Overview of docker architecture

At the very start of the IT revolution, most applications were deployed directly on physical hardware, over the host OS. Because of that single user space, runtime was shared between applications. The deployment was stable, hardware-centric, and had a long maintenance cycle. It was mostly managed by an IT department and gave a lot less flexibility to developers. In such cases, hardware resources were regularly underutilized.

The following diagram shows a Traditional application deployment:

The introduction of the virtual machines (VMs) we emulated the hardware for virtual machines (VMs) and deployed a guest OS on each virtual machine. With virtualization, applications are isolated at VM level and defined by the life cycle of VMs. This gives better return on investment and higher flexibility at the cost of increased complexity and redundancy.

The following diagram shows an application deployment with VMs:

After virtualization, we are now moving towards more application-centric IT. We have removed the hypervisor layer to reduce hardware emulation and complexity. The applications are packaged with their runtime environment and are deployed using containers. Containers are also considered less secure than VMs, because with containers, everything runs on the host OS. If a container gets compromised, then it might be possible to get full access to the host OS. It can be a bit too complex to set up, manage, and automate. These are a few reasons why we have not seen the mass adoption of containers in the last few years, even though we had the technology.

With Docker, containers suddenly became first-class citizens. All big corporations such as Google, Microsoft, Red Hat, IBM, and others are now working to make containers mainstream. Docker was started as an internal project by Solomon Hykes, who is the current CTO of Docker, Inc., at dotCloud. It was released as open source in March 2013 under the Apache 2.0 license. With dotCloud's platform as a service experience, the founders and engineers of Docker were aware of the challenges of running containers. So with Docker, they developed a standard way to manage containers.