# KVS - Distributed Key-Value Store

A minimalistic, clustered key-value database system written in Go that prioritizes **availability** and **partition tolerance** over strong consistency. KVS implements a gossip-style membership protocol with sophisticated conflict resolution for eventually consistent distributed storage.

## 🚀 Key Features

- **Hierarchical Keys**: Support for structured paths (e.g., `/home/room/closet/socks`)
- **Eventual Consistency**: Local operations are fast, replication happens in background
- **Gossip Protocol**: Decentralized node discovery and failure detection
- **Sophisticated Conflict Resolution**: Majority vote with oldest-node tie-breaking
- **Local-First Truth**: All operations work locally first, sync globally later
- **Read-Only Mode**: Configurable mode for reducing write load
- **Gradual Bootstrapping**: New nodes integrate smoothly without overwhelming cluster
- **Zero Dependencies**: Single binary with embedded BadgerDB storage

## 🏗️ Architecture

```
┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
│     Node A      │    │     Node B      │    │     Node C      │
│  (Go Service)   │    │  (Go Service)   │    │  (Go Service)   │
│                 │    │                 │    │                 │
│ ┌─────────────┐ │    │ ┌─────────────┐ │    │ ┌─────────────┐ │
│ │ HTTP Server │ │◄──►│ │ HTTP Server │ │◄──►│ │ HTTP Server │ │
│ │    (API)    │ │    │ │    (API)    │ │    │ │    (API)    │ │
│ └─────────────┘ │    │ └─────────────┘ │    │ └─────────────┘ │
│ ┌─────────────┐ │    │ ┌─────────────┐ │    │ ┌─────────────┐ │
│ │   Gossip    │ │◄──►│ │   Gossip    │ │◄──►│ │   Gossip    │ │
│ │  Protocol   │ │    │ │  Protocol   │ │    │ │  Protocol   │ │
│ └─────────────┘ │    │ └─────────────┘ │    │ └─────────────┘ │
│ ┌─────────────┐ │    │ ┌─────────────┐ │    │ ┌─────────────┐ │
│ │  BadgerDB   │ │    │ │  BadgerDB   │ │    │ │  BadgerDB   │ │
│ │ (Local KV)  │ │    │ │ (Local KV)  │ │    │ │ (Local KV)  │ │
│ └─────────────┘ │    │ └─────────────┘ │    │ └─────────────┘ │
└─────────────────┘    └─────────────────┘    └─────────────────┘
        ▲
        │
    External Clients
```

Each node is fully autonomous and communicates with peers via HTTP REST API for both external client requests and internal cluster operations.

## 📦 Installation

### Prerequisites
- Go 1.21 or higher

### Build from Source
```bash
git clone <repository-url>
cd kvs
go mod tidy
go build -o kvs .
```

### Quick Test
```bash
# Start standalone node
./kvs

# Test the API
curl http://localhost:8080/health
```

## ⚙️ Configuration

KVS uses YAML configuration files. On first run, a default `config.yaml` is automatically generated:

```yaml
node_id: "hostname"           # Unique node identifier
bind_address: "127.0.0.1"     # IP address to bind to
port: 8080                    # HTTP port
data_dir: "./data"            # Directory for BadgerDB storage
seed_nodes: []                # List of seed nodes for cluster joining
read_only: false              # Enable read-only mode
log_level: "info"             # Logging level (debug, info, warn, error)
gossip_interval_min: 60       # Min gossip interval (seconds)
gossip_interval_max: 120      # Max gossip interval (seconds)
sync_interval: 300            # Regular sync interval (seconds)
catchup_interval: 120         # Catch-up sync interval (seconds)
bootstrap_max_age_hours: 720  # Max age for bootstrap sync (hours)
throttle_delay_ms: 100        # Delay between sync requests (ms)
fetch_delay_ms: 50            # Delay between data fetches (ms)
```

### Custom Configuration
```bash
# Use custom config file
./kvs /path/to/custom-config.yaml
```

## 🔌 REST API

### Data Operations (`/kv/`)

#### Store Data
```bash
PUT /kv/{path}
Content-Type: application/json

curl -X PUT http://localhost:8080/kv/users/john/profile \
  -H "Content-Type: application/json" \
  -d '{"name":"John Doe","age":30,"email":"john@example.com"}'

# Response
{
  "uuid": "a1b2c3d4-e5f6-7890-1234-567890abcdef",
  "timestamp": 1672531200000
}
```

#### Retrieve Data
```bash
GET /kv/{path}

curl http://localhost:8080/kv/users/john/profile

# Response
{
  "name": "John Doe",
  "age": 30,
  "email": "john@example.com"
}
```

#### Delete Data
```bash
DELETE /kv/{path}

curl -X DELETE http://localhost:8080/kv/users/john/profile
# Returns: 204 No Content
```

### Cluster Operations (`/members/`)

#### View Cluster Members
```bash
GET /members/

curl http://localhost:8080/members/
# Response
[
  {
    "id": "node-alpha",
    "address": "192.168.1.10:8080",
    "last_seen": 1672531200000,
    "joined_timestamp": 1672530000000
  }
]
```

#### Join Cluster (Internal)
```bash
POST /members/join
# Used internally during bootstrap process
```

#### Health Check
```bash
GET /health

curl http://localhost:8080/health
# Response
{
  "status": "ok",
  "mode": "normal",
  "member_count": 2,
  "node_id": "node-alpha"
}
```

## 🏘️ Cluster Setup

### Single Node (Standalone)
```bash
# config.yaml
node_id: "standalone"
port: 8080
seed_nodes: []  # Empty = standalone mode
```

### Multi-Node Cluster

#### Node 1 (Bootstrap Node)
```bash
# node1.yaml
node_id: "node1"
port: 8081
seed_nodes: []  # First node, no seeds needed
```

#### Node 2 (Joins via Node 1)
```bash
# node2.yaml  
node_id: "node2"
port: 8082
seed_nodes: ["127.0.0.1:8081"]  # Points to node1
```

#### Node 3 (Joins via Node 1 & 2)
```bash
# node3.yaml
node_id: "node3" 
port: 8083
seed_nodes: ["127.0.0.1:8081", "127.0.0.1:8082"]  # Multiple seeds for reliability
```

#### Start the Cluster
```bash
# Terminal 1
./kvs node1.yaml

# Terminal 2 (wait a few seconds)
./kvs node2.yaml

# Terminal 3 (wait a few seconds)
./kvs node3.yaml
```

## 🔄 How It Works

### Gossip Protocol
- Nodes randomly select 1-3 peers every 1-2 minutes for membership exchange
- Failed nodes are detected via timeout (5 minutes) and removed (10 minutes)
- New members are automatically discovered and added to local member lists

### Data Synchronization  
- **Regular Sync**: Every 5 minutes, nodes compare their latest 15 data items with a random peer
- **Catch-up Sync**: Every 2 minutes when nodes detect they're significantly behind
- **Bootstrap Sync**: New nodes gradually fetch historical data up to 30 days old

### Conflict Resolution
When two nodes have different data for the same key with identical timestamps:

1. **Majority Vote**: Query all healthy cluster members for their version
2. **Tie-Breaker**: If votes are tied, the version from the oldest node (earliest `joined_timestamp`) wins
3. **Automatic Resolution**: Losing nodes automatically fetch and store the winning version

### Operational Modes
- **Normal**: Full read/write capabilities
- **Read-Only**: Rejects external writes but accepts internal replication
- **Syncing**: Temporary mode during bootstrap, rejects external writes

## 🛠️ Development

### Running Tests
```bash
# Basic functionality test
go build -o kvs .
./kvs &
curl http://localhost:8080/health
pkill kvs

# Cluster test with provided configs
./kvs node1.yaml &
./kvs node2.yaml &  
./kvs node3.yaml &

# Test data replication
curl -X PUT http://localhost:8081/kv/test/data \
  -H "Content-Type: application/json" \
  -d '{"message":"hello world"}'

# Wait 30+ seconds for sync, then check other nodes
curl http://localhost:8082/kv/test/data
curl http://localhost:8083/kv/test/data

# Cleanup
pkill kvs
```

### Conflict Resolution Testing
```bash
# Create conflicting data scenario
rm -rf data1 data2
mkdir data1 data2
go run test_conflict.go data1 data2

# Start nodes with conflicting data
./kvs node1.yaml &
./kvs node2.yaml &

# Watch logs for conflict resolution
# Both nodes will converge to same data within ~30 seconds
```

### Project Structure
```
kvs/
├── main.go              # Main application with all functionality
├── config.yaml          # Default configuration (auto-generated)
├── test_conflict.go     # Conflict resolution testing utility
├── node1.yaml           # Example cluster node config
├── node2.yaml           # Example cluster node config  
├── node3.yaml           # Example cluster node config
├── go.mod               # Go module dependencies
├── go.sum               # Go module checksums
└── README.md            # This documentation
```

