WASI: The Lightweight Challenger to Server Containers
For years, containers have been the de facto standard for deploying internet services, often orchestrated with Kubernetes. Now, a new and extremely lightweight alternative is emerging: the WebAssembly System Interface (WASI). While WebAssembly (WASM) is primarily known for browser-based applications, WASI enables the execution of WebAssembly modules on servers and at the edge.
What is WASI?
WASI is a system interface for WebAssembly that provides a standardized API for system resources like file I/O, networking, and environment variables. It achieves this without the complexity of traditional operating systems or containers. WASI modules are compiled binaries in the WebAssembly format. They don’t carry their own system libraries and interact with the host exclusively through well-defined interfaces.
WASI vs. Containers: A Fundamental Difference
In contrast, containers package an application along with all its dependencies, running in isolated user spaces on a host operating system. Although containers are lightweight compared to virtual machines, they still require a complete user-space image.
WASI modules, however, are significantly smaller, start almost instantaneously, and offer strong isolation thanks to WebAssembly’s sandboxed execution model.
Key Advantages of WASI
This opens up new possibilities for microservices, Function-as-a-Service (FaaS) platforms, and edge computing. Key advantages over containers include:
- Minimal Overhead: No need for operating system images, a shell, or package management.
- Blazing-Fast Startup Times: Measured in milliseconds.
- Tiny Artifact Sizes: Typically ranging from kilobytes to a few megabytes.
- Platform Independence: Wasm binaries run anywhere a compatible runtime exists.
- Strong Security Isolation: Inherent sandboxing prevents modules from accessing arbitrary system resources.
Development and Execution
These modules can be developed using languages like Rust, Go, and C or C++, which support WebAssembly as a compilation target. They are executed by a WASI runtime, such as Wasmtime, Wasmer, or WasmEdge.
Ecosystem and Orchestration
WASI is increasingly supported by major cloud providers (e.g., Cloudflare Workers, Fastly Compute, Deno Deploy), and projects like Krustlet are enabling the orchestration of WebAssembly workloads on Kubernetes.
Implications for Scalability and Density
Due to their minimal resource requirements, significantly more WASI instances can run in parallel on the same hardware compared to containers. This allows for finer-grained, faster, and more cost-effective scaling—ideal for bursty workloads or high-density multi-tenant environments.
Alignment with Modern App Principles
Many of the principles from the 12-Factor App methodology (e.g., configuration via environment variables, statelessness, disposability) can be applied with WASI. However, some areas, like logging, require new approaches since traditional stdout/stderr streams or sidecar patterns are not directly available.
Conclusion
WASI is not a complete replacement for containers but rather an attractive complement for specific use cases that demand maximum portability, strong isolation, and minimal overhead.