Virtual machines vs Containers

Virtual machines and containers are pretty similar, but they do possess some important differences. These differences will dictate which ones you decide to use. So, when you ask a question like 'virtual machines vs containers' there isn't necessarily going to be an outright winner - but there might be a winner for you in a given scenario.

Let's take a look at what a virtual machine is, exactly, what a container is, and how they compare - as well as the key differences between the two.

What is a virtual machine?

Virtual machines are a product of hardware virtualization. They sit on top of physical machines with the hypervisor or virtual machine manager in between, acting as a layer of abstraction between the virtual machine and the underlying hardware.

A virtualized physical machine can host multiple virtual machines, enabling better hardware utilization. Since the hypervisor abstracts the physical machine's hardware, it allows virtual machines to use a different operating system on the same host machine. The host operating system and virtual machine operating system run their own kernel.

All the communication between the virtual machines and the host machine occurs through the hypervisor, resulting in high level of isolation. This means if one virtual machine crashes, it would not affect other virtual machines running on the same physical machine.

Although the hypervisor's abstraction layer offers a high level of isolation, it also affects the performance. This problem can be solved by using a different virtualization technique.

What is a container?

Containers use lightweight operating system level virtualization. Similar to virtual machines, multiple containers can run on the same host machine. However, containers do not have their own kernel. They share the host machine's kernel, making them much smaller in size compared to virtual machines. They use process level isolation, allowing processes inside a container to be isolated from other containers.

The difference between virtual machines and containers

In his post Containers are not VMs, Mike Coleman use the analogy of houses and apartment buildings to compare virtual machines and containers.

Self-contained houses have their own infrastructure while apartments are built around shared infrastructure. Similarly, virtual machines have their own operating system, with kernel, binaries, libraries etc. While containers share the host operating system kernel with other containers.

Due to this, containers are much smaller in size allowing a physical machine to host more containers than virtual machines. Since containers use lightweight operating system level virtualization instead of a hypervisor, they are less resource intensive compared to virtual machines and offer better performance.

Compared to virtual machines, containers are faster, quicker to provision, and easy to scale. As spinning a new container is quick and easy when a patch or an update is required, it is easy to start a new container and stop the old one instead of updating a running container. This allows us to build immutable infrastructure, which is reliable, portable and easy to scale.

All of this makes containers a preferred choice for application deployment, especially with the teams that are using micro-services or similar architecture, where an application is composed of multiple small services instead of a monolith.

In microservice architecture, an application is built as a suite of independent, self-contained services. This allows the teams to work independent of each other and deliver features quicker. However, decomposing applications into multiple parts adds operational complexity and overhead. Containers solve this problem.

Containers can serve as a building block in the microservice world where each service can be packaged and deployed as a container. A container will have everything that is required to run a service, this includes service code, its dependencies, configuration files, libraries etc.

Packaging a service and all its dependencies as a container makes it easy to distribute and deploy a service. Since the container includes everything that is required to run a service, it can be deployed reliably in different environments. A service packaged as a container will run the same way locally on a developer's machine, in a test environment, and in production.

However, there are things to consider when using containers. Containers share the kernel and other components of the host operating system. This makes them less isolated compared to virtual machines, and thus less secure.

Since each virtual machine has its own kernel, we can run virtual machines with a different operating system on the same physical machine. However since containers share the host operating system kernel, only the guest operating system that can work with the host operating system can be installed in a container.

Virtual machines vs containers - in conclusion...

Compared to virtual machines, containers are lightweight, performant and easy to provision. While containers seem to be the obvious choice to build and deploy applications, virtual machines have their own advantages. Compared to physical machines, virtual machines have the better tooling and are easier to automate.

Virtual machines and containers can co-exist. Organizations with existing infrastructure built around virtual machines can take the benefits of containers by deploying them on virtual machines.