Containers are not revolutionary, ground-breaking concepts. They have been in action for quite a while. However, the world is witnessing the re-entry of containers, mainly due to the wide adoption of cloud computing. The shortcomings of traditional virtual machines in the cloud computing space also accelerated the use of containers. Container providers such as Docker simplified container technologies to a great extent, which also enabled a large adoption of container technologies in today's world. The recent popularity of DevOps and microservices also acted as a catalyst for the rebirth of container technologies.

So, what are containers? Containers provide private spaces on top of the operating system. This technique is also called operating system virtualization. In this approach, the kernel of the operating system provides isolated virtual spaces. Each of these virtual spaces is called a container or virtual engine (VE). Containers allow processes to run on an isolated environment on top of the host operating system. A representation of multiple containers running on the same host is shown as follows:

Containers are easy mechanisms to build, ship, and run compartmentalized software components. Generally, containers package all the binaries and libraries that are essential for running an application. Containers reserve their own filesystem, IP address, network interfaces, internal processes, namespaces, OS libraries, application binaries, dependencies, and other application configurations.

There are billions of containers used by organizations. Moreover, there are many large organizations heavily investing in container technologies. Docker is far ahead of the competition, supported by many large operating system vendors and cloud providers. Lmctfy, SystemdNspawn, Rocket, Drawbridge, LXD, Kurma, and Calico are some of the other containerization solutions. Open container specification is also under development.