What is Docker?
Docker is an open-source project for automating the deployment of applications as portable, self-sufficient containers that can run on the cloud or on-premises. Docker is also a company that promotes and evolves this technology, working in collaboration with cloud, Linux, and Windows vendors, including Microsoft.
Docker containers can run anywhere, on-premises in the customer datacenter, in an external service provider or in the cloud, on Azure. Docker image containers can run natively on Linux and Windows. However, Windows images can run only on Windows hosts and Linux images can run on Linux hosts and Windows hosts (using a Hyper-V Linux VM, so far), where host means a server or a VM.
Developers can use development environments on Windows, Linux, or macOS. On the development computer, the developer runs a Docker host where Docker images are deployed, including the app and its dependencies. Developers who work on Linux or on macOS use a Docker host that is Linux based, and they can create images only for Linux containers. (Developers working on macOS can edit code or run the Docker CLI from macOS, but as of the time of this writing, containers don't run directly on macOS.) Developers who work on Windows can create images for either Linux or Windows Containers.
To host containers in development environments and provide additional developer tools, Docker ships Docker Desktop for Windows or for macOS.
To run Windows Containers, there are two types of runtimes:
· Windows Server Containers provide application isolation through process and namespace isolation technology. A Windows Server Container shares a kernel with the container host and with all containers running on the host.
· Hyper-VContainers expand on the isolation provided by Windows Server Containers by running each container in a highly optimized virtual machine. In this configuration, the kernel of the container host isn't shared with the Hyper-V Containers, providing better isolation.
The images for these containers are created the same way and function the same. The difference is in how the container is created from the image running a Hyper-V Container requires an extra parameter
For VMs, there are three base layers in the host server, from the bottom-up: infrastructure, Host Operating System and a Hypervisor and on top of all that each VM has its own OS and all necessary libraries. For Docker, the host server only has the infrastructure and the OS and on top of that, the container engine, that keeps container isolated but sharing the base OS services.
