# Phase 3: Consumer Groups & Advanced Kafka Features

## Overview

Phase 3 transforms the Kafka Gateway from a basic producer/consumer system into a full-featured, production-ready Kafka-compatible platform with consumer groups, advanced APIs, and enterprise features.

## Goals

- **Consumer Group Coordination**: Full distributed consumer support
- **Advanced Kafka APIs**: Offset management, group coordination, heartbeats
- **Performance & Scalability**: Connection pooling, batching, compression
- **Production Features**: Metrics, monitoring, advanced configuration
- **Enterprise Ready**: Security, observability, operational tools

## Core Features

### 1. Consumer Group Coordination

**New Kafka APIs to Implement:**
- **JoinGroup** (API 11): Consumer joins a consumer group
- **SyncGroup** (API 14): Coordinate partition assignments 
- **Heartbeat** (API 12): Keep consumer alive in group
- **LeaveGroup** (API 13): Clean consumer departure
- **OffsetCommit** (API 8): Commit consumer offsets
- **OffsetFetch** (API 9): Retrieve committed offsets
- **DescribeGroups** (API 15): Get group metadata

**Consumer Group Manager:**
- Group membership tracking
- Partition assignment strategies (Range, RoundRobin)
- Rebalancing coordination
- Offset storage and retrieval
- Consumer liveness monitoring

### 2. Advanced Record Processing

**Record Batch Improvements:**
- Full Kafka record format parsing (v0, v1, v2)
- Compression support (gzip, snappy, lz4, zstd) — IMPLEMENTED
- Proper CRC validation — IMPLEMENTED
- Transaction markers handling
- Timestamp extraction and validation

**Performance Optimizations:**
- Record batching for SeaweedMQ
- Connection pooling to Agent
- Async publishing with acknowledgment batching
- Memory pooling for large messages

### 3. Enhanced Protocol Support

**Additional APIs:**
- **FindCoordinator** (API 10): Locate group coordinator
- **DescribeConfigs** (API 32): Get broker/topic configs
- **AlterConfigs** (API 33): Modify configurations
- **DescribeLogDirs** (API 35): Storage information
- **CreatePartitions** (API 37): Dynamic partition scaling

**Protocol Improvements:**
- Multiple API version support
- Better error code mapping
- Request/response correlation tracking
- Protocol version negotiation

### 4. Operational Features

**Metrics & Monitoring:**
- Prometheus metrics endpoint
- Consumer group lag monitoring
- Throughput and latency metrics
- Error rate tracking
- Connection pool metrics

**Health & Diagnostics:**
- Health check endpoints
- Debug APIs for troubleshooting
- Consumer group status reporting
- Partition assignment visualization

**Configuration Management:**
- Dynamic configuration updates
- Topic-level settings
- Consumer group policies
- Rate limiting and quotas

## Implementation Plan

### Step 1: Consumer Group Foundation (2-3 days)
1. Consumer group state management
2. Basic JoinGroup/SyncGroup APIs
3. Partition assignment logic
4. Group membership tracking

### Step 2: Offset Management (1-2 days)
1. OffsetCommit/OffsetFetch APIs
2. Offset storage in SeaweedMQ
3. Consumer position tracking
4. Offset retention policies

### Step 3: Consumer Coordination (1-2 days) 
1. Heartbeat mechanism
2. Group rebalancing
3. Consumer failure detection
4. LeaveGroup handling

### Step 4: Advanced Record Processing (2-3 days)
1. Full record parsing and real Fetch batch construction
2. Compression codecs and CRC (done) — focus on integration and tests
3. Performance optimizations
4. Memory management

### Step 5: Enhanced APIs (1-2 days)
1. FindCoordinator implementation
2. DescribeGroups functionality
3. Configuration APIs
4. Administrative tools

### Step 6: Production Features (2-3 days)
1. Metrics and monitoring
2. Health checks
3. Operational dashboards  
4. Performance tuning

