Podman vs Docker: Container Engines Compared

• 15/1/2024
• 6-minute read

Introduction

Container technology has revolutionized application development and deployment, providing consistency across environments and simplifying dependency management. While Docker has long dominated this space, Podman has emerged as a compelling alternative with distinct architectural advantages. This article compares these container engines to help developers and system administrators make informed decisions for their containerization needs.

Key Differences at a Glance

Architectural Differences

Docker Architecture

Docker uses a client-server architecture with three main components:

1. Docker Client: Command-line interface for users
2. Docker Daemon: Background service managing containers
3. Docker REST API: Communication interface between client and daemon

This architecture requires running a privileged daemon process, which presents both security concerns and a single point of failure.

# Docker architecture requires the daemon to be running
$ systemctl status docker
● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2024-01-15 08:30:15 UTC; 45min ago

Podman Architecture

Podman employs a daemonless architecture:

1. No background daemon: Commands execute directly
2. Rootless operation: Run containers as non-privileged users
3. Direct interaction: Works with OCI container runtime directly

This design eliminates privileged background processes and enhances security.

# Podman works without a daemon
$ podman info | grep ConmonVersion
  ConmonVersion: 2.1.7
$ ps aux | grep docker
# No docker daemon processes running

Rootless Containers: A Security Game-Changer

Both Podman and Docker now support rootless containers, though they implement this feature differently.

Podman’s Rootless Implementation

Podman was designed from the ground up with rootless containers as a primary feature:

# Run a container as a regular user with Podman
$ podman run --rm -it alpine:latest id
uid=1000(podman) gid=1000(podman) groups=1000(podman)

Docker’s Rootless Implementation

Since Docker 19.03, Docker has added support for rootless mode, allowing it to run without root privileges:

# Set up rootless Docker
$ dockerd-rootless-setuptool.sh install

# Run Docker in rootless mode
$ docker run --rm -it alpine:latest id
uid=1000(docker) gid=1000(docker) groups=1000(docker)

Key Differences in Rootless Implementations

While both engines now support rootless containers, there are important distinctions:

1. Setup complexity: Podman works rootless out-of-the-box, while Docker requires additional configuration
2. Maturity: Podman’s rootless mode has been a core feature longer and is more mature
3. Network limitations: Docker rootless mode has more networking limitations compared to Podman
4. Performance: Both have similar performance characteristics in rootless mode

Security Implications

With Docker in rootful mode:

• The daemon runs as root
• All users in the docker group effectively have root access
• Container breakout vulnerabilities have elevated privileges

With Docker in rootless mode or Podman:

• No daemon running as root
• User namespaces isolate container processes
• Container breakout limited to user permissions

Command Compatibility

Podman was designed to be a drop-in replacement for Docker, maintaining CLI compatibility:

# Docker command
$ docker run -d --name web -p 8080:80 nginx

# Equivalent Podman command
$ podman run -d --name web -p 8080:80 nginx

For teams transitioning from Docker to Podman, an alias can ease migration:

alias docker=podman

Pod Management

Podman introduces native support for Kubernetes-style pods, which Docker lacks:

# Create a pod with Podman
$ podman pod create --name webapp
$ podman run --pod webapp -d nginx
$ podman run --pod webapp -d redis

# Check the pod
$ podman pod ps
POD ID        NAME        STATUS    CREATED        # OF CONTAINERS
a9b22d8f231a  webapp      Running   5 minutes ago  3

This capability streamlines the development-to-production workflow when using Kubernetes in production.

Container Storage

Both engines use similar storage drivers but handle container storage differently:

# Docker storage location
$ ls -la /var/lib/docker/

# Podman storage for rootless containers
$ ls -la ~/.local/share/containers/

Podman stores rootless container data in the user’s home directory, aligning with the principle of least privilege.

Integration with Systemd

Podman offers superior integration with systemd for running containers as services:

# Generate a systemd service file for a container
$ podman generate systemd --name myapp --files

# Install and enable the service
$ mv container-myapp.service ~/.config/systemd/user/
$ systemctl --user enable container-myapp.service
$ systemctl --user start container-myapp.service

This provides a more standardized approach to container management on Linux systems.

Performance Considerations

The architectural differences between Podman and Docker result in some performance variations:

1. Container startup: Docker can be marginally faster due to the always-running daemon
2. Multiple operations: Podman may be slower without a persistent daemon
3. Memory usage: Podman uses less memory without a background daemon
4. Rootless overhead: Both engines have similar overhead in rootless mode, with some additional isolation costs

For most use cases, these differences are negligible, but high-throughput CI/CD systems may notice the impact.

When to Use Rootless Docker vs Podman

When choosing between rootless Docker and Podman, consider:

• Integration needs: If your workflow is heavily integrated with Docker ecosystem tools, rootless Docker may be preferable
• Setup simplicity: Podman offers easier rootless setup
• Host system: Some Linux distributions better support one or the other (Fedora/RHEL favor Podman, Ubuntu has good support for both)
• Production environment: Align development environment with production container runtime

# Example of setting up rootless Docker on Ubuntu
$ sudo apt-get install -y docker-ce-rootless-extras
$ dockerd-rootless-setuptool.sh install
$ systemctl --user start docker

# Verifying rootless Docker installation
$ docker context ls
NAME       DESCRIPTION                               DOCKER ENDPOINT                            KUBERNETES ENDPOINT
default *  Current DOCKER_HOST based configuration   unix:///run/user/1000/docker.sock          

Migration Path from Docker to Podman

For organizations considering a migration to Podman:

1. Test image compatibility:

   $ podman pull docker.io/yourorg/yourimage:latest
   

2. Ensure volume mounts work as expected:

   $ podman run -v /host/path:/container/path myimage
   

3. Adapt Docker Compose workflows:

   $ podman-compose up -d
   # or
   $ podman play kube docker-compose.yml
   

4. Update CI/CD pipelines to use Podman commands or aliases

Real-world Decision Factors

When choosing between Podman and Docker, consider:

• Security requirements: Both offer rootless options, with Podman providing a more mature implementation
• Existing workflows: Docker may be preferred for compatibility with existing tools
• Kubernetes integration: Podman provides a smoother path to Kubernetes
• Corporate support: Red Hat (Podman) vs Mirantis (Docker)
• Host OS: Podman is more aligned with modern Linux distributions, particularly Red Hat-based systems
• Daemon requirement: If avoiding a daemon is important, Podman is the better choice

Conclusion

Both Podman and Docker are excellent container engines with their own strengths. Docker retains its position as the most widely used container platform with extensive ecosystem support. While Docker has added rootless capabilities, Podman still offers advantages with its daemonless, rootless-by-default architecture and closer alignment with Kubernetes concepts.

For new projects with security focus, both engines can provide secure containerization, though Podman’s rootless implementation is more mature. For existing Docker workflows, either continuing with Docker (potentially in rootless mode) or migrating to Podman are viable options.

The choice ultimately depends on your specific requirements, security posture, and integration needs with existing systems.

Further Reading

• Official Podman Documentation
• Docker Documentation
• Rootless Docker Documentation
• Transitioning from Docker to Podman (Red Hat)
• Using Docker Compose with Podman
• Container Runtimes in Kubernetes