Firecracker MicroVMs vs Docker Containers: Which One Should You Use?

Docker containers and Firecracker microVMs both let you run isolated workloads. But they make very different tradeoffs. Choosing the wrong one wastes time, and in security-sensitive use cases, puts your infrastructure at risk.

This isn't a "microVMs are better" article. Both technologies have legitimate use cases. This guide helps you make the right choice for your specific situation.

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The fundamental difference

Docker containers isolate processes using Linux kernel features - namespaces (separate process trees, network stacks, mount points) and cgroups (resource limits). All containers on a host share the same kernel.

Firecracker microVMs are real virtual machines. Each one runs its own Linux kernel in a hardware-partitioned memory space, using KVM (the Linux hypervisor). A kernel exploit inside a microVM can't affect other VMs or the host.

Think of it this way:

• Containers are like apartments in a building - separate living spaces, but shared walls, shared plumbing, shared foundation
• MicroVMs are like separate houses - independent structures with independent everything

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Side-by-side comparison

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When Docker containers are the right choice

Deploying your own application code

If you wrote the code, audited the dependencies, and control the inputs, a container is fine. The isolation is mostly about packaging and consistency, not security.

Microservices you control

A Kubernetes cluster running your own microservices doesn't need VM-level isolation between services. Namespace isolation is sufficient when all services are trusted.

CI/CD pipelines with trusted inputs

Build pipelines that compile your own code from your own repos are fine in containers. The inputs are trusted.

Development environments on your own machine

Docker Desktop for local development is great. You trust yourself not to attack your own machine.

Resource-constrained environments

Containers have less memory overhead than VMs. If you're running hundreds of workloads on a single machine and memory is the bottleneck, containers let you pack more in.

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When Firecracker MicroVMs are the right choice

AI agent code execution

AI agents run code they generate or code users provide. That code is unaudited, unpredictable, and potentially malicious (via prompt injection). A shared kernel is too risky. Give each execution a separate VM.

Multi-tenant sandboxing

If multiple untrusted users share your infrastructure, you need hardware isolation. A bug in container isolation affects everyone on the host. A bug in one VM is contained to that VM.

Running untrusted code from any source

Code evaluation platforms, online IDEs, interview tools, educational sandboxes - anywhere users submit code that you execute. The code is hostile until proven otherwise.

Regulated environments

Healthcare, finance, government - if compliance requires you to demonstrate strong isolation between workloads, VMs are a clearer story than containers. "Separate kernels, separate memory, separate disks" is easier to audit than "namespaces plus seccomp plus AppArmor plus network policies."

When a container escape is catastrophic

If one workload accessing another's data would be a breach (not just a bug), containers aren't sufficient. The frequency of container escape CVEs should disqualify containers from security-critical isolation boundaries.

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"But VMs are slow" - Not anymore

The main argument against VMs has always been boot time. Traditional VMs take 30-60 seconds to start. For dynamic workloads, that's a dealbreaker.

Firecracker changed this:

• Millisecond cold start for a full Linux VM with own kernel, memory, and block device
• No BIOS/UEFI boot - direct kernel loading
• Minimal device model - only what's needed, nothing extra
• Optimized kernel - tailored for microVM workloads
• Fast init - minimal startup sequence

For comparison, a Docker container cold start (pulling from local cache, overlay setup, process start) is typically 200-500ms. A Firecracker microVM is in the same ballpark - or faster.

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"But VMs use more resources"

This is partially true, but the gap is smaller than you think:

Memory overhead:

• Docker container: ~10-30MB (kernel shared)
• Firecracker microVM: ~30-50MB (own kernel + startup overhead)
• Difference: 20-30MB per workload

At scale, this matters. If you're running 1,000 workloads, that's 20-30GB of extra memory. But for most use cases (tens to low hundreds of workloads), the difference is negligible compared to the application's own memory usage.

CPU overhead:

• Docker: Near-zero (syscalls go directly to shared kernel)
• Firecracker: Near-zero (KVM hardware virtualization, no emulation)
• Difference: Measurable only in micro-benchmarks

Disk overhead:

• Docker: Shared layers reduce total disk usage
• Firecracker: Each VM has its own block device with private storage

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The hybrid approach

You don't have to choose one for everything. Many production systems use both:

• Docker containers for your trusted application services (API servers, web frontends, background workers)
• Firecracker microVMs for untrusted code execution, user sandboxes, and AI agent tasks

This gives you the ecosystem and familiarity of Docker where security isn't the primary concern, and hardware isolation where it is.

Oblien takes this approach: you specify a Docker image when creating a workspace, and Oblien runs it inside a Firecracker microVM. All the Docker ecosystem compatibility, all the microVM security.

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Making the decision

Ask yourself three questions:

1. Who wrote the code that's running?

• You → Container is fine
• Your users or an AI agent → VM

2. What happens if the workload escapes its isolation?

• Inconvenient (restart it) → Container is fine
• Catastrophic (data breach, lateral movement) → VM

3. Do workloads need to be invisible to each other?

• Same trust boundary → Container is fine
• Different trust boundaries → VM

If you answered "VM" to any of these, you need hardware isolation. If you answered "container" to all three, Docker is the simpler choice.

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Summary

Docker containers and Firecracker microVMs aren't competitors - they're tools for different jobs. Containers package and deploy trusted applications. MicroVMs isolate untrusted workloads.

The boot time argument is dead. MicroVMs boot in milliseconds. The resource argument is shrinking. The security argument has always favored VMs.

For AI agents, multi-tenant platforms, and untrusted code execution, Firecracker microVMs are the right choice. For deploying your own application stack, Docker containers work great.

Just don't use containers as a security boundary. They weren't designed for it.

Try Firecracker-based workspaces on Oblien →