WebAssembly outside the browser

Level: 301 · deep dive

One line: WASI gives a module a standard set of host functions — arguments, clocks, files it was granted — so rustc hello.rs --target wasm32-wasip1 followed by wasmtime hello.wasm prints Hello, world! with no JavaScript anywhere, and the component model turns that idea into typed interfaces between modules.

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

What it has to cover

• The hello-world path exactly as wasmtime.dev ↗ prints it: rustup target add wasm32-wasip1, rustc hello.rs --target wasm32-wasip1, wasmtime hello.wasm. What does the module import to make println! work?
• WASI's three milestones as wasi.dev ↗ lists them — 0.1, 0.2 and 0.3, also written Preview 1/2/3 or P1/P2/P3, with 0.3 adding native async to the component model — and the Rust target for each on 1.98.0: wasm32-wasip1 and wasm32-wasip2 at Tier 2, wasm32-wasip3 at Tier 3.
• Preview 1 against preview 2, per the rustc book: wasm32-wasip1 produces a core module importing functions from wasi_snapshot_preview1 and is now kept for historical compatibility; wasm32-wasip2 produces a component, and the runtime must support components to run it. What can a component express that a core module cannot?
• Capabilities instead of ambient authority: a WASI program opens only what the runtime granted it. What does fs::read("/etc/hosts") return inside wasmtime without a --dir grant, and with one?
• The component model ↗: WIT files describing interfaces with strings, records and lists, and cargo-component ↗ — replacing the pointer-and-length conventions of Rust in the browser with a typed contract.
• Wasm as a plugin system: embedding the wasmtime ↗ crate (48.0.2 on crates.io) so a third-party module gets CPU and memory and nothing else. What stops a plugin's infinite loop or unbounded allocation — which limits does the host have to set? Extism ↗ packages the same idea as a framework.
• Threads: wasm32-wasip1-threads and what shared memory needs from the runtime.
• Against a container: start-up time, isolation and portability claims — as measurements the finished page makes, or as questions it leaves open.

The trap it exists for

Porting a command-line tool to wasm32-wasip1, watching it run under wasmtime, and assuming it now has the machine's filesystem. It has what the runtime was told to grant; everything else fails at run time, and the build gave no sign of the difference.

Where this sits

Compiling to wasm32 covers choosing among the targets; Rust in the browser covers the JavaScript host. This page covers only hosts that are not a browser.

See also

• Compiling to wasm32 — wasm32-wasip1 and wasm32-wasip2 among the six
• What WebAssembly is — imports, exports and the sandbox the capabilities rest on
• Calling C — the other way to load someone else's code into your process, without a sandbox
• The right to post is a value — permission as something held, the same idea as a pre-opened directory
• Standard error, and exit status — the streams and the number a WASI command hands back to its host
• Process ↗ — the isolation boundary a wasm instance is compared with

If you are coming from another language

• Java. A portable bytecode on a host runtime is the closest precedent, and the Security Manager was Java's in-process way of restricting what loaded code could touch — deprecated for removal in Java 17 (JEP 411 ↗) and later permanently disabled. WASI puts the restriction in the host interface itself: a module has no access until the runtime grants some.
• Go. GOOS=wasip1 GOARCH=wasm builds for the same WASI preview 1 host (go tool dist list, Go 1.25.5), so a Go module and a Rust module can run under the same wasmtime. That list has no preview 2 port, which is where wasm32-wasip2 and components go further.

Po polsku

Poza przeglądarką moduł WebAssembly dostaje od gospodarza standardowy zestaw funkcji — WASI (WebAssembly System Interface): argumenty, zegar i tylko te pliki, do których runtime dał mu dostęp. rustc hello.rs --target wasm32-wasip1 i wasmtime hello.wasm wypisują „Hello, world!” bez grama JavaScriptu. Wersje WASI to 0.1, 0.2 i 0.3 (Preview 1/2/3); od 0.2 opierają się na modelu komponentów (component model) z interfejsami opisanymi w plikach WIT, a osadzony w programie wasmtime pozwala wykonywać cudze wtyczki w piaskownicy.

Szukaj po polsku: WASI w Ruscie · WebAssembly na serwerze · wtyczki w piaskownicy · wasmtime wasi rust · webassembly component model wit · wasm32-wasip2 rust