Docker + WebAssembly (Wasm) Guide: Run Wasm in Docker

DALL·E 2025-02-07 22.42.02 - A conceptual illustration of Docker running WebAssembly (Wasm) workloads. The image should depict a modern cloud infrastructure with Docker containers.webp

Docker has revolutionized the way developers work through containerization. Now, WebAssembly (Wasm) is emerging as a lightweight, secure, and highly efficient alternative that seamlessly integrates with Docker, eliminating the need for a full Linux OS inside containers.

This guide explores how Docker's Wasm integration can enhance application performance, security, and portability.

🚨 Note: Docker's WebAssembly (Wasm) support is currently in beta, and its features may change over time. As improvements continue, the integration and functionality may evolve, requiring updates to workflows and best practices.

1. Understanding WebAssembly (Wasm) in Docker

WebAssembly (Wasm) is a portable, high-performance format that allows applications written in Rust, C, C++, Go, and JavaScript to execute securely in isolated environments. These isolated environments, also known as sandboxes, provide a secure execution environment by restricting the application's access to system resources. Initially built for web applications, Wasm is now expanding into serverless computing, edge deployments, and containerized environments.

Unlike traditional Docker containers that require a full OS layer, WebAssembly runs inside a dedicated runtime such as Wasmtime or WasmEdge, allowing it to execute faster, consume fewer resources, and maintain a stronger security model.

Why Use Wasm in Docker?

Fast Execution: Wasm applications start in milliseconds compared to traditional containers that need to initialize an OS.

Lightweight: Since Wasm apps do not require a full OS, the container size is significantly reduced.

Strong Security: The sandboxed nature of WebAssembly ensures that applications run with minimal attack surfaces.

Cross-Platform: A single Wasm binary can execute on multiple platforms without modification, making it ideal for multi-architecture support.

2. How Docker and WebAssembly Work Together

Traditional Docker Containers vs. Wasm Integration

In a traditional Docker container, the container runtime (runc) starts an instance of a Linux OS with system dependencies and libraries, and then the application runs inside that container. This means that the container has to load the OS, allocate resources, and manage processes just like a virtualized environment.

With Wasm integration, Docker uses runwasi instead of runc. This removes the need for an OS layer because WebAssembly applications do not rely on system calls and kernel features. Instead, they execute in a restricted sandbox with predefined capabilities provided by WASI (WebAssembly System Interface).

Key Differences Between Traditional Containers and Wasm

Feature	Traditional Docker (Linux Containers)	Docker + WebAssembly
Runtime	Uses `runc` (Linux-based)	Uses `runwasi` (Wasm runtime)
Startup Speed	500ms - 2s	~5ms - 50ms
System Access	Full Linux syscalls	Restricted by WASI
File System	Full OS access	Limited sandboxed access
Security	Kernel isolation	Fully sandboxed
Container Size	Large (includes OS dependencies)	Small (only Wasm binary)

3. Running a WebAssembly Application in Docker

Step 1: Install Docker With Wasm Support

Ensure you have the latest Docker Desktop with Wasm support installed.

docker --version

Step 2: Install Rust and the Wasm Target

To compile an application into WebAssembly, install Rust and add the required compilation target:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
rustup target add wasm32-wasip1

Step 3: Create a WebAssembly Rust Application

Create a new Rust project and modify the main.rs file:

cargo new wasm-docker-app
cd wasm-docker-app

Modify src/main.rs:

fn main() {
    println!("Hello from WebAssembly in Docker!");
}

Compile the Rust application to Wasm:

cargo build --target wasm32-wasip1 --release

Step 4: Package the Wasm App as a Docker Image

A Wasm application does not require a traditional OS-based container. Instead, we package it as a scratch-based Docker image:

FROM scratch
COPY target/wasm32-wasip1/release/wasm-docker-app.wasm /app.wasm
ENTRYPOINT ["/app.wasm"]

Build the Docker image:

docker buildx build --platform wasi/wasm -t my-wasm-app .

Run the Wasm container:

docker run --name my-wasm-app --rm --runtime=io.containerd.wasmedge.v1 --platform=wasi/wasm my-wasm-app

Expected Output:

Hello from WebAssembly in Docker!

Measure Startup Time in Docker

You can measure the startup time using the following methods:

Method 1: Using time Command

This will display the execution time.

time docker run --name my-wasm-app --rm --runtime=io.containerd.wasmedge.v1 --platform=wasi/wasm my-wasm-app

Method 2: Checking Docker Events

This logs the timestamp when the Wasm container starts.

docker events --filter container=my-wasm-app

4. Conclusion

Docker's WebAssembly integration provides a high-performance, lightweight, and secure alternative for running cloud-native applications. By leveraging Wasm inside Docker:

✅ Microservices run faster with near-instant startup times.
✅ Security is improved through isolated execution.
✅ Cross-platform portability is enhanced.

However, Wasm is not a full replacement for traditional containers. Applications requiring direct system calls or extensive OS integrations will still rely on Linux-based containers. But for microservices, serverless applications, and edge computing, WebAssembly offers an efficient and scalable solution.

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