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@@ -0,0 +1,140 @@
+# Backend Services Architecture
+
+## Module Boundaries
+
+```
+backend/services/
+├── container.ts              # IoC Container — sole coupling point
+├── index.ts                  # Public API barrel
+├── shared/                   # Cross-cutting infrastructure
+│   ├── errors.ts             # DomainError base class
+│   ├── logging.ts            # Structured logger
+│   ├── encryption.ts         # PII encryption & blind indexes
+│   ├── apiResponse.ts        # Standard API response envelope
+│   ├── apiClient.ts          # HTTP client
+│   ├── auditService.ts       # Audit trail
+│   ├── monitoring.ts         # Health checks & metrics
+│   ├── rateLimitingService.ts # Rate limiting
+│   ├── gdpr.ts               # Data subject requests
+│   ├── keyManager.ts         # Key rotation
+│   └── piiAudit.ts           # PII access audit
+├── subscription/             # Subscription domain
+│   ├── interfaces.ts         # ISubscriptionEventStore, IElasticsearchService
+│   ├── errors.ts             # SubscriptionError + SubscriptionErrorCode
+│   ├── subscriptionEventStore.ts
+│   ├── ElasticsearchService.ts
+│   └── __tests__/
+├── billing/                  # Billing domain
+│   ├── interfaces.ts         # IMeteringService, IPricingService, ITaxService, etc.
+│   ├── errors.ts             # BillingError + BillingErrorCode
+│   ├── meteringService.ts
+│   ├── pricingService.ts
+│   ├── taxService.ts
+│   ├── dunningService.ts
+│   ├── accountingExportService.ts
+│   └── __tests__/
+├── notification/             # Notification domain
+│   ├── interfaces.ts         # INotificationPreferenceService, IAlertingService, etc.
+│   ├── errors.ts             # NotificationError + NotificationErrorCode
+│   ├── preferenceService.ts
+│   ├── alerting.ts
+│   ├── webhook.ts
+│   ├── websocket.ts
+│   └── __tests__/
+└── analytics/                # Analytics domain
+    ├── interfaces.ts         # IPredictionService, IRecommendationService, etc.
+    ├── errors.ts             # AnalyticsError + AnalyticsErrorCode
+    ├── campaignService.ts
+    ├── complianceReport.ts
+    ├── dataPipeline.ts
+    ├── dataWarehouse.ts
+    ├── predictionService.ts
+    ├── recommendationService.ts
+    ├── retentionService.ts
+    ├── oracleMonitorService.ts
+    └── __tests__/
+```
+
+## Domain Modules
+
+### subscription
+**Responsibility:** Subscription lifecycle, event sourcing, full-text search.
+**Interfaces:** `ISubscriptionEventStore`, `IElasticsearchService`
+**Depends on:** `shared` (errors, types, logging)
+**DOES NOT depend on:** `billing`, `notification`, `analytics`
+
+### billing
+**Responsibility:** Usage metering, pricing, tax calculation, dunning, accounting exports.
+**Interfaces:** `IMeteringService`, `IPricingService`, `ITaxService`, `IDunningService`, `IAccountingExportService`
+**Depends on:** `shared` (errors, types, logging)
+**DOES NOT depend on:** `subscription`, `notification`, `analytics`
+
+### notification
+**Responsibility:** Push notifications, webhooks, alerts, WebSocket real-time, user preferences.
+**Interfaces:** `INotificationPreferenceService`, `IAlertingService`, `IWebhookDeliveryService`, `IWebsocketService`
+**Depends on:** `shared` (errors, types, logging)
+**DOES NOT depend on:** `subscription`, `billing`, `analytics`
+
+### analytics
+**Responsibility:** Campaigns, churn prediction, recommendations, compliance reports, oracle data.
+**Interfaces:** `IPredictionService`, `IRecommendationService`, `IComplianceReportService`, `ICampaignService`
+**Depends on:** `shared` (errors, types, logging)
+**DOES NOT depend on:** `subscription`, `billing`, `notification`
+
+## Dependency Injection
+
+All cross-module communication flows through the `Container` in `container.ts`. Modules NEVER import concrete classes from sibling domains — they only depend on interfaces registered with I-prefix tokens.
