How to Diagnose and Fix Docker Failures

Docker has revolutionized application deployment, but it creates a new layer of complexity for debugging. Even stable systems fail occasionally when containers restart, network connections drop, or logs fill up storage.

We have compiled a guide to the most common errors. Here is how to quickly diagnose failures and get your production environment back on track.

Container Starts and Exits Immediately

You launch a container, but it disappears from the list of active processes after a second.

What is the cause?

A Docker container remains alive only as long as its main process, known as PID 1, is running. If that process finishes or goes into the background, Docker considers the work done and stops the container.

How to fix it?

The most important step is to check what the container reported before it stopped. Use the command:

Solutions:

• If there are no logs: You likely started an empty OS image such as Ubuntu without a command. Add a command that keeps the process active, for example starting a web server.
• If there is an error: Fix the syntax in your Entrypoint startup script.

Sudden Crash with Exit Code 137

If your service was running normally and then silently died, check the exit code. Code 137 is a critical distress signal.

Diagnosis

This is the work of the Linux Out Of Memory Killer. When a container consumes more RAM than allowed or exceeds the server’s available memory, the system forcibly kills it to save other processes.

Verify this with the command:

What to do?

1. Set Limits: Never run containers without memory limits. Configure the memory parameters in your docker-compose or orchestration file.
2. Optimize Code: If the limits are adequate but the error persists, your application likely has a memory leak. At Optimum-Web, we use monitoring tools like Prometheus and Grafana to catch such leaks before they bring down the service.

Network Issues and Connection Refused Errors

Your application cannot see the database or does not respond to external requests.

Key Docker Networking Nuances

• Localhost is a Trap: Inside a container, localhost refers to the container itself. If you are trying to connect to a database on the host machine, localhost will not work. Instead, use the special address host.docker.internal when running Docker Desktop.
• Container-to-Container Communication: Use service names as hostnames. If your database service is named db in your project, the connection string should be postgres://db:5432 rather than an IP address.
• External Access: Ensure your application listens on 0.0.0.0 instead of 127.0.0.1. The latter restricts access to inside the container only.

Permission Denied Errors

You mounted a local volume to a container, but the application crashes with a file write error.

The Cause

By default, the process inside the container might run as the root user, while the files on your host server belong to your regular user account. This creates a permissions conflict.

The Solution

Do not run processes as root unless necessary.

• In Development: Run the container with the –user flag and pass your current user ID.
• In Dockerfile: Create a specific user and assign rights to the working directory using the chown command.

Slow Build Times

If your CI/CD pipeline stalls for 20 minutes building images, your business loses flexibility.

Optimization Tips from Optimum-Web

1. Use .dockerignore: This works just like .gitignore. Exclude the .git folder, local logs, and node_modules to prevent the Docker daemon from copying gigabytes of unnecessary data.
2. Layer Caching: In your Dockerfile, copy dependency files like package.json and install them first. Only copy your source code after that step. This allows Docker to avoid re-downloading libraries every time you change a single line of code.

Does Your Infrastructure Need Professional Attention?

Docker is a powerful tool, but improper configuration leads to security vulnerabilities, data loss, and business downtime.

If your team spends hours putting out fires instead of developing new features, it is time to delegate DevOps tasks to experts.

Why choose Optimum-Web?

• Audit and Optimization: We identify bottlenecks in your architecture.
• Reliability: We configure high-availability clusters and automated recovery.
• Cost Efficiency: Outsourcing DevOps is often more effective than maintaining and training an in-house engineering team for infrastructure support.

Contact us to make your deployment fast and your production stable.

Discuss your project with Optimum-Web →