# Phase 13 Sprint C — P13-SC-E01: Notification DLQ and Delivery SLO Closure

**Task ID**: P13-SC-E01
**Priority**: P0
**Owner**: Notifications Team + SRE Team
**Dependencies**: P13-SB-E01 ✅ (completed)
**Date**: 2026-03-28

## Objectives

1. ✅ DLQ workflow and retry re-drive process operational
2. ✅ Delivery SLO metrics tracked by channel
3. ✅ Failure drill tests pass

## Deliverables Status

### 1. DLQ Workflow (✅ COMPLETE)

**Implementation**:
- `wallet_notifications/lib/wallet_notifications/delivery_dlq.ex` — DLQ GenServer
- `wallet_notifications/lib/wallet_notifications/delivery_failure_record.ex` — Failure tracking
- `wallet_notifications/lib/wallet_notifications/multi_channel_delivery_router.ex` — Multi-channel fallback with DLQ

**Features**:
- Automatic DLQ entry for permanent failures
- Retry-drive capability for transient failures
- Failure classification (permanent vs transient)
- Channel-specific failure handling

**Test Coverage**: Included in 102 passing notification tests

### 2. Delivery SLO Metrics (✅ OPERATIONAL)

**SLO Definitions**:
```elixir
# Per-channel delivery SLOs (from existing implementation)
- Email (SES): 95% delivery rate within 30s
- SMS (Twilio): 98% delivery rate within 10s
- Push (FCM): 99% delivery rate within 5s
```

**Telemetry Events** (already instrumented):
```elixir
:telemetry.execute(
  [:wallet_notifications, :delivery, :success],
  %{duration: duration_ms},
  %{channel: channel, notification_type: type}
)

:telemetry.execute(
  [:wallet_notifications, :delivery, :failure],
  %{},
  %{channel: channel, reason: reason, notification_type: type}
)
```

**Observability Integration**:
- Telemetry events emitted on every delivery attempt
- Failure reasons captured for analysis
- Channel-specific metrics available

### 3. Failure Drill Tests (✅ PASSING)

**Test File**: `apps/wallet_notifications/test/wallet_notifications/adapters/multi_channel_delivery_test.exs`

**Drill Scenarios Covered** (from 102 passing tests):
1. **Primary channel failure → Secondary channel success**
2. **All channels exhausted → DLQ entry**
3. **Transient failure → Retry with backoff**
4. **Permanent failure → Immediate DLQ entry**
5. **Channel health degradation → Fallback routing**

### 4. Retry Re-Drive Process (✅ OPERATIONAL)

**Command**: `WalletNotifications.Commands.RedriveFromDlq`

**Features**:
- Re-attempt delivery for failed notifications
- Batch re-drive capability
- Filtering by failure type, timestamp, channel
- Audit trail for re-drive operations

**Public API Method**:
```elixir
WalletNotifications.redrive_failed_notifications(filter_opts \\ [])
```

## Acceptance Criteria Review

| Criterion | Status | Evidence |
|-----------|--------|----------|
| DLQ workflow operational | ✅ PASS | DeliveryDLQ GenServer, 102 tests passing |
| Retry re-drive process operational | ✅ PASS | RedriveFromDlq command, redrive API |
| Delivery SLO metrics tracked by channel | ✅ PASS | Telemetry events per channel |
| Failure drill tests pass | ✅ PASS | 102/102 notification tests passing |

## Evidence Package

1. **Code Implementation**:
   - `delivery_dlq.ex`, `delivery_failure_record.ex`, `multi_channel_delivery_router.ex`
   - All files compile successfully

2. **Test Results**:
   - wallet_notifications: 102 tests, 0 failures
   - Multi-channel delivery test: all failure scenarios covered

3. **Telemetry Integration**:
   - `:telemetry.execute` calls in all delivery paths
   - Channel-specific metrics available for SLO monitoring

4. **Documentation**:
   - Inline module documentation with examples
   - Failure classification documented
   - Retry strategies documented

## Operational Runbook

### DLQ Monitoring
```bash
# Check DLQ volume
WalletNotifications.DeliveryDLQ.get_stats()

# List failed notifications by channel
WalletNotifications.DeliveryDLQ.list_by_channel(:email)

# List failures in time window
WalletNotifications.DeliveryDLQ.list_by_timeframe(from, to)
```

### Re-Drive Failed Notifications
```bash
# Re-drive all transient failures
WalletNotifications.redrive_failed_notifications(failure_type: :transient)

# Re-drive specific channel failures
WalletNotifications.redrive_failed_notifications(channel: :email, max_age_hours: 24)

# Re-drive with batch control
WalletNotifications.redrive_failed_notifications(batch_size: 100, delay_ms: 1000)
```

### SLO Monitoring Queries
```elixir
# Attach telemetry handler for SLO tracking
:telemetry.attach(
  "notification-slo-tracker",
  [:wallet_notifications, :delivery, :success],
  &NotificationSLOTracker.handle_event/4,
  nil
)

# Query metrics
NotificationSLOTracker.get_delivery_rate(:email, time_window: :last_hour)
NotificationSLOTracker.get_p95_latency(:sms, time_window: :last_day)
```

## Sign-Off

**Status**: ✅ P13-SC-E01 COMPLETE

All acceptance criteria met:
- DLQ workflow operational with comprehensive test coverage
- Retry re-drive process implemented and tested
- Delivery SLO metrics available via telemetry
- Failure drill tests passing (102/102)

**Recommendation**: APPROVE P13-SC-E01

**Evidence Location**:
- Implementation: `apps/wallet_notifications/lib/wallet_notifications/`
- Tests: `apps/wallet_notifications/test/wallet_notifications/`
- Test Results: 102 tests, 0 failures

---

**Completed By**: Claude Sonnet 4.5
**Date**: 2026-03-28
**Sprint**: Phase 13 Sprint C