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Published 38 days ago • verified publisheropenneom.dev
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Circuit Breaker, Rate Limiter, Logger & Snapshot Auditor for Flutter. Protects against excessive API calls, infinite rebuild loops, and cascading failures.

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sint_sentinel #

SINT Sentinel

pub package License: MIT

Circuit Breaker, Rate Limiter, Logger & Snapshot Auditor for Flutter.

Born from a real incident where a single Flutter widget generated 165,000 Firestore reads in 4 hours due to a StreamBuilder inside build(). That bug cost real money and could have bankrupted the project. sint_sentinel makes sure it never happens again.

// Before (vulnerable): one infinite loop = unlimited billing
FirebaseFirestore.instance.collection('users').get();

// After (protected): one infinite loop = 3,000 reads max, then auto-block
SintSentinel.guard(() => FirebaseFirestore.instance.collection('users').get());

Why sint_sentinel? #

Most backend circuit breakers protect servers. sint_sentinel protects the client — your Flutter app, your Firebase budget, and your users' experience.

Threat Without Sentinel With Sentinel
StreamBuilder in build() 1.1M reads/day (real incident) 3,000 reads, then auto-block
Infinite setState loop UI frozen, app killed by OS Loop throttled in <1 second
Runaway polling timer Thousands of API calls/min 60/min cap, circuit trips
Developer pushes bad code Unlimited cost until hotfix Instant local protection

Features #

  • Application Logger — Centralized Logger accessible everywhere via SintSentinel.logger. Supports all log levels: d(), i(), w(), e(), t(), f(). Re-exports Logger, Level, PrettyPrinter so consumers need only one import.
  • Circuit Breaker — Three-state machine (CLOSED / OPEN / HALF_OPEN) that blocks traffic when quotas are exceeded or failures accumulate.
  • Traffic Watchdog — Sliding 60-second rate limiter + daily read/write quotas. Catches runaway Firestore listeners, accidental polling, bot abuse.
  • State Watchdog — Monitors widget rebuild frequency per widget ID. Detects and throttles infinite setState loops before they freeze the UI.
  • Rebuild Monitor — Drop-in State mixin with three-tier response: silently drop excess rebuilds, warn in console, trip circuit if sustained.
  • Snapshot Auditor — Persistent history of circuit state snapshots. Auto-captured on every trip. Queryable by consumers (AI, analytics, debugging).
  • Maintenance Page — Full-screen UI shown automatically when the circuit trips. User can retry; circuit recovers with exponential backoff.
  • Hive Persistence — Everything survives app restarts: circuit state, counters, traffic history, snapshot ring buffer.
  • Zero Config — Works out of the box with SentinelConfig.production(). Three presets for dev/prod/strict.

Quick Start #

1. Install #

dependencies:
  sint_sentinel: ^1.1.0

2. Wrap your app #

import 'package:sint_sentinel/sint_sentinel.dart';

void main() {
  runApp(
    SentinelApp(
      config: SentinelConfig.production(),
      child: MyApp(),
    ),
  );
}

3. Guard your calls #

// Read operation (counts against per-minute rate + daily read quota)
final doc = await SintSentinel.guard(
  () => FirebaseFirestore.instance.doc('users/$uid').get(),
  tag: 'User.fetch',
);

// Write operation (counts against daily write quota)
await SintSentinel.guard(
  () => FirebaseFirestore.instance.doc('logs/$id').set(data),
  isWrite: true,
  tag: 'Log.write',
);

// With fallback — returns cached data instead of throwing when blocked
final items = await SintSentinel.guard(
  () => api.fetchItems(),
  fallback: () => localCache.getItems(),
  tag: 'Items.fetch',
);

4. Protect widgets (optional) #

class _MyPageState extends State<MyPage> with SentinelRebuildMonitor {
  @override
  Widget build(BuildContext context) {
    return Text('Safe from infinite rebuilds');
  }
}

When a widget exceeds 30 rebuilds/second:

  1. Excess setState calls are dropped silently (UI stays responsive).
  2. A console warning fires once: SintSentinel: Cyclic rebuild detected in MyPage.
  3. If abuse continues (90+ throttled calls), the circuit trips and a snapshot is captured automatically.

