# ADR-0033: Service Discovery Implementation

## Status
Accepted

## Context
The platform follows a microservices architecture where services need to discover and communicate with each other. We need a service discovery mechanism that:

- Enables services to find each other dynamically
- Supports health checking and automatic deregistration
- Works in both development (Docker Compose) and production (Kubernetes) environments
- Provides service registration and discovery APIs
- Supports multiple service instances (load balancing)

Options considered:
1. **Consul** - HashiCorp's service discovery and configuration tool
2. **etcd** - Distributed key-value store with service discovery
3. **Kubernetes Service Discovery** - Native K8s service discovery
4. **Eureka** - Netflix service discovery (Java-focused)
5. **Custom Registry** - Build our own service registry

## Decision
Use **Consul** as the primary service discovery implementation with support for Kubernetes service discovery as an alternative.

### Rationale

1. **Mature and Production-Ready**:
   - Battle-tested in production environments
   - Active development and strong community
   - Comprehensive documentation

2. **Feature-Rich**:
   - Service registration and health checking
   - Key-value store for configuration
   - Service mesh capabilities (Consul Connect)
   - Multi-datacenter support
   - DNS-based service discovery

3. **Development-Friendly**:
   - Easy to run locally (single binary or Docker)
   - Docker Compose integration
   - Good for local development setup

4. **Production-Ready**:
   - Works well in Kubernetes (Consul K8s)
   - Can be used alongside Kubernetes service discovery
   - Supports high availability and clustering

5. **Language Agnostic**:
   - HTTP API for service registration
   - gRPC support
   - Go client library available

6. **Health Checking**:
   - Built-in health checking with automatic deregistration
   - Multiple health check types (HTTP, TCP, gRPC, script)
   - Health status propagation

## Architecture

### Service Registry Interface

```go
// pkg/registry/registry.go
type ServiceRegistry interface {
    // Register a service instance
    Register(ctx context.Context, service *ServiceInstance) error
    
    // Deregister a service instance
    Deregister(ctx context.Context, serviceID string) error
    
    // Discover service instances
    Discover(ctx context.Context, serviceName string) ([]*ServiceInstance, error)
    
    // Watch for service changes
    Watch(ctx context.Context, serviceName string) (<-chan []*ServiceInstance, error)
    
    // Get service health
    Health(ctx context.Context, serviceID string) (*HealthStatus, error)
}

type ServiceInstance struct {
    ID       string
    Name     string
    Address  string
    Port     int
    Tags     []string
    Metadata map[string]string
}
```

### Consul Implementation

```go
// internal/registry/consul/consul.go
type ConsulRegistry struct {
    client *consul.Client
    config *ConsulConfig
}

// Register service with Consul
func (r *ConsulRegistry) Register(ctx context.Context, service *ServiceInstance) error {
    registration := &consul.AgentServiceRegistration{
        ID:      service.ID,
        Name:    service.Name,
        Address: service.Address,
        Port:    service.Port,
        Tags:    service.Tags,
        Meta:    service.Metadata,
        Check: &consul.AgentServiceCheck{
            HTTP:     fmt.Sprintf("http://%s:%d/healthz", service.Address, service.Port),
            Interval: "10s",
            Timeout:  "3s",
            DeregisterCriticalServiceAfter: "30s",
        },
    }
    return r.client.Agent().ServiceRegister(registration)
}
```

## Implementation Strategy

### Phase 1: Consul Implementation (Epic 1)
- Create service registry interface in `pkg/registry/`
- Implement Consul registry in `internal/registry/consul/`
- Basic service registration and discovery
- Health check integration

### Phase 2: Kubernetes Support (Epic 6)
- Implement Kubernetes service discovery as alternative
- Service registry factory that selects implementation based on environment
- Support for both Consul and K8s in same codebase

### Phase 3: Advanced Features (Epic 6)
- Service mesh integration (Consul Connect)
- Multi-datacenter support
- Service tags and filtering
- Service metadata and configuration

## Configuration

```yaml
registry:
  type: consul  # or "kubernetes"
  consul:
    address: "localhost:8500"
    datacenter: "dc1"
    scheme: "http"
    health_check:
      interval: "10s"
      timeout: "3s"
      deregister_after: "30s"
  kubernetes:
    namespace: "default"
    in_cluster: true
```

