WASI — WebAssembly System Interface

• Capability-based security: WASM modules have no ambient authority
• Sandboxed by default: no fs, network, env without explicit grant
• WASI Preview 2 (0.2) = Component Model + API split into interfaces
• Preview 1 — legacy (wasi_snapshot_preview1), still works in Wasmtime
• Typical use: serverless (Fermyon Spin), CDN compute (Fastly), plugins (Shopify Functions)

// Rust → WASI binary
$ cargo build --target wasm32-wasi --release
$ wasmtime --dir=. target/wasm32-wasi/release/app.wasm

// Fermyon Spin app.toml
[[component.trigger]]
route = "/"
[component.source]
path = "target/wasm32-wasi/release/hello.wasm"

• Component Model
• Serverless containers

The WebAssembly System Interface (WASI) is designed to provide a standardized API for WebAssembly (WASM) modules to interact with the underlying system. This architecture allows WASM applications to run outside the browser environment, enabling a wide range of use cases from server-side applications to CLI tools and edge computing.

At its core, WASI separates the concerns of the application logic and the system calls. It does so through a capability-based access model, meaning that the WASM module must be provided with the necessary capabilities by the host environment. This ensures that the module can only access the resources that it has been explicitly granted permission to use.

WASI's architecture can be broken down into several key components:

• Modules: WASM modules that implement the business logic.
• Runtime: The environment that executes the WASM modules, such as Wasmtime or WasmEdge.
• Host: The system providing the capabilities, such as file systems, network sockets, and clocks.

This design allows for high portability of applications, as the same WASM module can run in different environments as long as the host implements the WASI standard correctly. The abstraction layer provided by WASI simplifies the development of cross-platform applications.

To effectively utilize the WebAssembly System Interface (WASI), developers can leverage various commands and configurations. Below are practical examples demonstrating how to execute WASM modules with WASI support.

1. **Compiling a WASM Module with WASI Support**

To compile a simple Rust application targeting WASI, use the following command:

This command compiles your_app.rs into a WASM module with the wasm32-wasi target.

2. **Running a WASM Module Using Wasmtime**

Once you have your WASM module, you can run it using the Wasmtime runtime:

This command executes the your_app.wasm module, allowing it to interact with the system via the WASI API.

3. **Passing Capabilities to a WASM Module**

When running a WASM module, you may want to provide it with specific capabilities, such as file system access. The following command demonstrates how to run a WASM module with read access to a specific directory:

This grants the module access to the current directory, allowing it to read files.

4. **Configuring Environment Variables**

You can also pass environment variables to the WASM module. For example:

This sets the environment variable VAR_NAME to value for the duration of the module's execution.

These commands illustrate the flexibility and power of using WASI for developing and deploying WASM applications in various environments.

The WebAssembly System Interface (WASI) opens up numerous possibilities for modern application development across several domains. By enabling WASM modules to interact with system resources outside of the browser, it paves the way for innovative solutions in various fields. Below are some notable use cases for WASI:

• Server-Side Applications: WASI allows developers to run WASM modules on the server, enabling high-performance web applications that can leverage existing server infrastructure while maintaining the security and portability of WebAssembly.
• CLI Tools: Developers can create command-line interfaces that are lightweight and easily deployable. For instance, a data processing tool written in Rust can be compiled to WASM and run as a self-contained binary with WASI support, making it platform-independent.
• Edge Computing: With the rise of edge computing, WASI enables the deployment of WASM modules on edge devices, allowing for low-latency processing of data closer to the source. This is particularly useful in IoT applications where resources are constrained.
• Game Development: WASI can be utilized in game development for building cross-platform games that can run on various devices without modification. The performance and security of WASM make it an attractive option for game engines.
• Microservices: In a microservices architecture, WASI allows the creation of lightweight microservices that can be deployed easily across different environments. This is particularly beneficial for organizations looking to maintain a consistent deployment strategy.

These use cases highlight the versatility of WASI in addressing modern development challenges, making it a valuable tool for developers looking to leverage the benefits of WebAssembly in various contexts.