Wasm

Overview

With WebAssembly, you can safely run code compiled in other languages. Runtimes execute WebAssembly Modules (Wasm), which are most often binaries with a .wasm extension.

The Wasm Binding allows you to invoke a program compiled to Wasm by passing commandline args or environment variables to it, similar to how you would with a normal subprocess. For example, you can satisfy an invocation using Python, even though Dapr is written in Go and is running on a platform that doesn’t have Python installed!

The Wasm binary must be a program compiled with the WebAssembly System Interface (WASI). The binary can be a program you’ve written such as in Go, or an interpreter you use to run inlined scripts, such as Python.

Minimally, you must specify a Wasm binary compiled with the canonical WASI version wasi_snapshot_preview1 (a.k.a. wasip1), often abbreviated to wasi.

Note: If compiling in Go 1.21+, this is GOOS=wasip1 GOARCH=wasm. In TinyGo, Rust, and Zig, this is the target wasm32-wasi.

You can also re-use an existing binary. For example, Wasm Language Runtimes distributes interpreters (including PHP, Python, and Ruby) already compiled to WASI.

Wasm binaries are loaded from a URL. For example, the URL file://rewrite.wasm loads rewrite.wasm from the current directory of the process. On Kubernetes, see How to: Mount Pod volumes to the Dapr sidecar to configure a filesystem mount that can contain Wasm binaries. It is also possible to fetch the Wasm binary from a remote URL. In this case, the URL must point exactly to one Wasm binary. For example:

• http://example.com/rewrite.wasm, or
• https://example.com/rewrite.wasm.

Dapr uses wazero to run these binaries, because it has no dependencies. This allows use of WebAssembly with no installation process except Dapr itself.

The Wasm output binding supports making HTTP client calls using the wasi-http specification. You can find example code for making HTTP calls in a variety of languages here:

• Golang
• C
• .NET
• TypeScript

Component format

To configure a Wasm binding, create a component of type bindings.wasm. See this guide on how to create and apply a binding configuration.

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: wasm
spec:
  type: bindings.wasm
  version: v1
  metadata:
    - name: url
      value: "file://uppercase.wasm"

Spec metadata fields

Binding support

This component supports output binding with the following operations:

• execute

Example request

The data field, if present will be the program’s STDIN. You can optionally pass metadata properties with each request:

• args any CLI arguments, comma-separated. This excludes the program name.

For example, consider binding the url to a Ruby interpreter, such as from webassembly-language-runtimes:

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: wasm
spec:
  type: bindings.wasm
  version: v1
  metadata:
  - name: url
    value: "https://github.com/vmware-labs/webassembly-language-runtimes/releases/download/ruby%2F3.2.0%2B20230215-1349da9/ruby-3.2.0-slim.wasm"

Assuming that you wanted to start your Dapr at port 3500 with the Wasm Binding, you’d run:

$ dapr run --app-id wasm --dapr-http-port 3500 --resources-path components

The following request responds Hello "salaboy":

$ curl -X POST http://localhost:3500/v1.0/bindings/wasm -d'
{
  "operation": "execute",
  "metadata": {
    "args": "-ne,print \"Hello \"; print"
  },
  "data": "salaboy"
}'

Related links

• Basic schema for a Dapr component
• Bindings building block
• How-To: Trigger application with input binding
• How-To: Use bindings to interface with external resources
• Bindings API reference