When Do You Need A Container vs. Virtual Machines?

Choosing between virtual machines and containers is an application-dependent decision. Containers provide speed, have a smaller footprint and are commonly used for cloud-native applications (or microservice apps). On the other hand, although containers provide isolation, virtual machines take it a step further; they allow you to run multiple OS on a physical machine and are commonly used by legacy applications.

Although the response above provides a good starting point for understanding when to use these two technologies, there's a bigger picture to it, and we'll get into that in this article. You'll understand the underlying technology that powers virtual machines - hypervisors. By dissecting these individual technologies, you'll see how they both work, as well as their advantages and use cases. By the end of this article, choosing between both technologies would be a no-brainer decision.

What are Hypervisors?

Hypervisors, or Virtual Machine Monitor (VMM), are specialized software or firmware components that create and manage virtual machines (VMs). There’s still so much to them, so let’s go deeper.

They sit between the physical hardware of a computer and the virtualized operating systems, allowing multiple operating systems to run concurrently on a single machine. Simply put, it's what allocates and controls the sharing of a machine's hardware.

Virtualization is the process of creating virtual instances of physical computers or software components, allowing multiple virtual environments to run on a single physical server. Hypervisors carry out virtualization by abstracting the underlying hardware resources such as CPU, memory, storage, and network. These resources are then presented as virtualized resources to the guest VMs.

Hypervisors also ensure that each VM operates in its isolated environment, unaware of other VMs running on the same machine. This isolation prevents interference and conflicts between VMs.

There are two main types of hypervisors:

1. Type 1 Hypervisors (Bare Metal Hypervisors): These hypervisors run directly on the host's hardware to control the hardware and manage guest operating systems. They are installed directly on the physical hardware, eliminating the need for a separate operating system. Examples include VMware ESXi and Xen.
2. Type 2 Hypervisors (Hosted Hypervisors): These hypervisors run on top of an operating system, allowing multiple operating systems to run as guests on the host system. They are typically used for development, testing, or desktop virtualization scenarios. Examples include VMware Workstation and Oracle VirtualBox.

Overview of Virtual Machines and Containers

Before we go into these technologies, it's good to understand how things were before they came into being.

Traditionally, organizations deployed their applications on servers. These servers could run only one application. That means your database, your web server, etc, would all have their separate server.

At this time, operating systems didn't have the capability of running multiple applications securely on a single server. If you needed to add more applications, you'd need to deploy them separately instead of deploying them on the same server.

Running one application on one server turned out to be a waste of money because you couldn't take full advantage of a server's capability. Leading us to the idea of virtual machines.

Virtual Machines (VMs)

VMs emulate physical hardware, including CPU, memory, etc, allowing multiple virtual instances of operating systems to run simultaneously on a single physical machine.

Each VM runs their own operating system, which is fully encapsulated within a virtualized environment. This means that VMs contain not only the application but also a complete copy of the operating system, along with any necessary libraries and dependencies.

Figure 1: Virtual machine architecture

VMs are typically slower to start and consume a lot of RAM and CPU power from the server. This, in turn, led us to the idea of containers.

Containers

A container encapsulates an application along with all the files, configurations and dependencies necessary for it to run, but they do not contain a separate guest operating system.

They are software that shares its host operating system kernel and use operating system-level virtualization to provide isolated environments for running applications, unlike VMs, which use hardware-level virtualization.

Figure 2: Container architecture

Containers are lightweight and efficient, as they do not require the overhead of running multiple operating systems. They can be started quickly and consume fewer resources compared to VMs.

They rely on a container runtime, such as Docker or Kubernetes, which manages the lifecycle of containers, including creating, starting, stopping, and deleting them.

Two features that allow for containerization are Cgroups and Namespaces. Cgroups, also known as control groups, enable administrators to manage resource allocation and enforce resource limits for containers. Namespaces provide process isolation and separation of resources between containers, allowing each container to have its own isolated view of the system.

Similarities Between Virtual Machines and Containers

Although they seem two worlds apart, VMs and containers are both virtualization technologies used to create isolated environments for running applications. This means they have some resembling characteristics:

• Isolation: They both provide isolation, which allows applications to run in isolated environments without interfering with each other.
• Flexibility: They both offer flexibility, allowing for easy provisioning, scaling, and migration of applications.
• Application deployment: VMs and containers are used to deploy and manage applications in cloud environments and data centres.

