Networking in Docker Compose — Service Discovery

Docker Compose default behaviour:
  → Creates one network for the entire compose file (named: _default)
  → All services on the default network can reach each other by service name
  → No ports are exposed to the host unless you specify "ports:"

Service discovery in containers:
  Host machine: services use "localhost:5000", "localhost:1433"
  Inside compose: services use the service name as hostname
    prescription-service → calls http://patient-service/api/patients
    prescription-service → connects to Server=sqlserver;Database=Clinical
    (where "sqlserver" is the compose service name)

# docker-compose.yml
services:
  prescription-service:
    image: clinical/prescription-service:latest
    environment:
      - ConnectionStrings__Clinical=Server=sqlserver;Database=ClinicalPrescriptions;User Id=sa;Password=${DB_PASSWORD}
      - Services__PatientService=http://patient-service
      - Services__LabService=http://lab-service
    ports:
      - "5001:8080"    # expose to host for testing
    depends_on:
      sqlserver:
        condition: service_healthy

  patient-service:
    image: clinical/patient-service:latest
    environment:
      - ConnectionStrings__Clinical=Server=sqlserver;Database=ClinicalPatients;...
      - Fhir__BaseUrl=http://fhir-mock   # another compose service
    # No "ports:" — not exposed to host. Only accessible to other containers.

  sqlserver:
    image: mcr.microsoft.com/mssql/server:2022-latest
    environment:
      - ACCEPT_EULA=Y
      - SA_PASSWORD=${DB_PASSWORD}
    # Also not exposed to host — only accessible to application services
    healthcheck:
      test: /opt/mssql-tools/bin/sqlcmd -S localhost -U sa -P ${DB_PASSWORD} -Q "SELECT 1"
      interval: 10s
      retries: 10
      start_period: 30s

# Separate frontend/backend networks — frontend services cannot directly access the DB
services:
  nginx:
    image: nginx:alpine
    networks: [frontend]
    ports:
      - "443:443"

  prescription-service:
    image: clinical/prescription-service:latest
    networks:
      - frontend    # accepts requests from nginx
      - backend     # can access DB and other services
    depends_on:
      - sqlserver
      - patient-service

  patient-service:
    image: clinical/patient-service:latest
    networks: [backend]  # not accessible from frontend directly

  sqlserver:
    image: mcr.microsoft.com/mssql/server:2022-latest
    networks: [backend]  # only accessible from backend services
    # Cannot be reached from nginx — frontend network has no route to it

networks:
  frontend:
    driver: bridge
  backend:
    driver: bridge
    internal: true  # no outbound internet from backend network

// In docker-compose, services call each other by service name
// Configure ASP.NET Core to use service names, not localhost

// appsettings.json for compose environment:
{
  "Services": {
    "PatientService":  "http://patient-service",
    "LabService":      "http://lab-service",
    "NotificationSvc": "http://notification-service"
  }
}

// Named HttpClient pointing to patient-service:
builder.Services.AddHttpClient<IPatientServiceClient, PatientServiceClient>(client =>
{
    client.BaseAddress = new Uri(
        builder.Configuration["Services:PatientService"] ?? "http://patient-service");
});

// The "patient-service" hostname resolves via Docker's internal DNS
// to the container's IP on the shared network

# Ports: "host:container"
services:
  prescription-service:
    ports:
      - "5001:8080"    # host port 5001 → container port 8080
      # Access from host: http://localhost:5001
      # Access from other containers: http://prescription-service:8080
      # (other containers use the service name + container port, not host port)

  patient-service:
    ports:
      - "5002:8080"    # different host port, same container port — no conflict
      # Multiple services CAN use port 8080 inside their containers
      # Only host port mappings must be unique

  sqlserver:
    ports:
      - "14330:1433"   # map to non-standard host port to avoid conflict
      # with local SQL Server running on 1433
      # Connection string from host: Server=localhost,14330;...
      # Connection string inside compose: Server=sqlserver,1433;... (container port)

# Network aliases — give a service an additional hostname on a specific network
services:
  fhir-service:
    image: clinical/fhir-service:latest
    networks:
      backend:
        aliases:
          - fhir              # accessible as "fhir" within the backend network
          - fhir.clinical.local  # FQDN alias

  prescription-service:
    image: clinical/prescription-service:latest
    networks: [backend]
    environment:
      - Fhir__BaseUrl=http://fhir  # uses the alias

# Useful when: migrating from a hostname to another without changing all consumers,
# or when a service should be reachable by multiple names