', ['w2', 'w2']], 'ack_deadline': [9, 10]}
check('valid operation', solve(r), True)
check('Repeated acknowledgements count as separate replicas', solve(dict(r, **{'distinct_replicas': [['a', 'a'], 2]})), False)
check('Volatile receipt acknowledgements satisfy durable concern', solve(dict(r, **{'durable_acks': [[True, False, False], 2]})), False)
check('Multiple copies in one failure domain satisfy zone concern', solve(dict(r, **{'failure_domains': [['z1', 'z1'], 2]})), False)
check('Replica acknowledgements refer to another write', solve(dict(r, **{'write_binding': ['w2', ['w1', 'w2']]})), False)
check('A write is reported successful after its response deadline', solve(dict(r, **{'ack_deadline': [10, 10]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'distinct_replicas': [['a', 'a'], 2], 'durable_acks': [[True, False, False], 2], 'failure_domains': [['z1', 'z1'], 2], 'write_binding': ['w2', ['w1', 'w2']], 'ack_deadline': [10, 10]}
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
Repeated acknowledgements count as separate replicas	`False`	`False`	Passed
Volatile receipt acknowledgements satisfy durable concern	`False`	`False`	Passed
Multiple copies in one failure domain satisfy zone concern	`False`	`False`	Passed
Replica acknowledgements refer to another write	`False`	`False`	Passed
A write is reported successful after its response deadline	`True`	`False`	Failed
unrelated tracing metadata	`True`	`True`	Passed
repeat validation is pure	`True`	`True`	Passed
two independent violations in variant	`False`	`False`	Passed

SHA-256 / ae6c980e568814019de56069de85c91e2c175755d05ac919fad0291139e62cf5

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(r):
    return (len(set(r['distinct_replicas'][0])) >= r['distinct_replicas'][1]) and (sum(r['durable_acks'][0]) >= r['durable_acks'][1]) and (len(set(r['failure_domains'][0])) >= r['failure_domains'][1]) and (all(x == r['write_binding'][0] for x in r['write_binding'][1])) and (r['ack_deadline'][0] < r['ack_deadline'][1])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
r = {'distinct_replicas': [['a', 'b'], 2], 'durable_acks': [[True, True, False], 2], 'failure_domains': [['z1', 'z2'], 2], 'write_binding': ['w2', ['w2', 'w2']], 'ack_deadline': [9, 10]}
check('valid operation', solve(r), True)
check('Repeated acknowledgements count as separate replicas', solve(dict(r, **{'distinct_replicas': [['a', 'a'], 2]})), False)
check('Volatile receipt acknowledgements satisfy durable concern', solve(dict(r, **{'durable_acks': [[True, False, False], 2]})), False)
check('Multiple copies in one failure domain satisfy zone concern', solve(dict(r, **{'failure_domains': [['z1', 'z1'], 2]})), False)
check('Replica acknowledgements refer to another write', solve(dict(r, **{'write_binding': ['w2', ['w1', 'w2']]})), False)
check('A write is reported successful after its response deadline', solve(dict(r, **{'ack_deadline': [10, 10]})), False)
check('unrelated tracing metadata', solve(dict(r, trace='run-'+str(N))), True)
check('repeat validation is pure', solve(r), True)
invalid = {'distinct_replicas': [['a', 'a'], 2], 'durable_acks': [[True, False, False], 2], 'failure_domains': [['z1', 'z1'], 2], 'write_binding': ['w2', ['w1', 'w2']], 'ack_deadline': [10, 10]}
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
Repeated acknowledgements count as separate replicas	`False`	`False`	Passed
Volatile receipt acknowledgements satisfy durable concern	`False`	`False`	Passed
Multiple copies in one failure domain satisfy zone concern	`False`	`False`	Passed
Replica acknowledgements refer to another write	`False`	`False`	Passed
A write is reported successful after its response deadline	`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 / db9f09117c9c355156490a5d2d48c5add06f61edc801b1cbadee59668a6c1532

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 2026-09-29T14:36:55.548154+00:00.

Case digest / c063d86f46906cc74ce75739d868f755119c40e33f96ca0b57c983f772ce7ff7