Benefits of Using Virtual Machines

Although containers address most of the pitfalls of VMS, they won't replace them. The following are some of the benefits of using VMs:

1. Hardware Abstraction: VMs abstract physical hardware resources, allowing multiple virtual instances to share the same physical hardware. This abstraction enables the efficient utilization of hardware resources, as numerous VMs can run concurrently on a single physical machine without conflicts.
2. Isolation: VMs provide strong isolation between workloads, with each VM running its own guest operating system and applications. This isolation prevents interference and conflicts between VMs, ensuring that a failure or security breach in one VM does not affect others. It also enhances security by sandboxing applications within their own virtualized environments.
3. Flexibility: VMs are highly flexible in how they allow you to build your machine. You can create a machine specific to your application's requirements. With VMs, you can specify how much RAM you want, processors, etc.
4. Legacy Application Support: VMs are well-suited for running legacy applications that may be incompatible with newer hardware or operating system environments. Organizations can maintain compatibility with older software by encapsulating the entire operating system and application stack within a VM while benefiting from modern hardware and infrastructure advantages.

Popular Containerization Tools

Container technology has become increasingly popular, and some tools have been created to simplify the process of building, deploying, and managing containerized applications.

The following are some of these tools and their features:

Docker

Docker is the leading platform for managing containerized applications. It simplifies the process of creating, running, and deploying containers by providing a comprehensive set of tools and APIs.

Docker uses container images to define the application environment, making it easy to deploy and scale applications consistently. It also enables declarative configuration through Dockerfiles and Docker Compose files, allowing developers to define the application's infrastructure and dependencies in a version-controlled, reproducible manner.

Docker containers can run on any platform that supports Docker, including laptops, servers, and cloud environments, providing flexibility and portability.

Kubernetes

Kubernetes is an open-source container orchestration platform that automates the deployment, scaling, and management of containerized applications.

In Kubernetes, your applications can scale automatically based on resource usage and demand, which ensures optimal performance and efficiency. It also provides built-in service discovery and load-balancing features.

Kubernetes monitors the health of your applications and automatically restarts or replaces any container that fails. It also uses declarative APIs to define the desired state of your application. This means you only need to specify the desired configuration, and Kubernetes will handle the rest.

Docker Compose

Docker Compose is a tool for defining and running multi-container Docker applications. It simplifies the process of managing complex applications composed of multiple containers.

Docker Compose uses YAML files called "compose files" to define the services, networks, and volumes that make up the application. It allows you to manage the entire application lifecycle with a single command, including building, starting, stopping, and removing containers.

Advantages of Using Containers

The following are some advantages of using containers:

1. Lightweight Resource Footprint: Containers share the host operating system's kernel and resources. Unlike VMs, which require a separate guest operating system for each instance, containers leverage the host OS kernel, resulting in more negligible resource overhead and improved resource utilization.
2. Fast Startup Times: Containers can be started and stopped quickly, typically in seconds, compared to VMs, which can take minutes to boot. This fast startup time makes containers well-suited for scaling applications based on demand, as containers can be spun up or down rapidly to handle fluctuations in traffic or workload.
3. Portability: Containers provide infinite portability because you define the steps of building an application in a single file. With this single file, you can run your application multiple times across different platforms.
4. Scalability and Resource Isolation: Containers offer built-in scalability and resource isolation, allowing applications to be scaled horizontally by running multiple instances of containers across a cluster of servers. They also provide process-level isolation, ensuring that applications run in isolated environments without interfering with each other.

When to Use VMs vs. Containers

As stated at the beginning of this article, making the decision between virtual machines (VMs) and containers is application-dependent, amongst other factors. These factors can include the level of isolation required, resource utilization, security, etc.

Below are some use cases for when to use each technology:

Use Cases for VMs

1. Legacy Applications: As stated earlier, VMs are well-suited for running legacy applications that require specific operating system environments or have compatibility constraints.
2. Isolation and Security: VMs provide more robust isolation between applications and workloads compared to containers because each VM runs its own separate kernel and operating system.
3. Diverse Operating Systems: VMs allow you to run multiple guest operating systems, such as Linux, macOS, and Windows, on the same physical machine. This can be very useful when testing and debugging applications.

Use Cases for Containers

1. Microservices-based Architectures: Containers are ideal for deploying microservices-based architectures, where applications are decomposed into small, loosely coupled services.
2. Speed: Since containers virtualise only the application layer, they have a faster startup time than VMs. This speed lets you quickly start a container in seconds, making it well-suited for agile development practices and CI/CD pipelines.
3. Resource efficiency: Containers share the same host kernel and have a smaller footprint than VMs. This allows more containers to run on the same piece of hardware with less overhead.

There are some cases where both approaches are used. This way, you get the management benefits of VMs and the flexibility of containers.

The architecture for this kind of approach would look like this:

Figure 3: VMs & Containers architecture

Conclusion

In this article, you learnt about virtual machines & containers and understood how they work. Through this process, you saw the advantages of relying on either technology. Finally, you understood some use cases for when to use each technology.

Both technologies have their pros and cons. Choosing between them boils down to the specific needs of your application or organization. In the end, both offer you top-notch features for managing your applications.

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