Building Spin components in Rust

Spin aims to have best-in-class support for building components in Rust, and writing such components should be familiar for Rust developers.

This guide assumes you are familiar with the Rust programming language, but if you are just getting started, be sure to check the official resources for learning Rust.

All examples from this page can be found in the Spin repository on GitHub.

In order to compile Rust programs to Spin components, you also need the wasm32-wasi target. You can add it using rustup:

$ rustup target add wasm32-wasi

HTTP components

In Spin, HTTP components are triggered by the occurrence of an HTTP request, and must return an HTTP response at the end of their execution. Components can be built in any language that compiles to WASI, but Rust has improved support for writing Spin components with the Spin Rust SDK.

Make sure to read the page describing the HTTP trigger for more details about building HTTP applications.

Building a Spin HTTP component using the Rust SDK means writing a single function that takes an HTTP request as a parameter, and returns an HTTP response — below is a complete implementation for such a component:

use anyhow::Result;
use spin_sdk::{
    http::{Request, Response},
    http_component,
};

/// A simple Spin HTTP component.
#[http_component]
fn hello_world(req: Request) -> Result<Response> {
    println!("{:?}", req);
    Ok(http::Response::builder()
        .status(200)
        .header("foo", "bar")
        .body(Some("Hello, Fermyon!".into()))?)
}

The important things to note in the implementation above:

• the spin_sdk::http_component macro marks the function as the entry point for the Spin component
• the function signature — fn hello_world(req: Request) -> Result<Response> — the Spin HTTP component uses the HTTP objects from the popular Rust crate http, and the request and response bodies are optionally using bytes::Bytes (spin_sdk::http::Request is a type alias for http::Request<Option<Bytes>>)
• the component returns a Rust anyhow::Result, so if there is an error processing the request, it returns an anyhow::Error.

Sending outbound HTTP requests

If allowed, Spin components can send outbound HTTP requests. Let’s see an example of a component that makes a request to an API that returns random dog facts and inserts a custom header into the response before returning:

#[http_component]
fn hello_world(_req: Request) -> Result<Response> {
    let mut res = spin_sdk::http::send(
        http::Request::builder()
            .method("GET")
            .uri("https://some-random-api.ml/facts/dog")
            .body(None)?,
    )?;

    res.headers_mut()
        .insert(http::header::SERVER, "spin/0.1.0".try_into()?);

    Ok(res)
}

Before we can execute this component, we need to add the some-random-api.ml domain to the application manifest allowed_http_hosts list containing the list of domains the component is allowed to make HTTP requests to:

# spin.toml
spin_version = "1"
name = "spin-hello-world"
trigger = { type = "http", base = "/" }
version = "1.0.0"

[[component]]
id = "hello"
source = "target/wasm32-wasi/release/spinhelloworld.wasm"
allowed_http_hosts = [ "some-random-api.ml" ]
[component.trigger]
route = "/hello"

Running the application using spin up --file spin.toml will start the HTTP listener locally (by default on localhost:3000), and our component can now receive requests in route /hello:

$ curl -i localhost:3000/hello
HTTP/1.1 200 OK
date: Fri, 18 Mar 2022 03:54:36 GMT
content-type: application/json; charset=utf-8
content-length: 185
server: spin/0.1.0

{"fact":"It's rumored that, at the end of the Beatles song, 
\"A Day in the Life,\" Paul McCartney recorded an ultrasonic whistle, 
audible only to dogs, just for his Shetland sheepdog."}

Without the allowed_http_hosts field populated properly in spin.toml, the component would not be allowed to send HTTP requests, and sending the request would result in a “Destination not allowed” error.

You can set allowed_http_hosts = ["insecure:allow-all"] if you want to allow the component to make requests to any HTTP host. This is NOT recommended for any production or publicly-accessible application.

We just built a WebAssembly component that sends an HTTP request to another service, manipulates that result, then responds to the original request. This can be the basis for building components that communicate with external databases or storage accounts, or even more specialized components like HTTP proxies or URL shorteners.

Redis components

Besides the HTTP trigger, Spin has built-in support for a Redis trigger — which will connect to a Redis instance and will execute Spin components for new messages on the configured channels.

See the Redis trigger for details about the Redis trigger.

Writing a Redis component in Rust also takes advantage of the SDK:

/// A simple Spin Redis component.
#[redis_component]
fn on_message(msg: Bytes) -> Result<()> {
    println!("{}", from_utf8(&msg)?);
    Ok(())
}

• the spin_sdk::redis_component macro marks the function as the entry point for the Spin component
• in the function signature — fn on_message(msg: Bytes) -> anyhow::Result<()> — msg contains the payload from the Redis channel
• the component returns a Rust anyhow::Result, so if there is an error processing the request, it returns an anyhow::Error.

