Containerization

Containerization or OS-level virtualization, according to the Wikipedia:

OS-level virtualization refers to an operating system paradigm in which the kernel allows the existence of multiple isolated user space instances.

And it continues:

Such instances [...] may look like real computers from the point of view of programs running in them. A computer program running on an ordinary operating system can see all resources (connected devices, files and folders, network shares, CPU power, quantifiable hardware capabilities) of that computer. However, programs running inside of a container can only see the container's contents and devices assigned to the container.

In general, this is where Docker (formerly dotCloud) found a developer focused niche. The UX Docker introduced allowed developers to:

• Define container images in a reproducible and in a very easy way
• Distribute these container images in a so-called registry
• Let developers run containers, based on container images:
  • Previously distributed to a shared registry
  • Locally built

As a result of this, it was much easier to reduce the friction and the dependency between the application and the system it was running on, because as with happens with virtual machine images, the container image includes all operating system components, except for the kernel.

Still, the friction was not completely removed. Many containerized applications expect environment variables, or certain volume mounting to be set up, else they won't work as expected.

In any case, containerization and specially the UX dotCloud introduced helped to make reproducible builds and environments easier with a greatly reduced friction surface.

Our containers can also run on top of virtualized machines: