Containers Overview

From a technical perspective, the concept of a container is rooted in the Namespaces and Cgroups concept in Linux, but the term is also inspired by the actual metal cargo shipping containers that you see on seafaring ships. Both kinds of containers share the ability to isolate contents, maintain carrier independence, offer portability, and much more.

Containers are a logical packaging mechanism. YYou can think of containers as a lightweight virtualization that runs an application and its dependencies in the same operating system, but in different contexts that remove the need to replicate an entire OS as shown in Figure 1. By doing this the application is confined in a lightweight package that can be developed and tested individually, then implemented and scaled much faster than the traditional VM. Developers just need to build and configure this lightweight piece of software so that most of the application is containerized and publicly available without the need to manage and support the application per OS.

Figure 1: VM versus Containers

Many developers would call the container runtime shown in Figure 1.1 as the Hypervisor of Containers. Although this term is not technically correct, it may be useful in visualizing the hierarchy.

As in many VM technologies, the most common hypervisors are KVM and VMware ESX/ESXi. In container technologies, Docker and Rkt are the most common, with Docker being the most widely deployed. Let’s review some useful numbers in comparing VMs with containers.