Docker Container Fundamentals

Introduction

Containers have become an increasingly popular choice for deploying and managing applications in recent years thanks to their portability, scalability, and ease of use.

In this article, we will delve into the technologies and concepts that make containers possible, including control groups, namespaces, Seccomp, and union file systems. We will also discuss the benefits of using containers and provide examples of common use cases for containers.

Control Groups

Control groups, or cgroups, are a feature in the Linux kernel that helps manage and control the usage of resources such as CPU, memory, and disk I/O by a group of processes. They are often used in containers to limit the resources available to certain processes and ensure that they do not consume too much of a particular resource.

In a Kubernetes environment, cgroups can be used to set limits on resources like CPU and memory for pods and ensure that they receive the appropriate Quality of Service (QoS).

In short, cgroups are a way to manage and control the resource usage of processes on a Linux system.

Namespaces

Namespaces are used to create a layer of isolation around the processes within a container, giving them their own "view" of the Linux filesystem. This helps to limit the resources that are available to the processes within the container, restricting their access to certain parts of the system.

There are several different types of namespaces in the Linux kernel that are used by containerization tools like Docker and Podman when creating containers. These namespaces include user, mnt, uts, ipc, pid, and net.

They work together to create a boundary around the processes within the container, limiting their access to certain resources and ensuring that they do not interfere with other processes on the system.

Seccomp

Seccomp stands for secure computing and is a feature in Linux that is used to limit the system calls that an application is allowed to make. It can be used to provide containers with access to only the resources that they need while blocking access to other resources that are not relevant.

For example, if a container does not need to change the time on the host machine, then it may not need access to the clock_adjtime and clock_settime system calls. Similarly, if a container does not need to change kernel modules, it may not need access to the create_module and delete_module system calls.

By using seccomp, you can further isolate and secure the processes within a container. The default seccomp profile for Docker, for instance, disables around 44 system calls out of the approximately 300 available.

Union File Systems

A union file system in Linux is a type of file system that combines the contents of multiple directories into a single view. This means that you can overlay one directory on top of another and access both as if they were a single directory.

Union file systems are useful when you want to access multiple directories in a unified way or when you want to make changes to a directory without modifying the original files. There are several different types of union file systems available in Linux, each with its specific features and capabilities.

Benefits of Containers

There are several benefits to using containers in application development :

• Portability :
  Containers allow developers to package an application and all its dependencies into a single container image, which can be easily moved between different environments (e.g. development, staging, production).
• Isolation :
  Containers provide a level of isolation for applications, allowing them to run independently of other applications on the same host. This can help to prevent conflicts and reduce the risk of errors.
• Resource efficiency :
  Containers allow developers to allocate specific resources (e.g. CPU, memory) to an application, ensuring that it has the resources it needs to run smoothly. This can help to improve the performance of the application.

Overall, using containers in application development can help to improve the portability, isolation, resource efficiency, ease of deployment, and security of applications.

Container Use Cases

Containers are commonly used in a variety of different scenarios, including :

• Application development and deployment :
  Containers are often used for packaging and deploying applications in a consistent, repeatable manner. This can help to improve the portability, isolation, and resource efficiency of applications.
• Microservices :
  Containers are often used to deploy microservices, which are small, independent units of functionality that can be easily scaled and managed.
• Continuous integration and delivery (CI/CD) :
  Containers can be used to automate the build, test, and deployment processes for software applications, helping to improve the speed and reliability of the delivery pipeline.

Overall, containers are used in a variety of different scenarios to improve the portability, isolation, resource efficiency, and deployment of applications.