Podman vs Docker: A Detailed Comparison of Container Runtimes

If you‘re involved in developing or deploying containerized applications, you‘ve likely encountered the two leading open-source container runtimes: Docker and Podman. Both serve similar purposes on the surface, but under the hood there are important architectural differences between these technologies.

In this in-depth comparison, we‘ll dig into the key characteristics and use cases of Docker versus Podman. Understanding the strengths and limitations of each will help you determine the best runtime for your container needs.

A Quick Intro to Containers

Before we dive into the Docker and Podman details, let‘s briefly recap how containers work.

Containers allow applications to run in isolated user-space instances called containers. All containers on a host share the same kernel, but have private filesystems and namespaces. This makes containers lightweight and fast to spawn compared to virtual machines.

Common use cases for containers include:

• Packaging and deploying applications as portable, self-contained units
• Enabling microservices architectures
• Achieving greater resource utilization for high density deployments
• Providing consistent dev, test and prod environments for apps

Running containers requires a container runtime to interface with the host OS. Docker and Podman are two of the most widely used open-source runtimes.

Docker Overview

Docker first brought containers into the mainstream in 2013. It remains by far the most popular and widely-supported container runtime.

Some key facts about Docker:

• Created by Docker Inc. and written in Go programming language
• Uses client-server architecture with persistent background Docker daemon process
• Leverages Linux kernel capabilities like control groups and namespaces
• Provides CLI and GUI tools for image building, sharing, and container lifecycle management
• Enables networking between containers across hosts via libnetwork overlay driver
• Supports Linux, Windows Server, and MacOS (via Docker Desktop app)

With over 5 million Dockerized applications, a huge ecosystem exists around Docker including public/private registries, orchestrators like Kubernetes, and third-party management tools.

Podman Overview

Podman was developed by Red Hat as a daemonless alternative to Docker for running OCI-compliant containers and pods. Some notable aspects of Podman:

• Written in Go and released in late 2018
• Manages containers using the libpod library rather than central daemon
• Runs containers and pods as child processes using a fork-exec model
• Restricts container permissions to the Podman user via user namespaces
• Currently limited to Linux hosts (clients for other OSes in progress)
• Integrates with systemd for container monitoring and management
• Supports Docker and OCI image formats

While Docker remains dominant, Podman adoption is growing especially on Red Hat Enterprise Linux due to its security and simplicity advantages. But it lacks some of Docker‘s maturity and ecosystem support.

Architectural Differences

The core architectural differences between Docker and Podman stem from Docker‘s use of a daemon versus Podman‘s fork-exec approach.

Docker Architecture

Docker utilizes a client-server architecture comprised of the Docker CLI client, Docker daemon, and containerd (formerly runc):

• Docker daemon – persistent background process and API for managing images, containers, networks and volumes
• containerd – lightweight runtime that handles low-level container lifecycle operations
• Docker client – CLI tool that communicates with daemon via API to direct container management

Docker architecture (image credit: Docker)

Clients issue commands like docker run which get converted to API requests handled by the daemon. The daemon then interfaces with containerd to perform container actions.

Podman Architecture

Rather than a centralized daemon, Podman uses independent tools that utilize the libpod library:

• libpod – library that handles lifecycle and runtime operations for containers and pods
• podman – CLI tool that calls libpod to manage pods, containers, images, networks, etc.
• buildah – tool for building OCI container images

Podman architecture (image credit: Red Hat)

When the podman command runs, it forks and execs container processes directly via libpod. Containers are child processes of the Podman parent.

This daemonless architecture has security and robustness advantages but lacks some consistency and coordination capabilities.

Key Comparison of Capabilities

Now that we‘ve looked at the architectural differences, let‘s explore how Docker and Podman compare across key functionality areas:

Container Networking

Out of the box, Docker offers seamless networking between containers using libnetwork overlay driver networks. This enables simple networking of distributed apps across multiple Docker hosts.

