Docker Compose depends_on Doesn’t Wait for Service Ready — Race Condition at Startup

Symptom

Agent container fails immediately on startup: Connection refused to database
docker-compose up starts all containers but agent fails before DB is ready
Works if you start database first, wait 10 seconds, then start agent
depends_on: postgres in docker-compose.yml doesn’t help
Fails intermittently — sometimes works, sometimes doesn’t (timing-dependent)

Root Cause

depends_on only waits for the container to start, not for the service inside to be ready to accept connections. PostgreSQL takes 2–5 seconds to initialize after the container starts. Redis takes 1–2 seconds. The agent tries to connect immediately and fails.

Fix

Option 1: Add healthchecks to services + condition in depends_on

# docker-compose.yml
services:
  agent:
    build: .
    depends_on:
      postgres:
        condition: service_healthy  # Wait for healthcheck to pass
      redis:
        condition: service_healthy
    environment:
      - DATABASE_URL=postgresql://user:pass@postgres:5432/agentdb
      - REDIS_URL=redis://redis:6379

  postgres:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: password
      POSTGRES_DB: agentdb
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres -d agentdb"]
      interval: 5s      # Check every 5 seconds
      timeout: 5s       # Fail if check takes more than 5s
      retries: 10       # Allow 10 failures before marking unhealthy
      start_period: 10s # Don't start checking for 10s after container starts

  redis:
    image: redis:7-alpine
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 5s

  mongodb:
    image: mongo:7
    healthcheck:
      test: ["CMD", "mongosh", "--eval", "db.adminCommand('ping')"]
      interval: 10s
      timeout: 5s
      retries: 5
      start_period: 15s

Option 2: Wait-for-it script in entrypoint

# Dockerfile
FROM python:3.12-slim

# Add wait-for-it utility
ADD https://raw.githubusercontent.com/vishnubob/wait-for-it/master/wait-for-it.sh /wait-for-it.sh
RUN chmod +x /wait-for-it.sh

COPY . /app
WORKDIR /app
RUN pip install -r requirements.txt

# Wait for postgres to be ready, then start agent
ENTRYPOINT ["/wait-for-it.sh", "postgres:5432", "--timeout=60", "--", "python", "agent.py"]

# Or in docker-compose command
services:
  agent:
    image: my-agent
    command: >
      sh -c "
        until nc -z postgres 5432; do
          echo 'Waiting for postgres...'
          sleep 2
        done
        echo 'Postgres is ready!'
        python agent.py
      "

Option 3: Retry logic in the agent itself (resilient startup)

import asyncpg, asyncio, os, time

async def connect_with_retry(dsn: str, max_attempts: int = 30, delay: float = 2.0):
    """Connect to database, retrying until successful or max_attempts exceeded"""
    for attempt in range(1, max_attempts + 1):
        try:
            conn = await asyncpg.connect(dsn)
            print(f"Database connected on attempt {attempt}")
            return conn
        except (asyncpg.CannotConnectNowError, ConnectionRefusedError, OSError) as e:
            if attempt == max_attempts:
                raise RuntimeError(f"Could not connect after {max_attempts} attempts: {e}")
            print(f"Attempt {attempt}/{max_attempts}: {e}. Retrying in {delay}s...")
            await asyncio.sleep(delay)

async def main():
    db = await connect_with_retry(os.environ["DATABASE_URL"])
    # Now safe to start agent
    await run_agent(db)

Option 4: Healthcheck for your own agent

from fastapi import FastAPI
import asyncpg, redis.asyncio as redis

app = FastAPI()

@app.get("/health")
async def health():
    """Liveness + readiness probe"""
    checks = {}

    # Check database
    try:
        conn = await asyncpg.connect(os.environ["DATABASE_URL"])
        await conn.fetchval("SELECT 1")
        await conn.close()
        checks["database"] = "ok"
    except Exception as e:
        checks["database"] = f"error: {e}"

    # Check Redis
    try:
        r = redis.from_url(os.environ["REDIS_URL"])
        await r.ping()
        await r.aclose()
        checks["redis"] = "ok"
    except Exception as e:
        checks["redis"] = f"error: {e}"

    all_ok = all(v == "ok" for v in checks.values())
    status_code = 200 if all_ok else 503
    return {"status": "healthy" if all_ok else "unhealthy", "checks": checks}

# Use your agent's own healthcheck in docker-compose
services:
  agent:
    build: .
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8080/health"]
      interval: 10s
      timeout: 5s
      retries: 5
      start_period: 30s

Option 5: Kubernetes equivalent — readinessProbe

# k8s deployment.yaml
spec:
  containers:
  - name: agent
    image: my-agent
    readinessProbe:
      httpGet:
        path: /health
        port: 8080
      initialDelaySeconds: 10
      periodSeconds: 5
      failureThreshold: 10  # 50 seconds total
    livenessProbe:
      httpGet:
        path: /health
        port: 8080
      initialDelaySeconds: 30
      periodSeconds: 10

Healthcheck Commands by Service

Service	Healthcheck command
PostgreSQL	`pg_isready -U $POSTGRES_USER -d $POSTGRES_DB`
MySQL	`mysqladmin ping -h localhost`
Redis	`redis-cli ping`
MongoDB	`mongosh --eval "db.adminCommand('ping')"`
Elasticsearch	`curl -f http://localhost:9200/_health`
RabbitMQ	`rabbitmq-diagnostics -q ping`
Kafka	`kafka-topics.sh --bootstrap-server localhost:9092 --list`

Expected Token Savings

Debugging intermittent startup race condition: ~6,000 tokens Healthcheck + condition: prevents the race entirely

Environment

Any Docker Compose setup with multiple services that depend on each other
Source: direct experience; depends_on confusion is one of the most common Docker mistakes

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