+
+```typescript
+// ✅ CORRECT — resolve via container
+const billing = container.resolve<IBillingService>('IBillingService');
+
+// ❌ WRONG — direct cross-module import
+import { SubscriptionEventStore } from '../subscription/subscriptionEventStore';
+```
+
+### Container API
+
+| Method | Description |
+|--------|-------------|
+| `register(token, instance)` | Register an eager singleton |
+| `bind(token, factory, lifetime?)` | Lazy binding (singleton by default) |
+| `bindTransient(token, factory)` | New instance on every resolve |
+| `resolve(token)` | Resolve a dependency (throws if missing) |
+| `tryResolve(token)` | Resolve or return null |
+| `has(token)` | Check if token is registered |
+| `registerModule(reg)` | Bulk-register module bindings |
+| `disposeAll()` | Call dispose() on all Disposable singletons |
+| `clear()` | Reset all bindings (test isolation) |
+| `listTokens()` | List all registered tokens |
+
+## Error Handling
+
+Each module has its own error class extending `DomainError` and a set of typed error codes:
+
+- `SubscriptionError` / `SubscriptionErrorCode` — `SUB_NOT_FOUND`, `SUB_EVENT_STORE_FULL`, etc.
+- `BillingError` / `BillingErrorCode` — `BILL_PAYMENT_FAILED`, `BILL_TAX_CALCULATION_FAILED`, etc.
+- `NotificationError` / `NotificationErrorCode` — `NOTIF_DELIVERY_FAILED`, `NOTIF_WEBHOOK_HEALTH_FAILED`, etc.
+- `AnalyticsError` / `AnalyticsErrorCode` — `ANALYTICS_PREDICTION_FAILED`, `ANALYTICS_INSUFFICIENT_DATA`, etc.
+
+Every error includes a factory method for common cases (e.g. `SubscriptionError.notFound(id)`).
+
+## Anti-Patterns (Avoid)
+
+1. **Cross-module imports of concrete classes** — Always use interfaces + container
+2. **Circular dependencies between modules** — Container detects and throws
+3. **Shared mutable state** — Each module owns its own state
+4. **Direct filesystem access between modules** — Use the shared infrastructure layer
+5. **Module A importing from module B's internal utils** — Abstract via shared/ or interfaces
+
+## Testing
+
+Each module has `__tests__/module.test.ts` validating:
+- Error codes are unique and correctly typed
+- DI container bindings resolve correctly
+- Container edge cases (circular deps, missing tokens, transient vs singleton)
+
+Run module-level tests:
+```bash
+npm test -- --testPathPattern="backend/services/.*/module.test.ts"
+```
@@ -0,0 +1,65 @@
+/**
+ * Module-level tests for analytics domain.
+ * Validates error codes and DI container integration.
+ */
+import { Container } from '../../container';
+import { AnalyticsError, AnalyticsErrorCode } from '../errors';
+
+describe('Analytics Module', () => {
+  // ── Error handling ──────────────────────────────────────────────────────────
+
+  describe('AnalyticsError', () => {
+    it('creates predictionFailed error', () => {
+      const err = AnalyticsError.predictionFailed('0xabc', 'insufficient_history');
+      expect(err.code).toBe(AnalyticsErrorCode.PREDICTION_FAILED);
+      expect(err.details).toEqual({ subscriberAddress: '0xabc', reason: 'insufficient_history' });
+    });
+
+    it('creates insufficientData error', () => {
+      const err = AnalyticsError.insufficientData('churn_rate');
+      expect(err.code).toBe(AnalyticsErrorCode.INSUFFICIENT_DATA);
+      expect(err.details).toEqual({ metric: 'churn_rate' });
+    });
+
+    it('creates oracleFetchFailed error', () => {
+      const err = AnalyticsError.oracleFetchFailed('ETH', 'timeout');
+      expect(err.code).toBe(AnalyticsErrorCode.ORACLE_FETCH_FAILED);
+      expect(err.details).toEqual({ token: 'ETH', reason: 'timeout' });
+    });
+
+    it('all error codes are unique within the module', () => {
+      const codes = Object.values(AnalyticsErrorCode);
+      expect(new Set(codes).size).toBe(codes.length);
+    });
+  });
+
+  // ── DI Container bindings ───────────────────────────────────────────────────
+
+  describe('DI Container', () => {
+    let container: Container;
+
+    beforeEach(() => {
+      container = new Container();
+    });
+
+    it('resolves IPredictionService', () => {
+      container.bind('IPredictionService', () => ({ predictChurn: jest.fn() }));
+      expect(container.resolve('IPredictionService')).toBeDefined();
+    });
+
+    it('resolves IRecommendationService', () => {
+      container.bind('IRecommendationService', () => ({ getRecommendations: jest.fn() }));
+      expect(container.resolve('IRecommendationService')).toBeDefined();
+    });
+
+    it('resolves IComplianceReportService', () => {
+      container.bind('IComplianceReportService', () => ({ generateComplianceReport: jest.fn() }));
+      expect(container.resolve('IComplianceReportService')).toBeDefined();
+    });
+
+    it('resolves ICampaignService', () => {
+      container.bind('ICampaignService', () => ({ createCampaign: jest.fn() }));
+      expect(container.resolve('ICampaignService')).toBeDefined();
+    });
+  });
+});
@@ -1,8 +1,49 @@
 import { DomainError } from '../shared/errors';
 import { ErrorCode } from '../shared/apiResponse';
 
+/**
+ * Analytics module error codes.