The component can be built with Cargo by executing:

$ cargo build --target wasm32-wasi --release

The manifest for a Redis application must contain the address of the Redis instance the trigger must connect to:

spin_version = "1"
name = "spin-redis"
trigger = { type = "redis", address = "redis://localhost:6379" }
version = "0.1.0"

[[component]]
id = "echo-message"
source = "target/wasm32-wasi/release/spinredis.wasm"
[component.trigger]
channel = "messages"

This application will connect to redis://localhost:6379, and for every new message on the messages channel, the echo-message component will be executed.

# first, start redis-server on the default port 6379
$ redis-server --port 6379
# then, start the Spin application
$ spin up --file spin.toml
# the application log file will output the following
INFO spin_redis_engine: Connecting to Redis server at redis://localhost:6379
INFO spin_redis_engine: Subscribed component 0 (echo-message) to channel: messages

For every new message on the messages channel:

$ redis-cli
127.0.0.1:6379> publish messages "Hello, there!"

Spin will instantiate and execute the component we just built, which will emit the println! message to the application log file:

INFO spin_redis_engine: Received message on channel "messages"
Hello, there!

If you would also like to see the println! messages echoed to the console as they happen, please include the additional --follow-all option, when starting the spin application. For example:

spin up --file spin.toml --follow-all

You can find a complete example for a Redis triggered component in the Spin repository on GitHub.

Storing data in Redis from Rust components

Using the Spin’s Rust SDK, you can use the Redis key/value store and to publish messages to Redis channels. This can be used from both HTTP and Redis triggered components.

Let’s see how we can use the Rust SDK to connect to Redis:

#[spin_sdk::http_component]
fn publish(_req: Request) -> Result<Response> {
    let address = std::env::var(REDIS_ADDRESS_ENV)?;
    let channel = std::env::var(REDIS_CHANNEL_ENV)?;

    // Get the message to publish from the Redis key "mykey"
    let payload = spin_sdk::redis::get(&address, &"mykey").map_err(|_| anyhow!("Error querying Redis"))?;

    // Set the Redis key "spin-example" to value "Eureka!"
    spin_sdk::redis::set(&address, &"spin-example", &b"Eureka!"[..])
        .map_err(|_| anyhow!("Error executing Redis command"))?;

    // Publish to Redis
    match spin_sdk::redis::publish(&address, &channel, &payload) {
        Ok(()) => Ok(http::Response::builder().status(200).body(None)?),
        Err(_e) => internal_server_error(),
    }
}

This HTTP component demonstrates fetching a value from Redis by key, setting a key with a value, and publishing a message to a Redis channel. The component is triggered by an HTTP request served on the route configured in the spin.toml:

[[component]]
environment = { REDIS_ADDRESS = "redis://127.0.0.1:6379", REDIS_CHANNEL = "messages" }
[component.trigger]
route = "/publish"

This HTTP component can be paired with a Redis component, triggered on new messages on the messages Redis channel.

You can find a complete example for using outbound Redis from an HTTP component in the Spin repository on GitHub.

Using external crates in Rust components

In Rust, Spin components are regular libraries that contain a function annotated using the http_component macro, compiled to the wasm32-wasi target. This means that any crate that compiles to wasm32-wasi can be used when implementing the component.

Troubleshooting

Sometimes things can go wrong, especially such early projects. If you bump into issues building and running your Rust component:

• ensure cargo is present in your path
• ensure your Rust version is recent, via rustup check
• ensure wasm32-wasi target is configured for your Rust installation — you can add it by running rustup target add wasm32-wasi
• build a release version of the component — all Spin application definitions (spin.toml files) reference build configuration for Rust, so make sure to run cargo build --release --target wasm32-wasi when building your components (you can validate modules are correctly being built by checking the contents of the target/wasm32-wasi/release directory and looking for .wasm files)
• make sure the path and name of the Wasm module in target/wasm32-wasi/release match source field in the component configuration (the source field contains the path to the Wasm module, relative to spin.toml)

Manually creating new projects with Cargo

The recommended way of creating new Spin projects is by starting from a template. This section shows how to manually create a new project with Cargo.

When creating a new Spin projects with Cargo, you should use the --lib flag.

$ cargo init --lib

A Cargo.toml with standard Spin dependencies looks like this:

[package]
name = "your-app"
version = "0.1.0"
edition = "2021"

[lib]
# Required to have a `cdylib` (dynamic library) to produce a Wasm module.
crate-type = [ "cdylib" ]

[dependencies]
# Useful crate to handle errors.
anyhow = "1"
# Crate to simplify working with bytes.
bytes = "1"
# General-purpose crate with common HTTP types.
http = "0.2"
# The Spin SDK.
spin-sdk = { git = "https://github.com/fermyon/spin" }
# Crate that generates Rust Wasm bindings from a WebAssembly interface.
wit-bindgen-rust = { git = "https://github.com/bytecodealliance/wit-bindgen", rev = "cb871cfa1ee460b51eb1d144b175b9aab9c50aba" }

At the time of this writing, wit-bindgen must be pinned to a specific rev. This will change in the future.