## Architecture Changes

### Consumer Group Coordinator
```
┌─────────────────────────────────────────────────┐
│                Gateway Server                    │
├─────────────────────────────────────────────────┤
│  Protocol Handler                               │
│  ├── Consumer Group Coordinator                 │
│  │   ├── Group State Machine                    │
│  │   ├── Partition Assignment                   │
│  │   ├── Rebalancing Logic                     │
│  │   └── Offset Manager                        │
│  ├── Enhanced Record Processor                  │
│  └── Metrics Collector                         │
├─────────────────────────────────────────────────┤
│  SeaweedMQ Integration Layer                    │
│  ├── Connection Pool                            │
│  ├── Batch Publisher                           │
│  └── Offset Storage                            │
└─────────────────────────────────────────────────┘
```

### Consumer Group State Management
```
Consumer Group States:
- Empty: No active consumers
- PreparingRebalance: Waiting for consumers to join
- CompletingRebalance: Assigning partitions
- Stable: Normal operation
- Dead: Group marked for deletion

Consumer States:  
- Unknown: Initial state
- MemberPending: Joining group
- MemberStable: Active in group
- MemberLeaving: Graceful departure
```

## Success Criteria

### Functional Requirements
- ✅ Consumer groups work with multiple consumers
- ✅ Automatic partition rebalancing
- ✅ Offset commit/fetch functionality
- ✅ Consumer failure handling
- ✅ Full Kafka record format support (v2 with real records)
- ✅ Compression support for major codecs (already available)

### Performance Requirements
- ✅ Handle 10k+ messages/second per partition
- ✅ Support 100+ consumer groups simultaneously
- ✅ Sub-100ms consumer group rebalancing
- ✅ Memory usage < 1GB for 1000 consumers

### Compatibility Requirements
- ✅ Compatible with kafka-go, Sarama, and other Go clients
- ✅ Support Kafka 2.8+ client protocol versions
- ✅ Backwards compatible with Phase 1&2 implementations

## Testing Strategy

### Unit Tests
- Consumer group state transitions
- Partition assignment algorithms  
- Offset management logic
- Record parsing and validation

### Integration Tests
- Multi-consumer group scenarios
- Consumer failures and recovery
- Rebalancing under load
- SeaweedMQ storage integration

### End-to-End Tests
- Real Kafka client libraries (kafka-go, Sarama)
- Producer/consumer workflows
- Consumer group coordination
- Performance benchmarking

### Load Tests
- 1000+ concurrent consumers
- High-throughput scenarios  
- Memory and CPU profiling
- Failure recovery testing

## Deliverables

1. **Consumer Group Coordinator** - Full group management system
2. **Enhanced Protocol Handler** - 13+ Kafka APIs supported
3. **Advanced Record Processing** - Compression, batching, validation
4. **Metrics & Monitoring** - Prometheus integration, dashboards
5. **Performance Optimizations** - Connection pooling, memory management
6. **Comprehensive Testing** - Unit, integration, E2E, and load tests
7. **Documentation** - API docs, deployment guides, troubleshooting

## Risk Mitigation

### Technical Risks
- **Consumer group complexity**: Start with basic Range assignment, expand gradually
- **Performance bottlenecks**: Profile early, optimize incrementally  
- **SeaweedMQ integration**: Maintain compatibility layer for fallback

### Operational Risks
- **Breaking changes**: Maintain Phase 2 compatibility throughout
- **Resource usage**: Implement proper resource limits and monitoring
- **Data consistency**: Ensure offset storage reliability

## Post-Phase 3 Vision

After Phase 3, the SeaweedFS Kafka Gateway will be:

- **Production Ready**: Handle enterprise Kafka workloads
- **Highly Compatible**: Work with major Kafka client libraries
- **Operationally Excellent**: Full observability and management tools
- **Performant**: Meet enterprise throughput requirements
- **Reliable**: Handle failures gracefully with strong consistency guarantees

This positions SeaweedFS as a compelling alternative to traditional Kafka deployments, especially for organizations already using SeaweedFS for storage and wanting unified message queue capabilities.