Podman networking is much more limited. It provides a simple bridge network per host by default. But implementing cross-host and cross-container networking requires third-party Container Network Interface (CNI) plugins.

Security

Podman implements user namespaces isolating container permissions to the user running Podman. This prevents containers from gaining root access to modify the host OS.

Docker does not restrict container capabilities by default. Starting the daemon requires root access, so containers inherit full privileges. Docker has added user namespace support, but this must be explicitly configured unlike Podman‘s default implementation.

Additionally, Podman‘s architecture prevents it from needing to modify host firewall rules for inter-container communication.

Platform Support

Docker supports Linux, Windows Server and macOS for both Docker hosts and clients. This allows Windows and macOS developers to run Docker natively via Docker Desktop.

Podman only runs on Linux currently. Podman remote clients are under development for macOS and Windows but not production-ready yet.

Ecosystem

Docker has a far richer ecosystem of tools and integrations available. For example:

• Registries – Docker Hub provides a public registry of container images. Docker also enables deploying private registries.

• Orchestration – Kubernetes is the de facto Docker orchestrator. Docker Swarm provides simple container orchestration out of the box.

• Desktop – Docker Desktop provides a polished desktop experience for Windows and macOS.

• CLI Tools – Docker Compose simplifies multi-container management.

Podman has fewer complementary tools. It does integrate with Kubernetes. Podman-compose provides similarity to Docker Compose. Overall, Docker has more prebuilt integrations and third-party support.

Maturity and Adoption

Docker‘s first-mover advantage gives it a 5+ year head start on community growth and adoption.

As of 2022, an estimated 4 million applications have been Dockerized making Docker the de facto standard for containerization. Major cloud providers all support Docker runtimes and workflows.

Podman adoption is growing, especially among Red Hat Enterprise Linux users since Docker support was replaced with Podman. But Podman still has a smaller user base and community support overall.

When Should You Use Docker vs Podman?

Now that we‘ve compared the Docker and Podman architectures and capabilities, when should you actually use each in practice?

Docker is the best choice for:

• Complex microservices apps that require networking between many containers
• Multi-OS environments (e.g. mix of Linux and Windows hosts)
• Workflows where Kubernetes orchestration will be implemented
• Teams wanting to leverage Docker‘s rich ecosystem and community resources

Podman is optimal for:

• Small-scale container deployments limited to just Linux hosts
• Use cases where avoiding a daemon process and root privileges are critical
• Red Hat Enterprise Linux workloads where Docker support was deprecated
• Situations where you want to avoid depending on the large Docker ecosystem

For most production container environments, Docker remains the standard and benefits from more implementations in the wild. But for particular Linux-centric or security-focused use cases, Podman offers a compelling daemonless alternative.

Migrating from Docker to Podman

For Linux users, transitioning fully from Docker to Podman requires moving container workloads over and adopting new workflows.

At the CLI, commands can often be interchanged by aliasing docker to podman. But Docker Compose, Docker Desktop and other ecosystem tools will need to be replaced.

Podman generates artifacts like pod specs that can assist with orchestrating via Kubernetes. But a full Kubernetes platform will still provide significant advantages over Podman alone.

So while Docker assets can be incrementally migrated to Podman, losing Docker‘s ecosystem should be carefully considered. Container management is often simplest by standardizing on just Docker or Podman across the organization.

Key Takeaways: Docker vs Podman

• Docker popularized software containers and remains the industry standard runtime.

• Podman offers a daemonless alternative with security advantages on Linux.

• Docker shines for complex containerized environments and cross-OS support.

• Podman is ideal for simpler Linux-based container needs.

• Migrating fully from Docker to Podman requires adapting tools and workflows.

Rather than a Docker versus Podman decision, the best approach is understanding the strengths of each. For most container use cases today, Docker is still the right choice. But Podman offers a compelling option for particular Linux and security-focused container workloads.