OS virtualization

The topic of this book is associated with OS virtualization. OS virtualization enables the kernel to be shared across multiple processes inside a single VM with isolation. OS virtualization is also called user-mode or user-space virtualization as it is one level up from the kernel. Inpidual user-space instances are called containers. The kernel provides all the features for resource management across containers.

This is highly helpful while consolidating a set of services spread across multiple servers into a single server. Few benefits of OS virtualization are high security due to reduced surface of contact for a breach or viruses, better resource management, easy migration of applications or services across hosts, and also instant and dynamic load balancing. OS virtualization does not require any hardware support, so it is easy to implement than other technologies. The most recent implementations of OS virtualization are Linux LXC, Docker, and Windows Server Containers.
This is what OS virtualization looks like:

Today's containers are not yet cross platform, which means a Linux container cannot be directly ported to Windows. Containers being an OS virtualization are tied up to the kernel features, this makes it difficult to pursue cross platform portability.