Alternative container runtimes

Table of contents

Docker Engine uses containerd for managing the container lifecycle, which includes creating, starting, and stopping containers. By default, containerd uses runc as its container runtime.

What runtimes can I use?

You can use any runtime that implements the containerd shim API. Such runtimes ship with a containerd shim, and you can use them without any additional configuration. See Use containerd shims.

Examples of runtimes that implement their own containerd shims include:

You can also use runtimes designed as drop-in replacements for runc. Such runtimes depend on the runc containerd shim for invoking the runtime binary. You must manually register such runtimes in the daemon configuration.

youki is one example of a runtime that can function as a runc drop-in replacement. Refer to the youki example explaining the setup.

Use containerd shims

containerd shims let you use alternative runtimes without having to change the configuration of the Docker daemon. To use a containerd shim, install the shim binary on PATH on the system where the Docker daemon is running.

To use a shim with docker run, specify the fully qualified name of the runtime as the value to the --runtime flag:

$ docker run --runtime io.containerd.kata.v2 hello-world

Use a containerd shim without installing on PATH

You can use a shim without installing it on PATH, in which case you need to register the shim in the daemon configuration as follows:

{
  "runtimes": {
    "foo": {
      "runtimeType": "/path/to/containerd-shim-foobar-v1"
    }
  }
}

To use the shim, specify the name that you assigned to it:

$ docker run --runtime foo hello-world

Configure shims

If you need to pass additional configuration for a containerd shim, you can use the runtimes option in the daemon configuration file.

  1. Edit the daemon configuration file by adding a runtimes entry for the shim you want to configure.

    • Specify the fully qualified name for the runtime in runtimeType key
    • Add your runtime configuration under the options key
    {
  "runtimes": {
    "gvisor": {
      "runtimeType": "io.containerd.runsc.v1",
      "options": {
        "TypeUrl": "io.containerd.runsc.v1.options",
        "ConfigPath": "/etc/containerd/runsc.toml"
      }
    }
  }
}

  2. Reload the daemon's configuration.

    # systemctl reload docker

  3. Use the customized runtime using the --runtime flag for docker run.

    $ docker run --runtime gvisor hello-world

For more information about the configuration options for containerd shims, see Configure containerd shims.

Examples

The following examples show you how to set up and use alternative container runtimes with Docker Engine.

youki

youki is a container runtime written in Rust. youki claims to be faster and use less memory than runc, making it a good choice for resource-constrained environments.

youki functions as a drop-in replacement for runc, meaning it relies on the runc shim to invoke the runtime binary. When you register runtimes acting as runc replacements, you configure the path to the runtime executable, and optionally a set of runtime arguments. For more information, see Configure runc drop-in replacements.

To add youki as a container runtime:

  1. Install youki and its dependencies.

    For instructions, refer to the official setup guide.

  2. Register youki as a runtime for Docker by editing the Docker daemon configuration file, located at /etc/docker/daemon.json by default.

    The path key should specify the path to wherever you installed youki.

    # cat > /etc/docker/daemon.json <<EOF
{
  "runtimes": {
    "youki": {
      "path": "/usr/local/bin/youki"
    }
  }
}
EOF

  3. Reload the daemon's configuration.

    # systemctl reload docker

Now you can run containers that use youki as a runtime.

$ docker run --rm --runtime youki hello-world

Wasmtime

Wasmtime is a Bytecode Alliance project, and a Wasm runtime that lets you run Wasm containers. Running Wasm containers with Docker provides two layers of security. You get all the benefits from container isolation, plus the added sandboxing provided by the Wasm runtime environment.

To add Wasmtime as a container runtime, follow these steps:

  1. Turn on the containerd image store feature in the daemon configuration file.

    Note

    This is an experimental feature.

    {
  "features": {
    "containerd-snapshotter": true
  }
}

  2. Restart the Docker daemon.

    # systemctl restart docker

  3. Install the Wasmtime containerd shim on PATH.

    The following command Dockerfile builds the Wasmtime binary from source and exports it to ./containerd-shim-wasmtime-v1.

    $ docker build --output . - <<EOF
FROM rust:latest as build
RUN cargo install \
    --git https://github.com/containerd/runwasi.git \
    --bin containerd-shim-wasmtime-v1 \
    --root /out \
    containerd-shim-wasmtime
FROM scratch
COPY --from=build /out/bin /
EOF

    Put the binary in a directory on PATH.

    $ mv ./containerd-shim-wasmtime-v1 /usr/local/bin

Now you can run containers that use Wasmtime as a runtime.

$ docker run --rm \
 --runtime io.containerd.wasmtime.v1 \
 --platform wasi/wasm32 \
 michaelirwin244/wasm-example

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