Pulse/docs/PHASE2_SUMMARY.md

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 ✅

• Metrics pipeline operational
• Scheduler produces valid task plans
• Queue respects adaptive intervals
• Circuit breakers prevent runaway failures
• Documentation enables ops team rollout
• 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