Running Components

You can "run" a component by calling one of its exports. In some cases, this requires a custom host. For "command" components, though, you can use the wasmtime command line. This can be a convenient tool for testing components and exploring the component model. Other runtimes are also available - see the "Runtimes" section of the sidebar for more info.

A "command" component is one that exports the wasi:cli/run interface, and imports only interfaces listed in the wasi:cli/command world.

You must use a recent version of wasmtime (v14.0.0 or greater), as earlier releases of the wasmtime command line do not include component model support.

If you build the wasmtime CLI from source, you must pass --features component-model to the build command.

To run your component, run:

wasmtime run --wasm component-model <path-to-wasm-file>

Running components with custom exports

If you're writing a library-style component - that is, one that exports a custom API - then you can run it in wasmtime by writing a "command" component that imports and invokes your custom API. By composing the command and the library, you can exercise the library in wasmtime.

  1. Write your library component. The component's world (.wit file) must export an interface. (Do not export functions directly, only interfaces.) See the language support guide for how to implement an export.

  2. Build your library component to a .wasm file.

  3. Write your command component. The component's world (.wit file) must import the interface exported from the library. Write the command to call the library's API. See the language support guide for how to call an imported interface.

  4. Build your command component to a .wasm file. You will not be able to run this in wasmtime yet, as its imports are not yet satisfied.

  5. Compose your command component with your library component by running wasm-tools compose <path/to/command.wasm> -d <path/to/library.wasm> -o main.wasm.

  6. Run the composed component using wasmtime run --wasm component-model main.wasm

See Composing Components for more details.