What are Linux Containers?

February 23,2021 - 12 min read
What are Linux Containers?

Containers have been widely used in the past couple of years. Linux containers, in particular, have become all the more popular. Containers are most commonly used in data centres and cloud computing environments, for example, Docker and Kubernetes.

There has also been a rise in a couple of container-based Linux distributions for embedded systems also. balenaOS and Linux microPlatform are some popular examples.

In order to comprehend the significance of Linux Containers, it is important to get a thorough understanding of containers, the problems they solve and the technologies involved. In this blog, we would talk about containers in general and then start our journey of exploring Linux containers. Let’s get started!

What are Containers?

Containers are basically a type of operating system virtualization. One container can be used to run several things, for example, a small microservice, software process or even a large application.

A container, in cloud computing, consists of all necessary executables, binary code, libraries, and even configuration files but not operating system images. Containers are lightweight, portable and also have significantly less overhead.

When dealing with the deployment of larger applications, multiple containers may be deployed as one or more container clusters. There are container orchestrators that manage the container clusters.

Benefits of Containers

We hope the concept of containers is crystal clear in your mind. It’s time to discuss the advantages that containers bring with themselves. Let’s enlist the various benefits of containers in computer science.

Containers build, test, deploy, and redeploy applications in multiple environments. The application is usually on a developer’s local space and is deployed to an on-premises data centre and even the cloud.

Containers have less overhead as they require fewer system resources compared to traditional hardware virtual machine environments. The main reason behind less overhead is that containers don’t include operating system images.

Containers have more portability. Applications running in containers are easily deployed to different operating systems as well as hardware platforms.

With Containers, the operations are more consistent. Applications in containers run the same, regardless of where they are deployed.

Development with containers is more efficient. Applications are rapidly deployed, patched and scaled. With containers, you can adapt agile and DevOps methodologies to accelerate application development.

As we have understood the concept of containers, let’s get to another field in which containers are being rapidly used. In the next segment, we will discuss the use of containers in embedded systems.

Embedded Systems

The concept of containers is not new; however, they are now being used in other domains in recent years. Several container solutions are now being used for embedded Linux. Such examples include balenaOS, Linux microPlatforms, Pantahub and Torizon.

However, there are still some challenges that the technology of containers needs to face in the embedded field. There is a lack of hardware resources required to run containers on embedded systems. There are issues of packaging a cross-compiled application into a container image and also managing the license of software artefacts, operating system etc.

Now that we have understood the concept of containers along with the containers for embedded systems, it is time to proceed to the concept of Linux containers, its concept, implementation, popularity and use cases.

Linux Containers

A Linux container only has the essential software components required to run one or more than one application. The kernel can run the container isolated from the system. It is basically a minimal filesystem.

Without Linux Containers, one had to distribute applications to environments with different configurations. Linux containers solve this problem very well. Also, as mentioned above, a container is lighter and requires fewer resources when compared to a virtual machine solution. The reason is that a container image is smaller than a virtual machine image and the kernel is shared with other processes and containers running on the same operating system.

A Linux container is an instance of the userspace layer. A number of resources are allocated to the Linux container and it runs in isolation from the rest of the system. A container is able to run just one application or the entire root filesystem, however, multiple containers can be started and run at the same time.

There are some important commands to know when reading about containers. If the terminal inside a container is accessed and the ps command is run, the result is the container processes. The result of the mount command is the mount points and the ls command displays the container filesystem(s). The reboot command is used to restart container applications will restart and probably in less than a second.

Now that we have understood the basic concept of Linux containers, in the next few paragraphs we will try to get a hang of the problem that Linux containers solve.

Think about an application that you are developing which has multiple dependencies on libraries, configuration files or even on some other applications. After you have developed this application, the next evident step would be to distribute this application to different environments. For instance, you have developed this application on Ubuntu 18.04 but you want to deploy this application on machines running on Fedora, Debian or some other version of Ubuntu. In order to successfully deploy your application, you will have to ensure that the execution environments of these different distributions match the dependencies of the application you have deployed.

This sounds like a problem and it is one. The source code of the developed application can be distributed along with documentation describing the installation process and a build system like autotools or cmake to that would verify and then notify the user if any dependency is not met.

The solution sounds good but it is not as easy as it sounds. It requires a lot of work to create and maintain the build system configuration files as the application is expected to change over time. Another problem of this solution is that the documentation of every single detail of the installation process of the application and its dependencies is a tedious task. Also, in this method, the user is forced to install a number of dependencies that they might not need but these dependencies can cause problems in their environment.

The main issue with this solution is the lack of isolation between the runtime environment and the application. It makes the process of deploy applications on GNU/Linux distributions difficult.

This problem can be solved by a virtual machine. It is a good way out, but the disadvantage is that virtual machine is heavy and the resource consumption is also very high as it runs a fully isolated instance of the operating system.

The solution to all these problems is simple and that is how Linux containers come into picture. The root filesystem is run isolated from the system, over the same kernel and all applications and libraries not needed are removed from the root filesystem.

Benefits of Linux Containers

I hope the concept of Linux container is now clear in your mind. In this segment, we will discuss the various advantages of Linux containers.

As discussed above, containers facilitate the distribution of applications in totally different environments. But, along with easy distribution, Linux containers also simplify the entire process of updating applications. In order to update the application and its dependencies, you only need to update the container image.

Also, with Linux containers, you can run and control multiple instances at the same time. Also, containers can run completely isolated from the operating system and hence the security of the system is quite high. The biggest advantage of using a Linux container is good performance and less resource-intensive when compared to a virtual machine solution.

In the next segment, we will discuss the implementation of Linux Containers.

Implementation of Linux Containers

Several Linux kernel features, including cgroups and namespaceS are used in the implementation of containers. cgroups are known as control groups, it is an interesting Linux kernel feature that allows the partition of system resources. Next feature, namespaceS isolate the execution of a process on Linux.

The features mentioned above and a few others are integral to the implementation of Linux container. The creation of a completely isolated execution environment for applications on Linux is possible with the help of different tools like LCX, systemd-nspawn, Docker etc.

Let’s discuss each of these tools in possible. The first one is LXC which is a userspace interface for the Linux kernel containment features and allows Linux users to easily create and manage system or application containers with the help of simple command-line tools.

Another tool is Systemd-nspawn which is a part of the system. It is a very simple and effective command-line tool that helps in running applications and full root filesystems inside containers.

The next tool that we mentioned is Docker. It is probably the most popular container management tool. Docker is friendly and has more features compared to other tools we discussed, LXC and systemd-nspawn.

Kubernetes is an open-source container-orchestration system. It is used for automating the deployment of applications along with its scaling and management. Docker and LXC create and run containers but Kubernetes can automate the management of containers.

Currently, there are two standard specifications. One is runtime specification which defines the standards for managing container execution. Another is image specification which standardizes the format of container images.

The blog above was an earnest effort to explain the concept of containers and Linux containers in particular. We hope that it helps you understand one of the most important technologies in today’s world.

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