What’s a Unikernel, Anyway?

A unikernel is a single-purpose, specialized operating system that is compiled with just the absolute bare minimum required to run a single application.

Imagine if, instead of running your app inside a full-blown OS (like Linux or Windows), you stripped the OS down to just the essential parts your app needs—nothing more, nothing less. That’s a unikernel!

How’s It Different from a Traditional OS?

Most operating systems are general-purpose, meaning they come with everything—including features you don’t even need. A unikernel, on the other hand, is like a custom-tailored suit for your application.

Here’s how it stacks up:

Why Should You Care About Unikernels?

Good question! Here’s why unikernels are gaining traction:

1. They’re Ridiculously Small

Because they only include what’s necessary, unikernels can be tiny—sometimes just a few megabytes. This means lower memory usage, faster boot times, and less attack surface.

2. They Boot Faster Than You Can Say ‘Microservices’

A unikernel can boot in milliseconds. Compare that to a full VM or even some containerized workloads, and you’ll see why people love them.

3. They’re More Secure

Since unikernels remove unnecessary components (like user accounts, shells, and package managers), attackers have way fewer entry points. No bloated OS, no unnecessary services—just pure application execution.

4. They Use Fewer Resources

Unikernels consume fewer CPU cycles, RAM, and storage compared to traditional OS-based applications. This makes them perfect for edge computing, IoT, and cloud-native applications.

How Unikernels Compare to Containers

Since unikernels are often mentioned in the same breath as containers, let’s clarify the differences.

The main difference? Containers rely on a shared OS kernel, while unikernels run their own minimal OS per instance. This makes unikernels even smaller and more secure than containers.

How Do You Build a Unikernel?

Most unikernel projects use specialized toolchains that compile applications into standalone, bootable images.

Here’s a simple example using MirageOS, one of the most popular unikernel frameworks (built in OCaml). Let’s create a basic web server:

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opam install mirage
mirage configure --unix
make depend
make

This compiles your application into a unikernel image that can run on bare metal or inside a lightweight VM.

Another popular tool is Unikraft, which lets you build unikernels from existing applications like Nginx or Redis:

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git clone https://github.com/unikraft/unikraft
cd unikraft
make menuconfig  # Select your app and dependencies
make

Boom! You’ve got a unikernel.

Where Are Unikernels Used?

Unikernels are already in production in a variety of fields:

• Edge Computing – Super lightweight workloads on IoT devices.
• Cloud Computing – High-performance applications with ultra-fast boot times.
• Security Applications – Sandboxed, minimal attack-surface environments.
• Embedded Systems – Devices that need a stripped-down OS.

The Downsides of Unikernels (Yes, There Are Some)

Nothing is perfect, and unikernels have their challenges:

• Not as mature as Linux or containers – The ecosystem is still growing.
• Debugging is tricky – No traditional shell access.
• Limited compatibility – Not all applications are unikernel-ready.
• Steep learning curve – Requires special compilation techniques.

Conclusion

Unikernels are a lightweight, fast, and highly secure alternative to traditional OSes and containers. They’re not a silver bullet for every workload, but for high-performance, low-footprint applications, they’re an exciting technology worth exploring.

Key Ideas

References

• Unikernel.org
• MirageOS
• Unikraft
• Xen Project
• Solo5