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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Assuming depends_on waits for a service to be ready, not just started

services:
  db:
    image: postgres:15
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 5s
      retries: 5
  api:
    build: .
    depends_on:
      db:
        condition: service_healthy

The Solution //

depends_on only guarantees Docker starts the db container before the api container — it does NOT wait for Postgres to finish initializing and accept connections. This causes intermittent ECONNREFUSED errors on 'docker-compose up'. Add a healthcheck to the db service and use the condition: service_healthy form of depends_on.

The Error //

Connecting to 127.0.0.1 or localhost from inside a container

// Wrong: only works if Postgres runs on the same host, outside Docker
const pool = new Pool({ host: '127.0.0.1', port: 5432 });

// Correct: resolves via Compose's internal DNS
const pool = new Pool({ host: 'db', port: 5432 });

The Solution //

Each container has its own isolated network namespace, so 'localhost' inside the api container refers to the api container itself, not the db container next to it. Use the service name defined in docker-compose.yml as the hostname — Compose's built-in DNS resolves it to the right container automatically.