{ { % experimental title="Beta" \%}} The Wasm feature is currently in Beta. We recommend that you do not use this feature in production environments as this feature may change or be removed from future releases. { { % /experimental \%}}

Wasm (short for WebAssembly) is a fast, light alternative to the Linux and Windows containers you’re using in Docker today (with some tradeoffs).

This page provides information about the new ability to run Wasm applications alongside your Linux containers in Docker.

Turn on Wasm workloads#

Wasm workloads require the containerd image store feature to be turned on. If you’re not already using the containerd image store, then pre-existing images and containers will be inaccessible.

  1. Navigate to Settings in Docker Desktop.
  2. In the General tab, check Use containerd for pulling and storing images.
  3. Go to Features in development and check the Enable Wasm option.
  4. Select Apply & restart to save the settings.
  5. In the confirmation dialog, select Install to install the Wasm runtimes.

Docker Desktop downloads and installs the following runtimes that you can use to run Wasm workloads:

  • io.containerd.slight.v1
  • io.containerd.spin.v2
  • io.containerd.wasmedge.v1
  • io.containerd.wasmtime.v1
  • io.containerd.lunatic.v1
  • io.containerd.wws.v1
  • io.containerd.wasmer.v1

Usage examples#

Running a Wasm application with docker run#

The following docker run command starts a Wasm container on your system:

$ docker run \
  --runtime=io.containerd.wasmedge.v1 \
  --platform=wasi/wasm \
  secondstate/rust-example-hello

After running this command, you can visit http://localhost:8080/ to see the "Hello world" output from this example module.

If you are receiving an error message, see the troubleshooting section for help.

Note the --runtime and --platform flags used in this command:

  • --runtime=io.containerd.wasmedge.v1: Informs the Docker engine that you want to use the Wasm containerd shim instead of the standard Linux container runtime
  • --platform=wasi/wasm: Specifies the architecture of the image you want to use. By leveraging a Wasm architecture, you don’t need to build separate images for the different machine architectures. The Wasm runtime takes care of the final step of converting the Wasm binary to machine instructions.

Running a Wasm application with Docker Compose#

The same application can be run using the following Docker Compose file:

services:
  app:
    image: secondstate/rust-example-hello
    platform: wasi/wasm
    runtime: io.containerd.wasmedge.v1

Start the application using the normal Docker Compose commands:

console $ docker compose up

Running a multi-service application with Wasm#

Networking works the same as you expect with Linux containers, giving you the flexibility to combine Wasm applications with other containerized workloads, such as a database, in a single application stack.

In the following example, the Wasm application leverages a MariaDB database running in a container.

  1. Clone the repository.

console $ git clone https://github.com/second-state/microservice-rust-mysql.git Cloning into 'microservice-rust-mysql'... remote: Enumerating objects: 75, done. remote: Counting objects: 100% (75/75), done. remote: Compressing objects: 100% (42/42), done. remote: Total 75 (delta 29), reused 48 (delta 14), pack-reused 0 Receiving objects: 100% (75/75), 19.09 KiB | 1.74 MiB/s, done. Resolving deltas: 100% (29/29), done.

  1. Navigate into the cloned project and start the project using Docker Compose.

console $ cd microservice-rust-mysql $ docker compose up [+] Running 0/1 ⠿ server Warning 0.4s [+] Building 4.8s (13/15) ... microservice-rust-mysql-db-1 | 2022-10-19 19:54:45 0 [Note] mariadbd: ready for connections. microservice-rust-mysql-db-1 | Version: '10.9.3-MariaDB-1:10.9.3+maria~ubu2204' socket: '/run/mysqld/mysqld.sock' port: 3306 mariadb.org binary distribution

If you run docker image ls from another terminal window, you can see the Wasm image in your image store.

console $ docker image ls REPOSITORY TAG IMAGE ID CREATED SIZE server latest 2c798ddecfa1 2 minutes ago 3MB

Inspecting the image shows the image has a wasi/wasm platform, a combination of OS and architecture:

console $ docker image inspect server | grep -A 3 "Architecture" "Architecture": "wasm", "Os": "wasi", "Size": 3001146, "VirtualSize": 3001146,

  1. Open the URL http://localhost:8090 in a browser and create a few sample orders. All of these are interacting with the Wasm server.

  2. When you're all done, tear everything down by hitting Ctrl+C in the terminal you launched the application.

Building and pushing a Wasm module#

  1. Create a Dockerfile that builds your Wasm application.

Exactly how to do this varies depending on the programming language you use.

  1. In a separate stage in your Dockerfile, extract the module and set it as the ENTRYPOINT.

dockerfile # syntax=docker/dockerfile:1 FROM scratch COPY --from=build /build/hello_world.wasm /hello_world.wasm ENTRYPOINT [ "/hello_world.wasm" ]

  1. Build and push the image specifying the wasi/wasm architecture. Buildx makes this easy to do in a single command.

console $ docker buildx build --platform wasi/wasm -t username/hello-world . ... => exporting to image 0.0s => => exporting layers 0.0s => => exporting manifest sha256:2ca02b5be86607511da8dc688234a5a00ab4d58294ab9f6beaba48ab3ba8de56 0.0s => => exporting config sha256:a45b465c3b6760a1a9fd2eda9112bc7e3169c9722bf9e77cf8c20b37295f954b 0.0s => => naming to docker.io/username/hello-world:latest 0.0s => => unpacking to docker.io/username/hello-world:latest 0.0s $ docker push username/hello-world

Troubleshooting#

This section contains instructions on how to resolve common issues.

Unknown runtime specified#

If you try to run a Wasm container without the containerd image store, an error similar to the following displays:

docker: Error response from daemon: Unknown runtime specified io.containerd.wasmedge.v1.

Turn on the containerd feature in Docker Desktop settings and try again.

Failed to start shim: failed to resolve runtime path#

If you use an older version of Docker Desktop that doesn't support running Wasm workloads, you will see an error message similar to the following:

docker: Error response from daemon: failed to start shim: failed to resolve runtime path: runtime "io.containerd.wasmedge.v1" binary not installed "containerd-shim-wasmedge-v1": file does not exist: unknown.

Update your Docker Desktop to the latest version and try again.

Known issues#

  • Docker Compose may not exit cleanly when interrupted
  • Workaround: Clean up docker-compose processes by sending them a SIGKILL (killall -9 docker-compose).
  • Pushes to Hub might give an error stating server message: insufficient_scope: authorization failed, even after logging in using Docker Desktop
  • Workaround: Run docker login in the CLI

Feedback#

Thanks for trying out Wasm workloads with Docker. Give feedback or report any bugs you may find through the issues tracker on the public roadmap item.