Composing Components

Because the WebAssembly component model packages code in a portable binary format, and provides machine-readable interfaces in WIT with a standardised ABI (Application Binary Interface), it enables applications and components to work together, no matter what languages they were originally written in. In the same way that, for example, a Rust package (crate) can be compiled together with other Rust code to create a higher-level library or an application, a Wasm component can be linked with other components.

Component model interoperation is more convenient and expressive than language-specific foreign function interfaces. A typical C FFI involves language-specific types, so it is not possible to link between arbitrary languages without at least some C-language wrapping or conversion. The component model, by contrast, provides a common way of expressing interfaces, and a standard binary representation of those interfaces. So if an import and an export have the same shape, they fit together directly.

What is composition?

When you compose components, you wire up the imports of one "primary" component to the exports of one or more other "dependency" components, creating a new component. The new component, like the original components, is a .wasm file, and its interface is defined as:

  • The new component exports the same exports as the primary component
  • The new component does not export the exports of the dependencies
  • The new component imports all the imports of the dependency components
  • The new component imports any imports of the primary component imports that the dependencies didn't satisfy
  • If several components import the same interface, the new component imports that interface - it doesn't "remember" that the import was declared in several different places

For example, consider two components with the following worlds:

// component `validator`
package docs:validator@0.1.0

interface validator {
    validate-text: func(text: string) -> string

world {
    export validator
    import docs:regex/match@0.1.0

// component 'regex'
package docs:regex@0.1.0

interface match {
    first-match: func(regex: string, text: string) -> string

world {
    export match

If we compose validator with regex, validator's import of docs:regex/match@0.1.0 is wired up to regex's export of match. The net result is that the composed component exports docs:validator/validator@0.1.0 and has no imports. The composed component does not export docs:regex/match@0.1.0 - that has become an internal implementation detail of the composed component.

Component composition tools are in their early stages right now. Here are some tips to avoid or diagnose errors:

  • Composition happens at the level of interfaces. If the initial component directly imports functions, then composition will fail. If composition reports an error such as "component path/to/component has a non-instance import named <name>" then check that all imports and exports are defined by interfaces.
  • Composition is asymmetrical. It is not just "gluing components together" - it takes a primary component which has imports, and satisfies its imports using dependency components. For example, composing an implementation of validator with an implementation of regex makes sense because validator has a dependency that regex can satisfy; doing it the other way round doesn't work, because regex doesn't have any dependencies, let alone ones that validator can satisfy.
  • Composition cares about interface versions, and current tools are inconsistent about when they infer or inject versions. For example, if a Rust component exports test:mypackage, cargo component build will decorate this with the crate version, e.g. test:mypackage@0.1.0. If another Rust component imports an interface from test:mypackage, that won't match test:mypackage@0.1.0. You can use wasm-tools component wit to view the imports and exports embedded in the .wasm files and check whether they match up.

Composing components with wasm-tools

The wasm-tools suite includes a compose command which can be used to compose components at the command line.

To compose a component with the components it directly depends on, run:

wasm-tools compose path/to/component.wasm -d path/to/dep1.wasm -d path/to/dep2.wasm -o composed.wasm

Here component.wasm is the component that imports interfaces from dep1.wasm and dep2.wasm, which export them. The composed component, with those dependencies satisfied and tucked away inside it, is saved to composed.wasm.

This syntax doesn't cover transitive dependencies. If, for example, dep1.wasm has unsatisfied imports that you want to satisfy from dep3.wasm, you'll need to use a configuration file. (Or you can compose dep1.wasm with dep3.wasm first, then refer to that composed component instead of dep1.wasm. This doesn't scale to lots of transitive dependencies though!)

For full information about wasm-tools compose including how to configure more advanced scenarios, see the wasm-tools compose documentation.

Composing components with a visual interface

You can compose components visually using the builder app at

  1. Use the Add Component Button to upload the .wasm component files you want to compose. The components appear in the sidebar.

  2. Drag the components onto the canvas. You'll see imports listed on the left of each component, and exports on the right.

  3. Click the box in the top left to choose the 'primary' component, that is, the one whose exports will be preserved. (The clickable area is quite small - wait for the cursor to change from a hand to a pointer.)

  4. To fulfil one of the primary component's imports with a dependency's export, drag from the "I" icon next to the export to the "I" item next to the import. (Again, the clickable area is quite small - wait for the cursor to change from a hand to a cross.)

  5. When you have connected all the imports and exports that you want, click the Download Component button to download the composed component as a .wasm file.