{"abstract":"The fastest replica returns data older than a version this session has already seen.","category":"Distributed coordination","checks":7,"contract":"Replicas are [ID,applied version,latency]. Return [ID,version] for the minimum (latency,ID) among versions >= session floor, or None if no replica qualifies. Version order is globally comparable in this model.","contract_signature":"floor, replicas","evaluation_group":"model-d101a1b7b33da9b3","failed_approach":"Always choosing the newest replica preserves monotonicity but violates the routing contract when a faster eligible replica exists.","family":"dist-monotonic-read-routing","id":"FA-071","implementations":{"attempt":{"sha256":"26bb8d3f828f51faea4aca780a5c84cfd998a1fc11fee198633b48a1aa9149c2","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(floor, replicas):\n    eligible = [r for r in replicas if r[1] >= floor]\n    return list(min(eligible, key=lambda r: (-r[1], r[2], r[0]))[:2]) if eligible else None\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nv = N+10\ncheck('stale fast replica', solve(v, [['a', v-1, 1], ['b', v, 10]]), ['b', v])\ncheck('fast enough versus newer', solve(v, [['a', v, N], ['b', v+N, 20]]), ['a', v])\ncheck('all replicas lag', solve(v, [['a', v-1, N]]), None)\ncheck('exact floor accepted', solve(v, [['a', v, N]]), ['a', v])\ncheck('stable latency tie', solve(v, [['b', v+1, N], ['a', v, N]]), ['a', v])\ncheck('empty discovery result', solve(v, []), None)\ncheck('zero-version session', solve(0, [['a', 0, N]]), ['a', 0])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"},"broken":{"sha256":"cff22615b81cbe823a002b4885533646aa728cb5894e26dde77af25e7b12bae3","source":"\"\"\"Failure Map reference implementation. Python standard library only.\"\"\"\nimport json\n\nN = 1\nobservations = []\ndef solve(floor, replicas):\n    eligible = replicas\n    return list(min(eligible, key=lambda r: (r[2], r[0]))[:2]) if eligible else None\ndef check(label, actual, expected):\n    observations.append({\"check\": label, \"actual\": actual, \"expected\": expected, \"passed\": actual == expected})\nv = N+10\ncheck('stale fast replica', solve(v, [['a', v-1, 1], ['b', v, 10]]), ['b', v])\ncheck('fast enough versus newer', solve(v, [['a', v, N], ['b', v+N, 20]]), ['a', v])\ncheck('all replicas lag', solve(v, [['a', v-1, N]]), None)\ncheck('exact floor accepted', solve(v, [['a', v, N]]), ['a', v])\ncheck('stable latency tie', solve(v, [['b', v+1, N], ['a', v, N]]), ['a', v])\ncheck('empty discovery result', solve(v, []), None)\ncheck('zero-version session', solve(0, [['a', 0, N]]), ['a', 0])\nprint(json.dumps({\"observations\": observations, \"passed\": all(x[\"passed\"] for x in observations)}, ensure_ascii=False))\nraise SystemExit(0 if all(x[\"passed\"] for x in observations) else 1)\n"}},"limitations":" 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.","method":"Deterministic executable model with adversarial boundary fixtures.","provenance":{"created_by":"Failure Map","dependencies":"Python standard library","family":"dist-monotonic-read-routing","generated_at":"2026-09-29T14:36:50.411210+00:00","license":"CC0-1.0","python":"3.12.14","seed":1,"split":"open-access"},"relevance":"Models session-consistent replica routing after a client moves between read endpoints, including the distinction between a required consistency floor and unnecessarily seeking the newest state.","root_cause":"Replica selection ignores the session's minimum acceptable applied version.","sha256":"67ca306cf9dadc511c0a577456c5f1f33bb37195272a17584d284a569cb7615f","title":"Failover routes a session behind its observed version · case 01","variant":1,"variant_policy":"Five numbered records share a model and may reuse boundary fixtures.","verified":true,"visibility":"public","verification":{"attempt":{"elapsed_ms":34.819,"exit_code":1,"observations":[{"actual":["b",11],"check":"stale fast replica","expected":["b",11],"passed":true},{"actual":["b",12],"check":"fast enough versus newer","expected":["a",11],"passed":false},{"actual":null,"check":"all replicas lag","expected":null,"passed":true},{"actual":["a",11],"check":"exact floor accepted","expected":["a",11],"passed":true},{"actual":["b",12],"check":"stable latency tie","expected":["a",11],"passed":false},{"actual":null,"check":"empty discovery result","expected":null,"passed":true},{"actual":["a",0],"check":"zero-version session","expected":["a",0],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"stale fast replica\", \"actual\": [\"b\", 11], \"expected\": [\"b\", 11], \"passed\": true}, {\"check\": \"fast enough versus newer\", \"actual\": [\"b\", 12], \"expected\": [\"a\", 11], \"passed\": false}, {\"check\": \"all replicas lag\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"exact floor accepted\", \"actual\": [\"a\", 11], \"expected\": [\"a\", 11], \"passed\": true}, {\"check\": \"stable latency tie\", \"actual\": [\"b\", 12], \"expected\": [\"a\", 11], \"passed\": false}, {\"check\": \"empty discovery result\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"zero-version session\", \"actual\": [\"a\", 0], \"expected\": [\"a\", 0], \"passed\": true}], \"passed\": false}\n"},"broken":{"elapsed_ms":33.55,"exit_code":1,"observations":[{"actual":["a",10],"check":"stale fast replica","expected":["b",11],"passed":false},{"actual":["a",11],"check":"fast enough versus newer","expected":["a",11],"passed":true},{"actual":["a",10],"check":"all replicas lag","expected":null,"passed":false},{"actual":["a",11],"check":"exact floor accepted","expected":["a",11],"passed":true},{"actual":["a",11],"check":"stable latency tie","expected":["a",11],"passed":true},{"actual":null,"check":"empty discovery result","expected":null,"passed":true},{"actual":["a",0],"check":"zero-version session","expected":["a",0],"passed":true}],"passed":false,"stderr":"","stdout":"{\"observations\": [{\"check\": \"stale fast replica\", \"actual\": [\"a\", 10], \"expected\": [\"b\", 11], \"passed\": false}, {\"check\": \"fast enough versus newer\", \"actual\": [\"a\", 11], \"expected\": [\"a\", 11], \"passed\": true}, {\"check\": \"all replicas lag\", \"actual\": [\"a\", 10], \"expected\": null, \"passed\": false}, {\"check\": \"exact floor accepted\", \"actual\": [\"a\", 11], \"expected\": [\"a\", 11], \"passed\": true}, {\"check\": \"stable latency tie\", \"actual\": [\"a\", 11], \"expected\": [\"a\", 11], \"passed\": true}, {\"check\": \"empty discovery result\", \"actual\": null, \"expected\": null, \"passed\": true}, {\"check\": \"zero-version session\", \"actual\": [\"a\", 0], \"expected\": [\"a\", 0], \"passed\": true}], \"passed\": false}\n"}},"member_only":{"stages":["fixed"],"fields":["implementations.fixed","verification.fixed","harness","repair"],"note":"The verified repair, its recorded checks, the repair description, and the scoring harness are available to members."}}