# SPEC-010: Advanced Retry and DLQ System

## Overview
Comprehensive retry mechanism and dead letter queue management system for robust message processing with configurable retry patterns and error handling.

## Components

### Retry Pattern Implementation
- **DLX + TTL Pattern**: Dead Letter Exchange with Time-To-Live for delayed retries
- **Exponential Backoff**: Configurable retry delays (5s → 15s → 60s → DLQ)
- **Retry Attempts**: Configurable maximum retry attempts per message
- **Retry Headers**: Automatic retry metadata in message headers (x-retry-count)

### Dynamic Retry Control
- **Per-Message Control**: MCP client decides retry behavior for each message
- **Immediate Retry**: Retry message immediately
- **Delayed Retry**: Retry after specified delay (N seconds)
- **DLQ Routing**: Send directly to dead letter queue
- **Custom Delays**: Configurable retry delays per message

### Dead Letter Queue Management
- **DLQ Operations**: List, requeue, purge, replay DLQ messages
- **Message Inspection**: View DLQ message content and retry history
- **Bulk Operations**: Process multiple DLQ messages efficiently
- **DLQ Statistics**: Track DLQ depth, age, and retry patterns

### Retry Configuration
- **Queue-Level Config**: Configure retry behavior per queue
- **Global Defaults**: Default retry settings for all queues
- **Runtime Updates**: Modify retry configuration without restart
- **Retry Monitoring**: Track retry success rates and patterns

### Internal Operations (AMQP Protocol - Manually Implemented)
**CRITICAL**: DLQ and retry operations use AMQP protocol features (DLX, TTL), NOT in OpenAPI specification.

- `dlq.configure-retry`: Set up DLX and TTL retry mechanism for queue (via AMQP)
- `dlq.move-to-dlq`: Send message directly to DLQ (via AMQP nack without requeue)
- `dlq.list-messages`: List messages in DLQ (via RabbitMQ Management API - OpenAPI)
- `dlq.requeue-message`: Move message back to main queue (via AMQP publish)
- `dlq.purge`: Clear all messages from DLQ (via RabbitMQ Management API - OpenAPI)
- `message.retry`: Retry message with custom delay (via AMQP + TTL queues)

**Implementation Note**: Combine AMQP protocol operations with HTTP API operations. Index all in vector database for semantic discovery.

## Technical Requirements

### Retry Mechanism
- Support for 1-10 retry attempts (configurable)
- Retry delays: 5s, 15s, 60s, 300s, 1800s (configurable)
- Automatic DLX and TTL queue creation
- Retry metadata preservation in message headers

### DLQ Management
- Support for multiple DLQ strategies
- Message age tracking and reporting
- Bulk requeue operations (up to 1000 messages)
- DLQ statistics and monitoring

### Performance
- Retry processing latency < 50ms
- DLQ operations complete within 2 seconds
- Support for 10,000+ messages in DLQ
- Efficient retry queue management

### Error Handling
- Graceful handling of retry failures
- DLQ overflow protection
- Retry loop detection and prevention
- Comprehensive error logging

## Acceptance Criteria

### Functional Requirements
- [ ] Retry mechanism works with DLX + TTL pattern
- [ ] Exponential backoff retry delays function correctly
- [ ] Per-message retry control is available
- [ ] DLQ operations (list, requeue, purge) work properly
- [ ] Retry metadata is preserved in message headers
- [ ] Retry configuration can be updated at runtime

### Performance Requirements
- [ ] Retry processing latency under 50ms
- [ ] DLQ operations complete within 2 seconds
- [ ] Supports 10,000+ messages in DLQ efficiently
- [ ] No memory leaks during extended retry operations

### Reliability Requirements
- [ ] No message loss during retry operations
- [ ] Retry loop detection prevents infinite retries
- [ ] DLQ overflow is handled gracefully
- [ ] Retry statistics are accurate and reliable

### Configuration Requirements
- [ ] Retry attempts and delays are configurable
- [ ] Queue-level and global retry settings work
- [ ] Runtime configuration updates function properly
- [ ] Retry monitoring provides useful insights

## Dependencies
- pika for RabbitMQ DLX/TTL support
- asyncio for concurrent retry processing
- structlog for retry operation logging

## Implementation Notes
- Use RabbitMQ DLX and TTL features for retry queues
- Implement retry queue naming conventions
- Track retry attempts in message headers
- Use async processing for retry operations
- Implement retry statistics and monitoring
- Set up DLQ cleanup and maintenance procedures
- Create retry configuration management system