Health Checks and Service Dependencies

Without health checks:
  docker compose up starts all services simultaneously
  prescription-service starts and immediately tries to connect to sqlserver
  sqlserver is still initialising — connection fails
  prescription-service crashes with "Cannot connect to SQL Server"
  Docker restarts it (if restart: unless-stopped)
  sqlserver finishes starting
  prescription-service starts again — works
  
  This wastes 20-40 seconds and fills logs with spurious errors.
  In CI/CD it can fail the pipeline before the service has a chance to start.

With health checks + depends_on:
  sqlserver starts
  Docker waits until sqlserver is healthy (health check passes)
  Only then does prescription-service start
  No connection failures, no restart loop

services:
  sqlserver:
    image: mcr.microsoft.com/mssql/server:2022-latest
    environment:
      - ACCEPT_EULA=Y
      - SA_PASSWORD=${DB_PASSWORD}
    healthcheck:
      test:
        - CMD
        - /opt/mssql-tools/bin/sqlcmd
        - -S
        - localhost
        - -U
        - sa
        - -P
        - ${DB_PASSWORD}
        - -Q
        - "SELECT 1"
      interval: 10s      # check every 10 seconds
      timeout: 5s        # fail if no response in 5 seconds
      retries: 10        # declare unhealthy after 10 consecutive failures
      start_period: 30s  # don't count failures in the first 30 seconds (SQL Server init time)

// Register health checks in Program.cs
builder.Services.AddHealthChecks()
    .AddSqlServer(
        connectionString: builder.Configuration.GetConnectionString("Clinical")!,
        name: "sql-server",
        tags: new[] { "ready" })
    .AddRedis(
        connectionString: builder.Configuration.GetConnectionString("Redis")!,
        name: "redis",
        tags: new[] { "ready" });

// Map endpoints
app.MapHealthChecks("/health/live", new HealthCheckOptions
{
    Predicate = _ => false  // no dependency checks — just "is the process running?"
});

app.MapHealthChecks("/health/ready", new HealthCheckOptions
{
    Predicate = check => check.Tags.Contains("ready"),
    ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
});

# Docker Compose: check the /health/live endpoint
services:
  prescription-service:
    image: clinical/prescription-service:latest
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8080/health/live"]
      interval: 15s
      timeout: 5s
      retries: 3
      start_period: 20s   # allow time for migrations to run on first start
    depends_on:
      sqlserver:
        condition: service_healthy  # wait for sqlserver to be healthy
      redis:
        condition: service_healthy

services:
  prescription-service:
    depends_on:
      sqlserver:
        condition: service_healthy    # wait for health check to pass
      patient-service:
        condition: service_started    # just wait for container to start (no health check on patient-service)
      migration-job:
        condition: service_completed_successfully  # wait for a one-shot migration job to exit 0

  # One-shot migration job that runs EF Core migrations
  migration-job:
    image: clinical/prescription-service:latest
    command: ["dotnet", "ef", "database", "update"]
    environment:
      - ConnectionStrings__Clinical=Server=sqlserver;...
    depends_on:
      sqlserver:
        condition: service_healthy
    restart: "no"   # don't restart — it's a one-shot job

# List containers with health status
docker compose ps
# Look for: STATUS column shows "(unhealthy)" or "(health: starting)"

# Inspect the last 5 health check results:
docker inspect --format='{{json .State.Health}}' clinical_sqlserver_1 | jq .

# Example output:
# {
#   "Status": "unhealthy",
#   "FailingStreak": 3,
#   "Log": [
#     {
#       "Start": "2026-03-15T10:00:00Z",
#       "End":   "2026-03-15T10:00:05Z",
#       "ExitCode": 1,
#       "Output": "Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server..."
#     }
#   ]
# }

# Stream logs from a specific service:
docker compose logs sqlserver -f --tail=50

# Execute a command inside a container to debug:
docker compose exec sqlserver bash
# Or for SQL Server specifically:
docker compose exec sqlserver \
  /opt/mssql-tools/bin/sqlcmd -S localhost -U sa -P $SA_PASSWORD -Q "SELECT @@VERSION"

services:
  redis:
    image: redis:7-alpine
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      # Returns "PONG" if Redis is up — exit code 0 = healthy
      interval: 10s
      timeout: 3s
      retries: 5

services:
  nginx:
    image: nginx:alpine
    depends_on:
      prescription-service:
        condition: service_healthy
      patient-service:
        condition: service_healthy
    # Nginx only starts after all backend services are healthy
    # Prevents nginx from routing traffic to services that aren't ready
    ports:
      - "443:443"