Containers are quite an old technology and existed in the form of chroot and FreeBSD Jails. Most of us have already used containers in some form or other. The rise of Docker gave containers the required adoption and popularity. Ubuntu has also released a new tool named LXD with Ubuntu 15.04.
A container is a lightweight virtual environment that contains a process or set of processes. You might already have used containers with chroot. Just as with containers, we create an isolated virtual environment to group and isolate a set of processes. The processes running inside the container are isolated from the base operating system environment, as well as other containers running on the same host. Such processes cannot access or modify anything outside the container. A recent development in the Linux kernel to support namespaces and cgroups has enabled containers to provide better isolation and resource-management capabilities.
One of the reasons for the widespread adoption of containers is the difference between containers and hypervisor-based virtualization, and the inefficiencies associated with virtual machines. A VM requires its own kernel, whereas containers share the kernel with the host, resulting in a fast and lightweight isolated environment. Sharing the kernel removes much of the overhead of VMs and improves resource utilization, as processes communicate with a single shared kernel. You can think of containers as OS-level virtualization.
With containers, the entire application can be started within milliseconds, compared to virtual minutes. Additionally, the image size becomes much smaller, resulting in easier and faster cloud deployments. The shared operating system results in smaller footprints, and saved resources can be used to run additional containers on the same host. It is normal to run hundreds of containers on your laptop.
However, containerization also has its own shortcomings. First, you cannot run cross-platform containers. That is, containers must use the same kernel as the host. You cannot run Windows containers on a Linux host, and vice versa. Second, the isolation and security is not as strong as hypervisor-based virtualization. Containers are largely divided into two categories: OS containers and application containers. As the name suggests, application containers are designed to host a single service or application. Docker is an application container. You can still run multiple processes in Docker, but it is designed to host a single process.
OS containers, on the other hand, can be compared to virtual machines. They provide user space isolation. You can install and run multiple applications and run multiple processes inside OS containers. LXC on Linux and Jails on BSD are examples of OS containers.
In this article, we will take a look at LXC, an OS container, and Docker, an application container. In the first part of the article, we will learn how to install LXC and deploy a containerized virtual machine. In subsequent recipes, we will work with Docker and related technologies. We will learn to create and deploy a container with Docker.