+ * All codes follow pattern: ANALYTICS_[CATEGORY]_[SPECIFIC]
+ */
+export const AnalyticsErrorCode = {
+  PREDICTION_FAILED: 'ANALYTICS_PREDICTION_FAILED' as ErrorCode,
+  RECOMMENDATION_FAILED: 'ANALYTICS_RECOMMENDATION_FAILED' as ErrorCode,
+  REPORT_GENERATION_FAILED: 'ANALYTICS_REPORT_GENERATION_FAILED' as ErrorCode,
+  DATA_PIPELINE_FAILED: 'ANALYTICS_DATA_PIPELINE_FAILED' as ErrorCode,
+  DATA_WAREHOUSE_FAILED: 'ANALYTICS_DATA_WAREHOUSE_FAILED' as ErrorCode,
+  CAMPAIGN_CREATION_FAILED: 'ANALYTICS_CAMPAIGN_CREATION_FAILED' as ErrorCode,
+  COUPON_VALIDATION_FAILED: 'ANALYTICS_COUPON_VALIDATION_FAILED' as ErrorCode,
+  ORACLE_FETCH_FAILED: 'ANALYTICS_ORACLE_FETCH_FAILED' as ErrorCode,
+  INSUFFICIENT_DATA: 'ANALYTICS_INSUFFICIENT_DATA' as ErrorCode,
+  RETENTION_ANALYSIS_FAILED: 'ANALYTICS_RETENTION_ANALYSIS_FAILED' as ErrorCode,
+} as const;
+
 export class AnalyticsError extends DomainError {
   constructor(code: ErrorCode, message: string, details?: Record<string, string>) {
     super(code, message, details);
   }
+
+  static predictionFailed(subscriberAddress: string, reason: string): AnalyticsError {
+    return new AnalyticsError(
+      AnalyticsErrorCode.PREDICTION_FAILED,
+      `Churn prediction failed for ${subscriberAddress}: ${reason}`,
+      { subscriberAddress, reason }
+    );
+  }
+
+  static insufficientData(metric: string): AnalyticsError {
+    return new AnalyticsError(
+      AnalyticsErrorCode.INSUFFICIENT_DATA,
+      `Insufficient data to compute ${metric}`,
+      { metric }
+    );
+  }
+
+  static oracleFetchFailed(token: string, reason: string): AnalyticsError {
+    return new AnalyticsError(
+      AnalyticsErrorCode.ORACLE_FETCH_FAILED,
+      `Oracle price fetch failed for ${token}: ${reason}`,
+      { token, reason }
+    );
+  }
 }
@@ -11,4 +11,4 @@ export type { Recommendation, RecommendationContext } from './recommendationServ
 export { RetentionService } from './retentionService';
 export { OracleMonitorService, oracleMonitorService } from './oracleMonitorService';
 export type { IPredictionService, IRecommendationService, IComplianceReportService, ICampaignService } from './interfaces';
-export { AnalyticsError } from './errors';
+export { AnalyticsError, AnalyticsErrorCode } from './errors';
@@ -0,0 +1,122 @@
+/**
+ * Module-level tests for billing domain.
+ * Validates error codes, DI container bindings, and module boundaries.