## Service Registration Flow

```mermaid
sequenceDiagram
    participant Service
    participant Registry[Service Registry Interface]
    participant Consul
    participant Health[Health Check]
    
    Service->>Registry: Register(serviceInstance)
    Registry->>Consul: Register service
    Consul->>Consul: Store service info
    Consul->>Health: Start health checks
    
    loop Health Check
        Health->>Service: GET /healthz
        Service-->>Health: 200 OK
        Health->>Consul: Update health status
    end
    
    Service->>Registry: Deregister(serviceID)
    Registry->>Consul: Deregister service
    Consul->>Consul: Remove service
```

## Service Discovery Flow

```mermaid
sequenceDiagram
    participant Client
    participant Registry[Service Registry]
    participant Consul
    participant Service1[Service Instance 1]
    participant Service2[Service Instance 2]
    
    Client->>Registry: Discover("auth-service")
    Registry->>Consul: Query service instances
    Consul-->>Registry: [instance1, instance2]
    Registry->>Registry: Filter healthy instances
    Registry-->>Client: [healthy instances]
    
    Client->>Service1: gRPC call
    Service1-->>Client: Response
```

## Development Setup

### Docker Compose

```yaml
services:
  consul:
    image: consul:latest
    ports:
      - "8500:8500"
    command: consul agent -dev -client=0.0.0.0
    volumes:
      - consul-data:/consul/data

volumes:
  consul-data:
```

### Local Development

```bash
# Run Consul in dev mode
consul agent -dev

# Or use Docker
docker run -d --name consul -p 8500:8500 consul:latest
```

## Production Deployment

### Kubernetes

```yaml
# Consul Helm Chart
helm repo add hashicorp https://helm.releases.hashicorp.com
helm install consul hashicorp/consul --set global.datacenter=dc1
```

### Standalone Cluster

- Deploy Consul cluster (3-5 nodes)
- Configure service discovery endpoints
- Set up Consul Connect for service mesh (optional)

## Consequences

### Positive
- **Dynamic Service Discovery**: Services can be added/removed without configuration changes
- **Health Checking**: Automatic removal of unhealthy services
- **Load Balancing**: Multiple service instances automatically discovered
- **Configuration Management**: Consul KV store for service configuration
- **Service Mesh Ready**: Can use Consul Connect for advanced features
- **Development Friendly**: Easy local setup with Docker

### Negative
- **Additional Infrastructure**: Requires Consul cluster in production
- **Network Dependency**: Services depend on Consul availability
- **Configuration Complexity**: Need to configure Consul cluster
- **Learning Curve**: Team needs to understand Consul concepts

### Mitigations
1. **High Availability**: Deploy Consul cluster (3+ nodes)
2. **Caching**: Cache service instances to reduce Consul queries
3. **Fallback**: Support Kubernetes service discovery as fallback
4. **Documentation**: Comprehensive setup and usage documentation
5. **Monitoring**: Monitor Consul health and service registration

## Alternative: Kubernetes Service Discovery

For Kubernetes deployments, we also support native Kubernetes service discovery:

```go
// internal/registry/kubernetes/k8s.go
type KubernetesRegistry struct {
    clientset kubernetes.Interface
    namespace string
}

func (r *KubernetesRegistry) Discover(ctx context.Context, serviceName string) ([]*ServiceInstance, error) {
    endpoints, err := r.clientset.CoreV1().Endpoints(r.namespace).Get(ctx, serviceName, metav1.GetOptions{})
    // Convert K8s endpoints to ServiceInstance
}
```

## Service Registry Factory

```go
// internal/registry/factory.go
func NewServiceRegistry(cfg *config.Config) (registry.ServiceRegistry, error) {
    switch cfg.Registry.Type {
    case "consul":
        return consul.NewRegistry(cfg.Registry.Consul)
    case "kubernetes":
        return kubernetes.NewRegistry(cfg.Registry.Kubernetes)
    default:
        return nil, fmt.Errorf("unknown registry type: %s", cfg.Registry.Type)
    }
}
```

## References
- [ADR-0029: Microservices Architecture](./0029-microservices-architecture.md)
- [ADR-0030: Service Communication Strategy](./0030-service-communication-strategy.md)
- [ADR-0031: Service Repository Structure](./0031-service-repository-structure.md)
- [Consul Documentation](https://www.consul.io/docs)
- [Consul Go Client](https://github.com/hashicorp/consul/api)
- [Consul Kubernetes](https://www.consul.io/docs/k8s)