Overview of Container Architecture

Objectives

After completing this section, you should be able to:

  • Describe the architecture of Linux containers.
  • Describe the podman tool for the managing containers.

Introducing Container History

Containers have quickly gained popularity in recent years. However, the technology behind containers has been around for a relatively long time. In 2001, Linux introduced a project named VServer. VServer was the first attempt at running complete sets of processes inside a single server with a high degree of isolation.

From VServer, the idea of isolated processes further evolved and became formalized around the following features of the Linux kernel:

Namespaces

A namespace isolates specific system resources usually visible to all processes. Inside a namespace, only processes that are members of that namespace can see those resources. Namespaces can include resources like network interfaces, the process ID list, mount points, IPC resources, and the system’s hostname information.

Control groups (cgroups)

Control groups partition sets of processes and their children into groups to manage and limit the resources they consume. Control groups place restrictions on the amount of system resources processes might use. Those restrictions keep one process from using too many resources on the host.

Seccomp

Developed in 2005 and introduced to containers circa 2014, Seccomp limits how processes could use system calls. Seccomp defines a security profile for processes, whitelisting the system calls, parameters, and file descriptors they are allowed to use.

SELinux

Security-Enhanced Linux (SELinux) is a mandatory access control system for processes. Linux kernel uses SELinux to protect processes from each other and to protect the host system from its running processes. Processes run as a confined SELinux type that has limited access to host system resources.

All of these innovations and features focus on a basic concept: enabling processes to run isolated while still accessing system resources. This concept is the foundation of container technology and the basis for all container implementations. Nowadays, containers are processes in the Linux kernel making use of those security features to create an isolated environment. This environment forbids isolated processes from misusing system or other container resources.

A common use case of containers is having several replicas of the same service (for example, a database server) in the same host. Each replica has isolated resources (file system, ports, memory), so there is no need for the service to handle resource sharing. Isolation guarantees that a malfunctioning or harmful service does not impact other services or containers in the same host, nor in the underlying system.

Describing Linux Container Architecture

From the Linux kernel perspective, a container is a process with restrictions. However, instead of running a single binary file, a container runs an image. An image is a file-system bundle that contains all dependencies required to execute a process: files in the file system, installed packages, available resources, running processes, and kernel modules.

Like executable files are the foundation for running processes, images are the foundation for running containers. Running containers use an immutable view of the image, allowing multiple containers to reuse the same image simultaneously. As images are files, they can be managed by versioning systems, improving automation on container and image provisioning.

Container images need to be locally available for the container runtime to execute them, but the images are usually stored and maintained in an image repository. An image repository is just a service — public or private — where images can be stored, searched, and retrieved. Other features provided by image repositories are remote access, image metadata, authorization or image version control.

There are many different image repositories available, each one offering different features:

Managing Containers with Podman

Containers, images, and image registries need to be able to interact with each other. For example, you need to be able to build images and put them into image registries. You also need to be able to retrieve an image from the image registry and build a container from that image.

Podman is an open-source tool for managing containers and container images and interacting with image registries. It offers the following key features:

  • It uses the image format specified by the Open Container Initiative (OCI). Those specifications define a standard, community-driven, non-proprietary image format.
  • Podman stores local images in the local file system. Doing so avoids unnecessary client/server architecture or having daemons running on the local machines.
  • Podman follows the same command patterns as the Docker CLI, so there is no need to learn a new toolset.
  • Podman is compatible with Kubernetes. Kubernetes can use Podman to manage its containers.

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