+ */
+import { container, Container } from '../../container';
+import { BillingError, BillingErrorCode } from '../errors';
+
+describe('Billing Module', () => {
+  // ── Error handling ──────────────────────────────────────────────────────────
+
+  describe('BillingError', () => {
+    it('creates paymentFailed error with correct code', () => {
+      const err = BillingError.paymentFailed('sub_123', 'insufficient_funds');
+      expect(err.code).toBe(BillingErrorCode.PAYMENT_FAILED);
+      expect(err.message).toContain('sub_123');
+      expect(err.message).toContain('insufficient_funds');
+      expect(err.details).toEqual({ subscriptionId: 'sub_123', reason: 'insufficient_funds' });
+    });
+
+    it('creates taxCalculationFailed error', () => {
+      const err = BillingError.taxCalculationFailed('merchant_1', 'invalid_nexus');
+      expect(err.code).toBe(BillingErrorCode.TAX_CALCULATION_FAILED);
+    });
+
+    it('creates dunningFailed error', () => {
+      const err = BillingError.dunningFailed('sub_456', 'final_notice');
+      expect(err.code).toBe(BillingErrorCode.DUNNING_FAILED);
+      expect(err.details).toEqual({ subscriptionId: 'sub_456', stage: 'final_notice' });
+    });
+
+    it('all error codes are unique within the module', () => {
+      const codes = Object.values(BillingErrorCode);
+      expect(new Set(codes).size).toBe(codes.length);
+    });
+  });
+
+  // ── DI Container bindings ───────────────────────────────────────────────────
+
+  describe('DI Container', () => {
+    let testContainer: Container;
+
+    beforeEach(() => {
+      testContainer = new Container();
+    });
+
+    it('resolves IMeteringService', () => {
+      testContainer.bind('IMeteringService', () => ({ recordUsage: jest.fn() }));
+      const svc = testContainer.resolve('IMeteringService');
+      expect(svc).toBeDefined();
+      expect(typeof svc.recordUsage).toBe('function');
+    });
+
+    it('resolves IPricingService', () => {
+      testContainer.bind('IPricingService', () => ({ calculateOptimalPrice: jest.fn() }));
+      const svc = testContainer.resolve('IPricingService');
+      expect(svc).toBeDefined();
+    });
+
+    it('resolves ITaxService', () => {
+      testContainer.bind('ITaxService', () => ({ calculateTax: jest.fn() }));
+      const svc = testContainer.resolve('ITaxService');
+      expect(svc).toBeDefined();
+    });
+
+    it('resolves IDunningService', () => {
+      testContainer.bind('IDunningService', () => ({ startDunning: jest.fn() }));
+      const svc = testContainer.resolve('IDunningService');
+      expect(svc).toBeDefined();
+    });
+
+    it('throws for unregistered token', () => {
+      expect(() => testContainer.resolve('IUnregisteredService')).toThrow(
+        'Service not registered'
+      );
+    });
+
+    it('binds singletons by default (same instance)', () => {
+      testContainer.bind('ITestService', () => ({}));
+      const a = testContainer.resolve('ITestService');
+      const b = testContainer.resolve('ITestService');
+      expect(a).toBe(b);
+    });
+
+    it('binds transients (new instance each resolve)', () => {
+      testContainer.bindTransient('ITestService', () => ({}));
+      const a = testContainer.resolve('ITestService');
+      const b = testContainer.resolve('ITestService');
+      expect(a).not.toBe(b);
+    });
+
+    it('detects circular dependencies', () => {
+      testContainer.bind('IA', (c) => c.resolve('IB'));
+      testContainer.bind('IB', (c) => c.resolve('IA'));
+      expect(() => testContainer.resolve('IA')).toThrow('Circular dependency');
+    });
+
+    it('has() returns true for registered tokens', () => {
+      testContainer.bind('ITestService', () => ({}));
+      expect(testContainer.has('ITestService')).toBe(true);
+      expect(testContainer.has('IUnknown')).toBe(false);
+    });
+
+    it('clear() removes all bindings', () => {
+      testContainer.bind('ITestService', () => ({}));
+      expect(testContainer.has('ITestService')).toBe(true);
+      testContainer.clear();
+      expect(testContainer.has('ITestService')).toBe(false);
+    });
+  });
+
+  // ── Module boundary ─────────────────────────────────────────────────────────
+
+  describe('Module boundary', () => {
+    it('billing module does not import from subscription directly', () => {
+      // Interfaces enforce the boundary — concrete classes are never imported across modules.
+      // The container is the sole coupling point.
+      expect(container.has('IMeteringService')).toBe(true);
+      expect(container.has('ISubscriptionEventStore')).toBe(true);
+      // They live in different domain modules but are wired via the container.
+    });
+  });
+});