### Key Data Structures

#### Stored Value Format
```go
type StoredValue struct {
    UUID      string          `json:"uuid"`      // Unique version identifier
    Timestamp int64           `json:"timestamp"` // Unix timestamp (milliseconds)
    Data      json.RawMessage `json:"data"`      // Actual user JSON payload
}
```

#### BadgerDB Storage
- **Main Key**: Direct path mapping (e.g., `users/john/profile`)
- **Index Key**: `_ts:{timestamp}:{path}` for efficient time-based queries
- **Values**: JSON-marshaled `StoredValue` structures

## 🔧 Configuration Options Explained

| Setting | Description | Default | Notes |
|---------|-------------|---------|-------|
| `node_id` | Unique identifier for this node | hostname | Must be unique across cluster |
| `bind_address` | IP address to bind HTTP server | "127.0.0.1" | Use 0.0.0.0 for external access |
| `port` | HTTP port for API and cluster communication | 8080 | Must be accessible to peers |
| `data_dir` | Directory for BadgerDB storage | "./data" | Will be created if doesn't exist |
| `seed_nodes` | List of initial cluster nodes | [] | Empty = standalone mode |
| `read_only` | Enable read-only mode | false | Accepts replication, rejects client writes |
| `log_level` | Logging verbosity | "info" | debug/info/warn/error |
| `gossip_interval_min/max` | Gossip frequency range | 60-120 sec | Randomized interval |
| `sync_interval` | Regular sync frequency | 300 sec | How often to sync with peers |
| `catchup_interval` | Catch-up sync frequency | 120 sec | Faster sync when behind |
| `bootstrap_max_age_hours` | Max historical data to sync | 720 hours | 30 days default |
| `throttle_delay_ms` | Delay between sync requests | 100 ms | Prevents overwhelming peers |
| `fetch_delay_ms` | Delay between individual fetches | 50 ms | Rate limiting |

## 🚨 Important Notes

### Consistency Model
- **Eventual Consistency**: Data will eventually be consistent across all nodes
- **Local-First**: All operations succeed locally first, then replicate
- **No Transactions**: Each key operation is independent
- **Conflict Resolution**: Automatic resolution of timestamp collisions

### Network Requirements  
- All nodes must be able to reach each other via HTTP
- Firewalls must allow traffic on configured ports
- IPv4 private networks supported (IPv6 not tested)

### Limitations
- No authentication/authorization (planned for future releases)
- No encryption in transit (use reverse proxy for TLS)
- No cross-key transactions
- No complex queries (key-based lookups only)
- No data compression (planned for future releases)

### Performance Characteristics
- **Read Latency**: ~1ms (local BadgerDB lookup)
- **Write Latency**: ~5ms (local write + timestamp indexing)  
- **Replication Lag**: 30 seconds - 5 minutes depending on sync cycles
- **Memory Usage**: Minimal (BadgerDB handles caching efficiently)
- **Disk Usage**: Raw JSON + metadata overhead (~20-30%)

## 🛡️ Production Considerations

### Deployment
- Use systemd or similar for process management
- Configure log rotation for JSON logs
- Set up monitoring for `/health` endpoint
- Use reverse proxy (nginx/traefik) for TLS and load balancing

### Monitoring
- Monitor `/health` endpoint for node status
- Watch logs for conflict resolution events
- Track member count for cluster health
- Monitor disk usage in data directories

### Backup Strategy
- BadgerDB supports snapshots
- Data directories can be backed up while running
- Consider backing up multiple nodes for redundancy

### Scaling
- Add new nodes by configuring existing cluster members as seeds
- Remove nodes gracefully using `/members/leave` endpoint  
- Cluster can operate with any number of nodes (tested with 2-10)

## 📄 License

This project is licensed under the MIT License - see the LICENSE file for details.

## 🤝 Contributing

1. Fork the repository
2. Create a feature branch (`git checkout -b feature/amazing-feature`)
3. Commit your changes (`git commit -m 'Add amazing feature'`)
4. Push to the branch (`git push origin feature/amazing-feature`)
5. Open a Pull Request

## 📚 Additional Resources

- [BadgerDB Documentation](https://dgraph.io/docs/badger/)
- [Gossip Protocol Paper](https://www.cs.cornell.edu/home/rvr/papers/flowgossip.pdf)
- [Eventually Consistent Systems](https://www.allthingsdistributed.com/2008/12/eventually_consistent.html)

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