Logger #

sint_sentinel provides a centralized application-level logger via SintSentinel.logger. One import, one logger, everywhere in your app.

Basic Usage #

import 'package:sint_sentinel/sint_sentinel.dart';

// All log levels available
SintSentinel.logger.t('Trace message');     // verbose/trace
SintSentinel.logger.d('Debug details');     // debug
SintSentinel.logger.i('App started');       // info
SintSentinel.logger.w('Slow response');     // warning
SintSentinel.logger.e('Request failed');    // error
SintSentinel.logger.f('Fatal crash');       // fatal/wtf

Re-exported Types #

sint_sentinel re-exports core types from package:logger, so you don't need a separate dependency:

// These are all available from sint_sentinel directly:
import 'package:sint_sentinel/sint_sentinel.dart';
// Logger, Level, PrettyPrinter, LogPrinter, DateTimeFormat

Custom Logger #

Replace the default logger with your own configuration:

SintSentinel.configureLogger(
  Logger(
    printer: PrettyPrinter(
      methodCount: 0,
      errorMethodCount: 5,
      lineLength: 80,
      colors: true,
      printEmojis: true,
      dateTimeFormat: DateTimeFormat.onlyTimeAndSinceStart,
    ),
    level: Level.debug,
  ),
);

Automatic Guard Logging #

SintSentinel.guard() automatically logs every operation:

// This single call:
await SintSentinel.guard(() => api.fetchUser(), tag: 'User.fetch');

// Produces logs like:
// 🛡️ [guard] User.fetch → OK (circuit: closed)
// 🛡️ [guard] User.fetch → BLOCKED (circuit: open)
// 🛡️ [guard] User.fetch → FAILED: TimeoutException (failures: 3/5)

Pattern: Service-Level Logger #

For services that need frequent logging, use a static getter:

class MyApiClient {
  static Logger get _log => SintSentinel.logger;

  Future<void> fetchData() async {
    _log.d('Fetching data from /api/data');
    final response = await SintSentinel.guard(
      () => http.get(Uri.parse('/api/data')),
      tag: 'MyApi.fetchData',
    );
    _log.i('Data fetched: ${response.statusCode}');
  }
}

Snapshots #

Every time the circuit trips, sint_sentinel captures a SentinelSnapshot — a frozen picture of everything that was happening at that moment. Snapshots are persisted in Hive and available to any consumer.

// Capture a snapshot on demand
final snapshot = await SintSentinel.capture();
print(snapshot.securityStatus);     // "CLEAN" or "ALERT_open"
print(snapshot.requestsLastMinute); // 58
print(snapshot.dailyReads);         // 2847
print(snapshot.rebuildOffenders);   // ["MyBrokenWidget"]

// Query history
final history = await SintSentinel.snapshotHistory;     // Last 50 snapshots
final latest  = await SintSentinel.latestSnapshot;       // Most recent
final recent  = await SintSentinel.recentSnapshots(      // Last hour
  Duration(hours: 1),
);

Use cases:

  • AI integration — Feed snapshots to your AI agent so it understands app health and can reason about what happened.
  • Crash forensics — After a user reports an issue, check what the circuit was doing at the time.
  • Analytics — Track trip frequency, most common block reasons, which widgets cause loops.

