# Pulse Adaptive Polling – Phase 2 Summary

## Executive Summary
Phase 2 delivers adaptive polling infrastructure that dynamically adjusts monitoring intervals based on instance freshness, error rates, and system load. The scheduler replaces fixed cadences with intelligent priority-based execution, dramatically improving resource efficiency while maintaining data freshness.

## Completed Tasks (9/10 - 90%)

### ✅ Task 1: Poll Cycle Metrics
- 7 new Prometheus metrics (duration, staleness, queue depth, in-flight, errors)
- Per-instance tracking with histogram/counter/gauge types
- Integrated into all poll functions (PVE/PBS/PMG)
- Metrics server on port 9091

### ✅ Task 2: Adaptive Scheduler Scaffold
- Pluggable interfaces (StalenessSource, IntervalSelector, TaskEnqueuer)
- BuildPlan generates ordered task lists with NextRun times
- FilterDue/DispatchDue for queue management
- Default no-op implementations for gradual rollout

### ✅ Task 3: Configuration & Feature Flags
- `ADAPTIVE_POLLING_ENABLED` feature flag (default: false)
- Min/max/base interval tuning (5s / 5m / 10s defaults)
- Environment variable overrides
- Persisted in system.json
- Validation logic (min ≤ base ≤ max)

### ✅ Task 4: Staleness Tracker
- Per-instance freshness metadata (last success/error/mutation)
- SHA1 change hash detection
- Normalized staleness scoring (0-1 scale)
- Integration with PollMetrics for authoritative timestamps
- Updates from all poll result handlers

### ✅ Task 5: Adaptive Interval Logic
- EMA smoothing (alpha=0.6) to prevent oscillations
- Staleness-based interpolation (min-max range)
- Error penalty (0.6x per failure) for faster recovery detection
- Queue depth stretch (0.1x per task) for backpressure
- ±5% jitter to avoid thundering herd

### ✅ Task 6: Priority Queue Execution
- Min-heap (container/heap) ordered by NextRun + Priority
- Worker goroutines with WaitNext() blocking
- Tasks only execute when due (respects adaptive intervals)
- Automatic rescheduling after execution
- Upsert semantics prevent duplicates

### ✅ Task 7: Error Handling & Circuit Breakers
- Circuit breaker with closed/open/half-open states
- Trips after 3 consecutive failures
- Exponential backoff (5s initial, 2x multiplier, 5min max)
- ±20% jitter on retry delays
- Dead-letter queue after 5 transient failures
- Error classification (transient vs permanent)

### ✅ Task 8: API Surfaces
- GET /api/monitoring/scheduler/health endpoint (auth required)
- Scheduler health snapshot (queue, dead-letter, breakers, staleness)
- Snapshot methods: StalenessTracker.Snapshot(), TaskQueue.Snapshot()
- Circuit breaker state export via circuitBreaker.State()
- Dead-letter queue inspection (top 25 tasks with error details)

### ✅ Task 9: Unit Testing (Partial)
- Backoff logic tests (13 test cases): exponential growth, jitter, capping
- Circuit breaker tests (10 test cases): state machine, retry intervals, half-open window
- Staleness tracker tests (17 test cases): scoring, normalization, snapshot API
- **Total: 40+ unit tests** covering core scheduling math
- ✅ All tests passing (`go test ./internal/monitoring`)

### ✅ Task 10: Documentation
- Architecture guide in `docs/monitoring/ADAPTIVE_POLLING.md`
- API endpoint specification with example responses
- Configuration reference
- Metrics catalog
- Operational guidance & troubleshooting
- Rollout plan (dev → staged → full)

## Deferred Tasks (1/10 - 10%)

### ⏭ Task 9b: Integration & Soak Testing (Future Phase)
- Integration tests with mock PVE/PBS clients
- Soak tests for queue stability over extended runtime
- Regression suite for Phase 1 hardening features

## Key Metrics Delivered

| Metric | Purpose |
|--------|---------|
| `pulse_monitor_poll_total` | Success/error rate tracking |
| `pulse_monitor_poll_duration_seconds` | Latency per instance |
| `pulse_monitor_poll_staleness_seconds` | Data freshness indicator |
| `pulse_monitor_poll_queue_depth` | Backpressure monitoring |
| `pulse_monitor_poll_inflight` | Concurrency tracking |
| `pulse_monitor_poll_errors_total` | Error type classification |
| `pulse_monitor_poll_last_success_timestamp` | Recovery timeline |

## Technical Achievements

**Performance:**
- Adaptive intervals reduce unnecessary polls on idle instances
- Queue-based execution prevents task pile-up
- Circuit breakers stop hot loops on failing endpoints

**Reliability:**
- Exponential backoff with jitter prevents thundering herd
- Dead-letter queue isolates persistent failures
- Transient error retry logic (5 attempts before DLQ)

**Observability:**
- 7 Prometheus metrics for complete visibility
- Structured logging for all state transitions
- Tamper-evident audit trail (from Phase 1)

## Deployment Status

**Current State:** Feature flag disabled by default (`ADAPTIVE_POLLING_ENABLED=false`)

**Activation Path:**
1. Enable flag in dev/QA environment
2. Observe metrics for 24-48 hours
3. Staged rollout to subset of production clusters
4. Full activation after validation

**Rollback:** Set `ADAPTIVE_POLLING_ENABLED=false` to revert to fixed intervals

## Git Commits

1. `c048e7b9b` - Tasks 1-3: Metrics + scheduler + config
2. `8ce93c1df` - Task 4: Staleness tracker
3. `e8bd79c6c` - Task 5: Adaptive interval logic
4. `1d6fa9188` - Task 6: Priority queue execution
5. `7d9aaa406` - Task 7: Circuit breakers & backoff
6. `fdd2bebe4` - Task 10: Documentation
7. `1b969d008` - Task 8: Scheduler health API endpoint
8. `938a8153f` - Task 9a: Backoff & circuit breaker tests
9. `6d3786041` - Task 9b: Staleness tracker tests

## Phase 2 Success Criteria ✅

- [x] Metrics pipeline operational
- [x] Scheduler produces valid task plans
- [x] Queue respects adaptive intervals
- [x] Circuit breakers prevent runaway failures
- [x] Documentation enables ops team rollout
- [x] Feature flag allows safe activation

## Known Limitations

- Dead-letter queue state lost on restart (no persistence yet)
- No integration/soak test coverage (deferred from Task 9)
- Queue depth metric updated per-cycle (not real-time within cycle)
- API endpoint read-only (no circuit breaker reset or DLQ management)

## Next Steps

**Immediate (Post-Phase 2):**
- Deploy to dev environment with flag enabled
- Configure Grafana dashboards for new metrics
- Set alerting thresholds (queue depth >50, staleness >60s)

**Future Phases:**
- Task 9b: Integration tests with mock clients + soak testing
- Phase 3: External sentinels and cross-cluster coordination
- API enhancements: Write endpoints for circuit breaker reset and DLQ management