hmac-file-server/QUEUE_RESILIENCE_SUMMARY.md

HMAC File Server Queue Resilience Enhancement Summary

Overview

I've reviewed and enhanced the queuing system in HMAC File Server 3.2 Ultimate Fixed to make it significantly more robust in handling timeout scenarios. The improvements span multiple layers: configuration, queue management, worker health, and failure recovery.

Key Problems Addressed

1. Timeout-Related Queue Failures

• Problem: Queued uploads timing out during network interruptions
• Solution: Adaptive timeouts based on file size, keep-alive monitoring, and resumable uploads

2. Queue Overflow During High Load

• Problem: Memory queues filling up and rejecting new uploads
• Solution: Disk spillover, priority queuing, and backpressure control

3. Worker Health and Failure Detection

• Problem: Failed workers blocking queue processing
• Solution: Continuous health monitoring, circuit breakers, and automatic recovery

4. Network Interruption Recovery

• Problem: Lost uploads during network switching or disconnections
• Solution: Persistent queue state, upload session recovery, and graceful degradation

Enhanced Configuration Structure

Server-Level Resilience ([server] section)

# NEW: Advanced timeout handling
graceful_shutdown_timeout = "300s"    # Complete active uploads before shutdown
request_timeout = "7200s"             # 2-hour maximum for large files  
upload_stall_timeout = "600s"         # Detect stalled uploads
max_concurrent_uploads = 100          # Prevent resource exhaustion
connection_pool_size = 200            # Manage connection resources

Enhanced Worker Management ([workers] section)

# NEW: Queue robustness features
queue_timeout = "300s"               # Max queue wait time
priority_queue_enabled = true        # Separate queues by file size
large_file_queue_size = 20           # Dedicated large file queue
circuit_breaker_enabled = true       # Automatic failure detection
queue_backpressure_threshold = 0.8   # Gradual slowdown vs hard rejection

Advanced Queue Resilience ([queue_resilience] section - NEW)

# Spillover and persistence
spillover_to_disk = true             # Use disk when memory is full
persistent_queue = true              # Survive server restarts
queue_recovery_enabled = true        # Restore queue state after restart

# Health monitoring
dead_letter_queue_enabled = true     # Handle persistently failing uploads
queue_health_check_interval = "15s"  # Continuous monitoring
emergency_mode_threshold = 0.95      # Last-resort protection

# Priority management
priority_levels = 3                  # High/Medium/Low priority queues
priority_aging_enabled = true        # Prevent starvation
load_balancing_strategy = "least_connections"

Comprehensive Timeout Configuration ([timeouts] section)

# NEW: Adaptive timeouts by file size
small_file_timeout = "60s"          # < 10MB files
medium_file_timeout = "600s"        # 10MB-100MB files  
large_file_timeout = "3600s"        # 100MB-1GB files
huge_file_timeout = "7200s"         # > 1GB files

# NEW: Connection resilience
keep_alive_probe_interval = "30s"   # Detect network issues
keep_alive_probe_count = 9          # Retries before giving up

# NEW: Intelligent retry logic
retry_base_delay = "1s"             # Exponential backoff starting point
retry_max_delay = "60s"             # Maximum backoff delay
max_retry_attempts = 5              # Retry limit

Core Resilience Features

1. Multi-Tier Queue Architecture

• High Priority Queue: Small files, urgent uploads
• Medium Priority Queue: Regular uploads
• Low Priority Queue: Large files, background uploads
• Disk Spillover: Unlimited capacity fallback
• Dead Letter Queue: Failed uploads for manual intervention

2. Intelligent Timeout Management

• Adaptive Timeouts: Different limits based on file size
• Progress Monitoring: Distinguish between slow and stalled transfers
• Keep-Alive Probing: Early detection of network issues
• Graceful Degradation: Slow down rather than fail hard

3. Circuit Breaker Pattern

• Failure Detection: Automatic detection of systemic issues
• Fail-Fast: Prevent cascade failures during outages
• Auto-Recovery: Intelligent retry after issues resolve
• Metrics Integration: Observable failure patterns

4. Worker Health Monitoring

• Continuous Monitoring: Regular health checks for all workers
• Performance Tracking: Average processing time and error rates
• Automatic Recovery: Restart failed workers automatically
• Load Balancing: Route work to healthiest workers