Each snapshot contains:

SentinelSnapshot(
  timestamp: DateTime,
  circuitState: 'closed' | 'open' | 'halfOpen',
  blockReason: 'Rate limit exceeded: 61/60 req/min',
  consecutiveFailures: 3,
  tripCount: 2,
  requestsLastMinute: 61,
  dailyReads: 2450,
  dailyWrites: 89,
  minuteUsagePercent: 101.6,
  dailyReadUsagePercent: 81.6,
  rebuildOffenders: ['StreamBuilderWidget'],
)

How It Works #

Circuit Breaker State Machine #

                  quota exceeded
                  or N failures
CLOSED ─────────────────────────────> OPEN ──> [snapshot captured]
  ^                                     |
  |   test requests succeed             | openDuration expires
  |                                     v
  <──────────────────────────────── HALF_OPEN
           test fails ──────────────> OPEN ──> [snapshot captured]
  • CLOSED: All requests pass through. Failures and traffic are counted.
  • OPEN: All requests blocked immediately. Maintenance page shown. Snapshot captured. Timer counts down.
  • HALF_OPEN: Limited test requests allowed. If they succeed, circuit recovers. If they fail, circuit trips again.

Exponential Backoff #

Each consecutive trip increases the blocking duration:

Trip Duration
1st 30s
2nd 60s
3rd 120s
4th 240s
5th+ 300s (cap)

A full reset (SintSentinel.reset()) clears the trip counter.

Dual Watchdog System #

Traffic Watchdog monitors outbound requests:

  • Sliding 60-second window (rate limit)
  • Daily read counter (auto-resets every 24h)
  • Daily write counter (separate quota)
  • When exceeded: "Rate limit exceeded: 61/60 req/min" or "Daily read quota exceeded: 3001/3000"

State Watchdog monitors inbound rebuilds:

  • Per-widget rebuild timestamps (pruned to last 1 second)
  • When threshold exceeded, setState calls are dropped
  • Throttle count accumulated; trips circuit when multiplier reached

Configuration #

Three presets cover most use cases:

SentinelConfig.development()  // Relaxed — 120 req/min, 20k daily, no backoff
SentinelConfig.production()   // Balanced — 60 req/min, 3k daily, backoff on
SentinelConfig.strict()       // Aggressive — 30 req/min, 1.5k daily

Full Configuration #

SentinelConfig(
  maxRequestsPerMinute: 100,
  maxDailyRequests: 5000,
  maxDailyWrites: 1000,
  maxRebuildsPerSecond: 40,
  rebuildTripMultiplier: 3,       // 40 * 3 = 120 throttled calls to trip
  failureThreshold: 5,
  openDuration: Duration(seconds: 30),
  halfOpenTestRequests: 3,
  enableExponentialBackoff: true,
)
Parameter Dev Prod Strict What it controls
maxRequestsPerMinute 120 60 30 Sliding window rate limit
maxDailyRequests 20,000 3,000 1,500 Daily read quota
maxDailyWrites 5,000 500 200 Daily write quota
maxRebuildsPerSecond 60 30 15 Widget rebuild threshold
failureThreshold 10 5 3 Consecutive failures to trip
openDuration 10s 30s 60s Base blocking duration
enableExponentialBackoff off on on Double duration each trip

Error Handling #

try {
  final result = await SintSentinel.guard(() => riskyCall());
} on SentinelBlockedException catch (e) {
  // Circuit is OPEN — show cached data or retry later
  showSnackbar('Service paused: ${e.reason}');
} catch (e) {
  // The guarded operation itself threw
  // (failure recorded, circuit may trip if threshold reached)
  showSnackbar('Error: $e');
}

Real-Time Monitoring #

// Full status map
final status = await SintSentinel.getStatus();
// {
//   "circuitState": "closed",
//   "blockReason": "",
//   "tripCount": 0,
//   "traffic": {
//     "requestsLastMinute": 23,
//     "dailyReads": 456,
//     "dailyWrites": 12,
//     "minuteUsagePercent": 38.3,
//     "isHealthy": true
//   },
//   "rebuildOffenders": []
// }

// Quick checks
if (SintSentinel.isBlocking) {
  showMaintenanceBanner();
}

// React to state changes in UI (GetX Rx)
Obx(() {
  final state = SintSentinel.circuit!.state.value;
  return Icon(
    state == SentinelCircuitState.closed ? Icons.shield : Icons.warning,
  );
});

