Containers vs. VMs: Which Should You Use?

Both containers and virtual machines are virtual environments that comprise a number of computing components and are independent in nature, thereby allowing developers to scale applications in isolated runtimes. Both of these concepts aim at providing independent sets of resources to individual computing environments to ensure quick and reliable application performance. However, even though containers and virtual machines both facilitate the more efficient use of resources, the two differ in a lot of aspects. In the article below we’ll walk through an overview of each, and then take a deeper dive into the differences that set these virtual environments apart.

What is a Container?

What is a Container?
How do Containers Work?
Container Architecture

What is a Virtual Machine?
How do Virtual Machines Work?
Virtual Machine Architecture

Key Differences between Containers and VMs
Which One Should I Use?

What is a Container?

A container is a packaging mechanism -- consisting of application code alongside all of its dependencies, libraries, configuration files, binary code, etc. -- that decouples applications from the environments within which they run in order to facilitate cross-environment efficiency. Containers virtualize an operating system (operating system) and, by doing so, allow developers to run their applications without worrying about operating system specifications or the runtime environment that the host machine has. They can be used to run anything from a small micro-service to a large-scale application and, because they don’t contain the operating system kernel, they’re lighter weight than other forms of virtualization, including virtual machines.

Additionally, because containers are an abstraction on the application level and package only the application code and dependencies, machines can run multiple containers at the same time while sharing available hardware resources with all of them. Overall, containers are highly valued for the streamlining they provide while redeploying applications from a developer’s local machine to the production environment.

How do Containers Work?

Containers contain a copy of the file system, environment variables, libraries, etc. of the guest operating system without containing the actual operating system kernel. Each container has its own fresh copy of the system variables, which satisfies the need for isolation of runtimes by allowing modification of one container’s variables to leave variables in other containers unchanged.

At any given time there can be tens or hundreds of containers running on a single host, and the lifecycle of these simultaneous containers can be managed in a centralized way with a container engine. Container engines organize and facilitate communication between all containers running on a system, keeping track of those that have been launched, those that have been shut down, and by providing the ability to launch or shut down containers as needed. Containers managed within a container engine helps developers break larger applications down into smaller micro-applications, which increases the overall efficiency of application development and maintenance. 

Containers are constituted via “images” or a bunch of files that define a container’s details, such as: the environment, the application, the dependencies, etc. This opens up a lot of possibilities in domains for bug tracking and dependency management by allowing containers to be both easily shared across development platforms and compared against one another.

Container Architecture

undefined
The container architecture of deploying multiple applications.

As shown in the diagram, the containers share the same Host operating system, or kernel. The container engine is responsible for setting the resource limits and managing the lifecycle of the containers. Because all of the containers share the same kernel, they are much lighter weight than virtual machines.

What is a Virtual Machine?

A virtual machine, on the flip side, is an imitation of the actual hardware associated with a computing system. Virtual machines are meant to emulate an exact computing environment with a separate, independent operating system and segregated access to actual hardware. Virtual machines allow you to “clone” a computer system by creating a copy of the file system, the system kernel, and a virtual copy of the system’s hardware resources.

How do Virtual Machines Work?

Similar to how a container engine manages and governs multiple containers, a hypervisor serves as the managing authority of virtual machines. Analogous to how a container engine sits between the host operating system and the containers, a hypervisor sits between the host operating system and the guest operating systems. This implies that, in a system composed of virtual machines, there is one set of hardware resources, which are controlled and managed by a host operating system via the hypervisor kernel, and this host operating system houses a hypervisor process that manages launching and destroying virtual machines.

Each of these virtual machines has its own set of virtual hardware resources. Internally, they access the same set of physical hardware resources attached to the host operating system, and the access is moderated by the hypervisor process.

Virtualized hardware resources with independent operating system images provide greater freedom for  individual runtimes, but are also more demanding on the system as a larger number of resource-intensive operating systems have to be supported on the same physical hardware.

Virtual Machine Architecture

undefined
The virtual machine architecture of deploying multiple environments.

Contrary to the container architecture above, virtual machine (VM) architecture attaches an additional layer of guest operating system to every separate runtime. This additional layer is responsible for providing direct access to a simulated set of hardware resources for its applications, which means that -- in addition to binaries, libraries, system variables, etc. unique to each runtime -- an instance of a full-fledged kernel is also attached.

Key Differences between Containers and VMs

Containers are application images; VMs are emulated machine environments

To summarize the above definitions of both containers and virtual machines: containers are a group of files that save environment data about applications (more often known as “application images”), while virtual machines store an entire computer system with a copy of the kernel as well. 

Owing to this, deploying containers in a cloud environment is a much simpler process as developers only have to take care of their application and not the underlying runtime environment. Developers can package their dependencies and app state as in development, and it will be scaled exactly the same in production. With virtual machines, however, developers must provision a virtual machine in the cloud and maintain the runtime environment manually. 

Containers are extremely lightweight; VMs measure in Gigabytes

Keeping in mind the fact that containers are images while virtual machines comprise actual operating systems, it can be understood that this addition of an operating system layer adds a huge chunk of weight to the virtual machine instances. For this reason, virtual machines are much less compact than containers, positioning containers as the preferred choice in situations that require the frequent transfer of source data. Since containers only add runtime information to an application bundle, and are therefore mostly the same size as the underlying applications, they are much easier to port. This lighter weight means that deploying a container from a local development environment to a remote production environment is much simpler than transferring an entire virtual machine from a local environment to a remote one.

Containers are quick to load; VMs are slow and heavy

When considering the actual operating systems in the case of virtual machines, lifecycle event durations associated with kernels must be considered as well. Virtual machines are very slow in this aspect, as they incur the same boot-up time, installation, and maintenance requirements of a normal operating system. Containers, on the other hand, are just applications wrapped with environment data, making their load time very similar to the raw application’s load time and much faster overall.

Containers are expected to be more secure than VMs

Conventional applications inside a virtual machine are not isolated from each other in any sense. This leaves room for malicious applications to be installed on virtual machines as a way to breach the target application. In the case of containerization, each application is isolated from others, be it on a virtual machine or a real one. 

A major part of containerization security reinforcements arise due to the ease with which containers can be maintained. CI/CD pipelines can be configured to update the images with new artifacts as soon as they’re published, and this ease of maintenance promotes highly frequent updates and checks. This in turn ensures that the packages used in the application are up-to-date, cutting off a very prominent source of vulnerability in applications. With virtual machines, however, proper administration is required to keep the environment up-to-date and secure.

Which One Should I Use?

The answer to this question depends on the specific requirements at hand. Is portability a priority in your development cycle? Are you looking to push frequent updates? Do you want a convenient CI/CD configuration for your development life cycle? Do you want to build an application that heavily depends on loosely bound micro-services? If you answer yes to most of these questions, containerization is likely the best choice.

On the other hand, if your application needs control over hardware resources, or involves running a different operating system on top of the host, virtual machines might be a better option.

All in all, both of these techniques can be used interchangeably in most cases, and a final decision will rely largely on project requirements. However, because containers offer features most relevant to the contemporary development lifecycle, containerization is largely recognized as both the most efficient and secure option.