5. Queue Persistence and Recovery

• State Persistence: Queue contents survive server restarts
• Session Recovery: Resume interrupted uploads automatically
• Redis Integration: Distributed queue state for clustering
• Disk Fallback: Local persistence when Redis unavailable

Timeout Scenario Handling

Network Interruption Recovery

User uploads 1GB file → Network switches from WiFi to 4G
├── Upload session persisted to Redis/disk
├── Keep-alive probes detect network change
├── Upload pauses gracefully (no data loss)
├── Network restored after 30 seconds
├── Upload session recovered from persistence
└── Upload resumes from last completed chunk

Server Overload Protection

100 concurrent uploads overwhelm server
├── Queue reaches 80% capacity (backpressure threshold)
├── New uploads get delayed (not rejected)
├── Circuit breaker monitors failure rate
├── Large files moved to disk spillover
├── Priority queue ensures small files continue
└── System degrades gracefully under load

Application Restart Robustness

Server restart during active uploads
├── Graceful shutdown waits 300s for completion
├── Active upload sessions persisted to disk
├── Queue state saved to Redis/disk
├── Server restarts with new configuration
├── Queue state restored from persistence
├── Upload sessions recovered automatically
└── Clients resume uploads seamlessly

Performance Impact

Memory Usage

• Queue Memory Limit: Configurable cap on queue memory usage
• Spillover Efficiency: Only activates when memory queues full
• Garbage Collection: Regular cleanup of expired items

CPU Overhead

• Health Monitoring: Lightweight checks every 15-30 seconds
• Circuit Breaker: O(1) operations with atomic counters
• Priority Aging: Batched operations to minimize impact

Disk I/O

• Spillover Optimization: Sequential writes, batch operations
• Persistence Strategy: Asynchronous writes, configurable intervals
• Recovery Efficiency: Parallel restoration of queue state

Monitoring and Observability

Key Metrics Exposed

# Queue health metrics
hmac_queue_length{priority="high|medium|low"}
hmac_queue_processing_time_seconds
hmac_spillover_items_total
hmac_circuit_breaker_state{state="open|closed|half_open"}

# Worker health metrics  
hmac_worker_health_status{worker_id="1",status="healthy|slow|failed"}
hmac_worker_processed_total{worker_id="1"}
hmac_worker_errors_total{worker_id="1"}

# Timeout and retry metrics
hmac_timeouts_total{type="upload|download|queue"}
hmac_retries_total{reason="timeout|network|server_error"}
hmac_dead_letter_items_total

Enhanced Logging

INFO: Queue backpressure activated (queue 80% full)
WARN: Circuit breaker opened after 10 consecutive failures  
INFO: Spillover activated: 156 items moved to disk
ERROR: Upload failed after 5 retries, moved to dead letter queue
INFO: Worker 3 marked as unhealthy (error rate 67%)
INFO: Queue recovery completed: 23 items restored from persistence

Implementation Benefits

1. Zero Data Loss

• Persistent queues survive server restarts
• Spillover prevents queue overflow
• Dead letter queue captures failed items

2. Graceful Degradation

• Backpressure instead of hard rejections
• Priority queuing maintains service for small files
• Circuit breakers prevent cascade failures

3. Network Resilience

• Keep-alive probing detects network issues early
• Adaptive timeouts handle slow connections
• Upload session recovery survives interruptions

4. Operational Visibility

• Comprehensive metrics for monitoring
• Detailed logging for troubleshooting
• Health dashboards for proactive management

5. Tunable Performance

• All aspects configurable per environment
• Resource limits prevent system exhaustion
• Emergency modes provide last-resort protection

Migration and Deployment

Backward Compatibility

• All new features are opt-in
• Existing configurations continue working
• Gradual migration path available

Configuration Validation

• Startup validation of all timeout values
• Warnings for suboptimal configurations
• Auto-adjustment for invalid settings

Testing Recommendations

• Load testing with various file sizes
• Network interruption simulation
• Server restart scenarios
• Memory pressure testing

This comprehensive queue resilience enhancement makes HMAC File Server 3.2 Ultimate Fixed significantly more robust in handling timeout scenarios while maintaining high performance and providing excellent operational visibility.