{
  "schema_version": "1.3.0",
  "slug": "beyond-what-concurrency-thresholds-must-an-instance-serialize-state-access-to-prevent-check-then-act-races-and-lost-updates",
  "title": "Beyond what concurrency thresholds must an instance serialize state access to prevent check-then-act races and lost updates?",
  "date_of_record": "2026 October 20",
  "scope": "* **Applies when:** Reading and mutating shared state whenever two or more processes, threads, HTTP handlers, queue workers, or async coroutines (`await` between read and write) can execute concurrently (`concurrency_degree >= 2`). * **Does",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/beyond-what-concurrency-thresholds-must-an-instance-serialize-state-access-to-prevent-check-then-act-races-and-lost-updates.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/beyond-what-concurrency-thresholds-must-an-instance-serialize-state-access-to-prevent-check-then-act-races-and-lost-updates.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/beyond-what-concurrency-thresholds-must-an-instance-serialize-state-access-to-prevent-check-then-act-races-and-lost-updates.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 2435,
    "python_slice_tokens": 409,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "In any environment where two or more processes, threads, async coroutines, or agent instances can touch the same state (`concurrent_actors >= 2`), splitting a state transition into a separate **Read/Check step** (`val = get(k); if val > 0:`) followed milliseconds later by a **Write/Act step** (`set(k, val - 1)`) creates a classic **Time-of-Check to Time-of-U...",
    "what_was_observed": "When autonomous instances synthesize state-mutation code for web handlers, async tasks, or multi-agent workflows without serializing access at `concurrent_writers >= 2`, four primary race failure modes occur: 1. **Classic Lost Update on Concurrent JSON/Row Edits:** Worker A and Worker B both read `document_v1` at `t=0`. Worker A appends tag `\"urgent\"` and writes back at `t=10ms`; Worker B appends tag `\"reviewed\"` to ...",
    "condition_of_validity": "* **Applies when:** Reading and mutating shared state whenever two or more processes, threads, HTTP handlers, queue workers, or async coroutines (`await` between read and write) can execute concurrently (`concurrency_degree >= 2`). * **Does not apply when:** Mutating strictly thread-local variables with zero `await` yields and zero cross-process sharing (`co...",
    "decision_boundary": "1. **Is there any network call, database round-trip, or `await` statement between the line where you check a value and the line where you update it?** If so, you have a check-then-act race condition that must be serialized with a CAS version check, a database lock, or an atomic `WHERE` predicate. 2. **Did you check `cursor.rowcount == 1` after running a cond...",
    "operational_effect": "1. **Total Elimination of Lost Updates:** Concurrent writers cannot overwrite each other's changes without triggering an immediate CAS version conflict. 2. **Immunity to Quota & Balance TOCTOU Exploits:** Precondition checks execute atomically inside the storage engine at the exact instant of mutation. 3. **Safe Asyncio & Multi-Agent Concurrency:** Interleav..."
  },
  "matched_failure_symptoms": [
    "LOST_UPDATE_OR_RACE_CONDITION"
  ],
  "composition_edges": {
    "primary_domain": "concurrency_locking_and_shared_resources",
    "upstream_precondition_residuals": [
      "https://corpusfetch.com/c/beneath-what-concurrency-and-backpressure-ceilings-must-an-instance-throttle-fan-out-tasks-to-prevent-file-descriptor-and-connection-pool-starvation.md",
      "https://corpusfetch.com/c/across-what-mandatory-tenant-predicate-scoping-and-row-level-security-gates-must-an-instance-isolate-multi-tenant-data-to-prevent-cross-tenant-data-leaks.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/across-what-readiness-unregistration-and-in-flight-drain-gates-must-an-instance-sequence-sigterm-shutdown-to-prevent-dropped-requests-and-truncated-writes.md",
      "https://corpusfetch.com/c/before-what-pre-flight-and-dry-run-gates-must-an-instance-stage-bulk-or-irreversible-mutations-to-prevent-unrecoverable-blast-radius-cascade.md"
    ],
    "co_occurring_sibling_residuals": [
      "https://corpusfetch.com/c/across-what-bounded-queue-admission-control-and-load-shedding-gates-must-an-instance-manage-concurrency-to-prevent-unbounded-memory-growth-and-tail-latency-collapse.md",
      "https://corpusfetch.com/c/across-what-concurrent-pipe-drain-and-ring-buffer-boundaries-must-an-instance-consume-subprocess-stdout-and-stderr-to-prevent-os-pipe-deadlocks-and-unbounded-output-oom.md"
    ]
  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "8dd89c03c49411f3eb7d3f9735c3b7c4d191acc2052e1eec8ae81f43e8ec138c",
    "verified_classes": [
      "ConcurrentLostUpdateError",
      "VersionedStateRecord",
      "SerializedStateMutator"
    ],
    "verified_functions": [
      "__init__",
      "put_initial",
      "read_snapshot",
      "atomic_cas_mutate"
    ],
    "verified_functions_count": 4,
    "defensive_guards_count": 6,
    "raised_exceptions": [
      "ConcurrentLostUpdateError",
      "KeyError",
      "ValueError"
    ],
    "sample_fail_closed_invariants": [
      "expected_version must be >= 1 for CAS serialization."
    ],
    "stdlib_dependencies": [
      "dataclasses",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}