To fully understand what Docker really is, first we need to understand the difference between traditional virtualization and containerization. Let's compare those two technologies now.
A traditional virtual machine, which represents the hardware-level virtualization, is basically a complete operating system running on top of the host operating system. There are two types of virtualization hypervisor: Type 1 and Type 2. Type 1 hypervisors provide server virtualization on bare metal hardware—there is no traditional end user's operating system. Type 2 hypervisors, on the other hand, are commonly used as a desktop virtualization—you run the virtualization engine on top of your own operating system. There are a lot of use cases that would take advantage of using virtualization—the biggest asset is that you can run many virtual machines with totally different operating systems on a single host.
Virtual machines are fully isolated, hence very secure. But nothing comes without a price. There are many drawbacks—they contain all the features that an operating system needs to have: device drivers, core system libraries, and so on. They are heavyweight, usually resource-hungry, and not so easy to set up—virtual machines require full installation. They require more computing resources to execute. To successfully run an application on a virtual machine, the hypervisor needs to first import the virtual machine and then power it up, and this takes time. Furthermore, their performance gets substantially degraded. As a result, only a few virtual machines can be provisioned and made available to work on a single machine.
The Docker software runs in an isolated environment called a Docker container. A Docker container is not a virtual machine in the popular sense. It represents operating system virtualization. While each virtual machine image runs on an independent guest OS, the Docker images run within the same operating system kernel. A container has its own filesystem and environment variables. It's self-sufficient. Because of the containers run within the same kernel, they utilize fewer system resources. The base container can be, and usually is, very lightweight. It's worth knowing that Docker containers are isolated not only from the underlying operating system, but from each other as well. There is no overhead related to a classic virtualization hypervisor and a guest operating system. This allows achieving almost bare metal, near native performance. The boot time of a dockerized application is usually very fast due to the low overhead of containers. It is also possible to speed up the roll-out of hundreds of application containers in seconds and to reduce the time taken provisioning your software.
the traditional virtualization engines. Be aware that containers cannot substitute virtual machines for all use cases. A thoughtful evaluation is still required to determine what is best for your application. Both solutions have their advantages. On one hand we have the fully isolated, secure virtual machine with average performance and on the other hand, we have the containers that are missing some of the key features (such as total isolation), but are equipped with high performance that can be provisioned swiftly. Let's see what other benefits you will get when using Docker containerization.
As you can see, Docker is quite different from the traditional virtualization engines. Be aware that containers are not substitutes for virtual machines for all use cases. A thoughtful evaluation is still required to determine what is best for your application. Both solutions have their advantages. On one hand we have the fully isolated, secure virtual machine with average performance, and on the other hand, we have containers that are missing some of the key features (such as total isolation), but are equipped with high performance and can be provisioned swiftly.
Let's see what other benefits you will get when using Docker containerization.