Important

This is a fork of apple/containerization, modified for Nucleic. It is not Apple's distribution, it is not maintained by Apple, and it is not a drop-in replacement for upstream. Apple's original README follows below; anything specific to this fork is described under Nucleic fork. If you want the unmodified package, use apple/containerization.

The Containerization package allows applications to use Linux containers. Containerization is written in Swift and uses Virtualization.framework on Apple silicon.

Looking for command line binaries for running containers?
They are available in the dedicated apple/container repository.

Nucleic fork

Nucleic embeds this package to run Linux workloads on macOS. Upstream targets the general case of short-lived, one-process-per-container usage; Nucleic drives it much harder than that, and the divergence exists to make those conditions survivable. Broadly, the changes fall into a few areas:

Host and guest I/O robustness. Hardening of the stdio and socket relay paths so that one slow, stalled, or dying stream cannot block, starve, or tear down the others that share a transport.
Lifecycle bounds. Deadlines, cancellation, and teardown guarantees on operations that upstream allows to block indefinitely, plus independent escape hatches for stopping a virtual machine that is no longer answering.
Per-workload resource isolation. Finer-grained cgroup placement and limits inside the guest, so resource exhaustion is contained to the workload that caused it.
Host integration. Additional configuration surface the embedding application needs, and non-interactive behavior in places where upstream may prompt the user.

The details are deliberately kept out of this file. Every divergence is enumerated in PATCHES.md, and every edit site in the sources is marked with a [Nucleic vendored patch] comment, so upstream code and fork code stay easy to tell apart.

Two consequences worth knowing before you use this fork:

It tracks a pinned upstream commit rather than upstream main, and Tests/, docs/, examples/, and images/ are trimmed from the tree.
Some changes live in the guest agent (vminitd/) and are inert until the init filesystem image is rebuilt from this source and the consuming application is pointed at it.

Containerization provides APIs to:

Manage OCI images.
Interact with remote registries.
Create and populate ext4 file systems.
Interact with the Netlink socket family.
Create an optimized Linux kernel for fast boot times.
Spawn lightweight virtual machines and manage the runtime environment.
Spawn and interact with containerized processes.
Use Rosetta 2 for running linux/amd64 containers on Apple silicon.

Please view the API documentation for information on the Swift packages that Containerization provides.

Design

Containerization executes each Linux container inside of its own lightweight virtual machine. Clients can create dedicated IP addresses for every container to remove the need for individual port forwarding. Containers achieve sub-second start times using an optimized Linux kernel configuration and a minimal root filesystem with a lightweight init system.

vminitd is a small init system, which is a subproject within Containerization. vminitd is spawned as the initial process inside of the virtual machine and provides a GRPC API over vsock. The API allows the runtime environment to be configured and containerized processes to be launched. vminitd provides I/O, signals, and events to the calling process when a process is run.

Requirements

To build the Containerization package, you need:

Mac with Apple silicon
macOS 26
Xcode 26

Older versions of macOS are not supported.

Example Usage

For examples of how to use the libraries' API surface, the cctl executable is a good start. This app is a useful playground for exploring the API. It contains commands that exercise some of the core functionality of the various products, such as:

Linux kernel

A Linux kernel is required for spawning lightweight virtual machines on macOS. Containerization provides an optimized kernel configuration located in the kernel directory.

This directory includes a containerized build environment to easily compile a kernel for use with Containerization.

The kernel configuration is a minimal set of features to support fast start times and a lightweight environment.

While this configuration will work for the majority of workloads we understand that some will need extra features. To solve this Containerization provides first class APIs to use different kernel configurations and versions on a per container basis. This enables containers to be developed and validated across different kernel versions.

See the README in the kernel directory for instructions on how to compile the optimized kernel.

Kernel Support

Containerization allows user provided kernels but tests functionality starting with kernel version 6.14.9.

Pre-built Kernel

If you wish to consume a pre-built kernel, make sure it has VIRTIO drivers compiled into the kernel (not merely as modules).

The Kata Containers project provides a Linux kernel that is optimized for containers, with all required configuration options enabled. The releases page contains downloadable artifacts, and the image itself (vmlinux.container) can be found in the /opt/kata/share/kata-containers/ directory.

Prepare to build package

Install the recommended version of Xcode.

Set the active developer directory to the installed Xcode (replace <PATH_TO_XCODE>):

sudo xcode-select -s <PATH_TO_XCODE>

Install Swiftly, Swift, and Static Linux SDK:

make cross-prep

If you use a custom terminal application, you may need to move this command from .zprofile to .zshrc (replace <USERNAME>):

# Added by swiftly
. "/Users/<USERNAME>/.swiftly/env.sh"

Restart the terminal application. Ensure this command returns /Users/<USERNAME>/.swiftly/bin/swift (replace <USERNAME>):

which swift

If you've installed or used a Static Linux SDK previously, you may need to remove older SDK versions from the system (replace <SDK-ID>):

swift sdk list
swift sdk remove <SDK-ID>

Build the package

Build Containerization from sources:

make all

Test the package

After building, run basic and integration tests:

make test integration

A kernel is required to run integration tests. If you do not have a kernel locally, a default kernel can be fetched using the make fetch-default-kernel target.

Fetching the default kernel only needs to happen after an initial build or after a make clean.

make fetch-default-kernel
make all test integration

Protobufs

Containerization depends on specific versions of grpc-swift and swift-protobuf. You can install them and re-generate RPC interfaces with:

make protos

Building a kernel

If you'd like to build your own kernel please see the instructions in the kernel directory.

Pre-commit hook

Run make pre-commit to install a pre-commit hook that ensures that your changes have correct formatting and license headers when you run git commit.

Documentation

Generate the API documentation for local viewing with:

make docs
make serve-docs

Preview the documentation by running in another terminal:

open http://localhost:8000/containerization/documentation/

Contributing

Contributions that are not specific to Nucleic belong upstream, at apple/containerization — see CONTRIBUTING.md; anything landed upstream reaches this fork on the next re-vendor. Changes to the fork's own divergence should come with a corresponding entry in PATCHES.md and a [Nucleic vendored patch] marker at each edit site, so the next re-vendor can re-apply them.

Project Status

This fork carries no stability guarantees of its own: it exists to serve Nucleic, and its API may change with Nucleic's needs. Upstream's own status statement follows.

Version 0.1.0 is the first official release of Containerization. Earlier versions have no source stability guarantees.

Because the Containerization library is under active development, source stability is only guaranteed within minor versions (for example, between 0.1.1 and 0.1.2). If you don't want potentially source-breaking package updates, you can specify your package dependency using .upToNextMinorVersion(from: "0.1.0") instead.

Future minor versions of the package may introduce changes to these rules as needed.