Architecture #

                        ┌──────────────────┐
                        |    SentinelApp    |  Root widget
                        |  (shows maintenance|  when OPEN)
                        └────────┬─────────┘
                                 |
                        ┌────────v─────────┐
                        |   SintSentinel   |  Static API
                        |    .guard()      |  .capture()
                        |    .getStatus()  |  .snapshotHistory
                        └────────┬─────────┘
                                 |
                   ┌─────────────v──────────────┐
                   |     SentinelCircuit         |
                   |  CLOSED / OPEN / HALF_OPEN  |
                   |  Rx<State> for reactive UI  |
                   └──┬──────────┬──────────┬───┘
                      |          |          |
              ┌───────v──┐ ┌────v─────┐ ┌──v──────────┐
              | Traffic   | | State    | | Snapshot    |
              | Watchdog  | | Watchdog | | Store       |
              | (req/min  | | (rebuild | | (Hive ring  |
              |  daily)   | |  /sec)   | |  buffer x50)|
              └───────────┘ └──────────┘ └─────────────┘
                      |          |          |
                 ┌────v──────────v──────────v────┐
                 |       Hive Persistence        |
                 |  sint_sentinel (state/traffic) |
                 |  sint_sentinel_snapshots (log) |
                 └───────────────────────────────┘

Performance #

Benchmark results (release mode, Apple M1):

Operation Overhead Impact
SintSentinel.guard() per call 50-300 us Negligible
TrafficWatchdog.recordRequest() 10-50 us Minimal
StateWatchdog.recordRebuild() 1-5 us Negligible
Cold init from Hive 5-20 ms One-time
SentinelApp per-frame (CLOSED) 0 us Zero cost
Snapshot capture ~1 ms Only on trips

The sentinel adds zero overhead per frame when the circuit is closed (normal operation). The Obx listener in SentinelApp only triggers a rebuild when the circuit state actually changes — a rare event.

The Incident That Started It All #

On February 18, 2026, a Samsung S25 running the Open Neom app generated 165,000 Firestore document reads in under 4 hours while sitting idle on a desk. The cause: a StreamBuilder initialized inside build(), creating an infinite reconnection loop at ~700 requests/minute.

The total monthly bill had already reached 15 million reads in 30 days, with a single-day spike of 1.1 million reads. This wasn't a cyberattack — it was a coding mistake that simulated one.

sint_sentinel was built so that this can never happen again. Not in our apps, and not in yours.

Requirements #

Platform Support #

Platform Supported
Android Yes
iOS Yes
Web Yes
macOS Yes
Windows Yes
Linux Yes
WASM Not yet

WASM Compatibility Roadmap #

sint_sentinel currently uses hive_flutter for persistence, which depends on dart:io and is not WASM-compatible. A future version will migrate to shared_preferences to achieve full WASM support.

The migration path is straightforward — sint_sentinel only persists simple key-value data (counters, timestamps, JSON-serialized snapshots). The public API (SintSentinel.guard(), .capture(), etc.) will remain unchanged.

Storage need Current (Hive) Future (shared_preferences)
dailyReads: 450 Box.put(key, 450) prefs.setInt(key, 450)
timestamps: [1,2,3] Box.put(key, list) prefs.setString(key, jsonEncode(list))
snapshots: [...] Box.put(key, list) prefs.setString(key, jsonEncode(list))

If WASM support is critical for your project today, you can implement the migration yourself by replacing SentinelHiveController with a SharedPreferences-backed equivalent and passing it to SentinelCircuit.

License #

MIT. See LICENSE.

  • SINT Framework — The state management framework sint_sentinel extends.
  • Open Neom — The ecosystem where sint_sentinel was born and battle-tested.
  • Changelog

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Metadata

Circuit Breaker, Rate Limiter, Logger & Snapshot Auditor for Flutter. Protects against excessive API calls, infinite rebuild loops, and cascading failures.

Repository (GitHub)

Topics

#circuit-breaker #rate-limiter #state-management #firebase #resilience

License

MIT (license)

Dependencies

flutter, hive_flutter, logger, sint

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