String Input (WASM Component)

This example demonstrates passing a string input to a WASM Component Model export via propeller. The component takes a name and returns a personalised greeting.

Source Code

The source code is available in the examples/greet-component directory.

The WIT interface defines a single exported function:

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The Rust implementation:

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Build the Component

You need the following tools installed:

Rust with the wasm32-wasip2 target: rustup target add wasm32-wasip2

cd propeller/examples/greet-component
cargo build --target wasm32-wasip2 --release

The task name must exactly match the exported function name in the WIT interface. The proplet uses the task name to look up and call the component export — if the names don't match, the task will fail.

String inputs require WAVE encoding — see below for details.

curl -X POST "http://localhost:7070/tasks" \
  -H "Content-Type: application/json" \
  -d '{"name": "greet", "inputs": ["\"John Doe\""]}'

Your output should look like this:

{
    "id": "6381afd0-cefd-434e-85d2-473576a81432",
    "name": "greet",
    "kind": "standard",
    "state": 0,
    "cli_args": null,
    "inputs": [
        "\"John Doe\""
    ],
    "daemon": false,
    "encrypted": false,
    "start_time": "0001-01-01T00:00:00Z",
    "finish_time": "0001-01-01T00:00:00Z",
    "created_at": "2026-03-26T10:22:23.697115725Z",
    "updated_at": "0001-01-01T00:00:00Z",
    "next_run": "0001-01-01T00:00:00Z",
    "priority": 50
}

Upload Wasm

curl -X PUT "http://localhost:7070/tasks/6381afd0-cefd-434e-85d2-473576a81432/upload" \
  -F "file=@target/wasm32-wasip2/release/greet_component.wasm"

Your output should look like this:

{
    "id": "6381afd0-cefd-434e-85d2-473576a81432",
    "name": "greet",
    "kind": "standard",
    "state": 0,
    "file": "...<redacted>...",
    "cli_args": null,
    "inputs": [
        "\"John Doe\""
    ],
    "daemon": false,
    "encrypted": false,
    "start_time": "0001-01-01T00:00:00Z",
    "finish_time": "0001-01-01T00:00:00Z",
    "created_at": "2026-03-26T10:22:23.697115725Z",
    "updated_at": "2026-03-26T10:22:27.341182514Z",
    "next_run": "0001-01-01T00:00:00Z",
    "priority": 50
}

Start Task

curl -X POST "http://localhost:7070/tasks/6381afd0-cefd-434e-85d2-473576a81432/start"

Your output should look like this:

{ "started": true }

The proplet logs will show the component export being invoked:

2026-03-26T10:22:37.451615Z  INFO Received start command for task: 6381afd0-cefd-434e-85d2-473576a81432
2026-03-26T10:22:37.457237Z  INFO Decoded wasm binary, size: 72591 bytes
2026-03-26T10:22:37.459183Z  INFO Executing task 6381afd0-cefd-434e-85d2-473576a81432 in spawned task
2026-03-26T10:22:37.462876Z  INFO Starting Wasmtime runtime app: task_id=6381afd0-cefd-434e-85d2-473576a81432, function=greet, daemon=false, wasm_size=72591, is_component=true, is_proxy=false
2026-03-26T10:22:37.463406Z  INFO Compiling WASM component for custom export 'greet' for task: 6381afd0-cefd-434e-85d2-473576a81432
2026-03-26T10:22:37.858028Z  INFO Export 'greet' for task 6381afd0-cefd-434e-85d2-473576a81432 completed, result: "Hello, John Doe!"
2026-03-26T10:22:37.936202Z  INFO Task 6381afd0-cefd-434e-85d2-473576a81432 completed successfully. Result: "Hello, John Doe!"
2026-03-26T10:22:37.936288Z  INFO Publishing result for task 6381afd0-cefd-434e-85d2-473576a81432
2026-03-26T10:22:37.936324Z  INFO Successfully published result for task 6381afd0-cefd-434e-85d2-473576a81432

Get Results

curl -X GET "http://localhost:7070/tasks/6381afd0-cefd-434e-85d2-473576a81432"

Your output should look like this:

{
    "id": "6381afd0-cefd-434e-85d2-473576a81432",
    "name": "greet",
    "kind": "standard",
    "state": 3,
    "cli_args": null,
    "inputs": [
        "\"John Doe\""
    ],
    "daemon": false,
    "encrypted": false,
    "proplet_id": "fecdb9c3-be8c-458b-a188-f974a2aa93c7",
    "results": "\"Hello, John Doe!\"",
    "start_time": "2026-03-26T10:22:37.454060334Z",
    "finish_time": "2026-03-26T10:22:37.971575688Z",
    "created_at": "2026-03-26T10:22:23.697115725Z",
    "updated_at": "2026-03-26T10:22:37.971575412Z",
    "next_run": "0001-01-01T00:00:00Z",
    "priority": 50
}

State 3 means the task completed successfully. The results field shows "\"Hello, John Doe!\"" — the return value of greet("John Doe").

WAVE Encoding for String Inputs

The proplet passes inputs to component exports using WAVE (WebAssembly Value Encoding). In WAVE, a string value must include its own surrounding quotes — "John Doe" — so the parser knows it is a string and not an identifier, enum variant, or other type.

Because every element in the inputs JSON array is already a JSON string, you end up with two layers of quoting:

WAVE layer — the value itself must be "John Doe" (with quotes) to be a valid WAVE string
JSON layer — to embed that quoted value inside a JSON string, the inner quotes must be escaped: \"John Doe\"

The final JSON input for the string John Doe is therefore "\"John Doe\"".

What happens if you omit the inner quotes?

If you send "John Doe" instead of "\"John Doe\"", the WAVE parser receives John Doe with no surrounding quotes. It treats that as an identifier — not a string literal — and rejects it. The task will fail with a WAVE parse error before the component is even called.

Wasm type	JSON input value	WAVE value received by component
`string`	`"\"John Doe\""`	`"John Doe"`
`s32`	`"42"`	`42`
`f32`	`"3.14"`	`3.14`
`bool`	`"true"`	`true`

Non-string types (s32, f32, bool) do not need inner quotes because their WAVE representations have no quoting requirement.

Invoking Using Wasmtime

You can test the component binary directly without propeller:

wasmtime run --invoke 'greet("John Doe")' target/wasm32-wasip2/release/greet_component.wasm

The output should look like this:

"Hello, John Doe!"

String Input (WASM Component)

Source Code

Build the Component

Create Task

Upload Wasm

Start Task

Get Results

WAVE Encoding for String Inputs

Invoking Using Wasmtime

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