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 have Spin installed. If this is your first encounter with Spin, please see the Quick Start, which includes information about installing Spin with the Rust templates, installing required tools, and creating Rust applications.

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.

Prerequisites

Install the Templates

You don’t need the Spin Rust templates to work on Rust components, but they speed up creating new applications and components. You can install them as follows:

$ spin templates install --git https://github.com/fermyon/spin --update
Copying remote template source
Installing template redis-rust...
Installing template http-rust...
... other templates omitted ...
+------------------------------------------------------------------------+
| Name                Description                                        |
+========================================================================+
| ... other templates omitted ...                                        |
| http-rust           HTTP request handler using Rust                    |
| redis-rust          Redis message handler using Rust                   |
| ... other templates omitted ...                                        |
+------------------------------------------------------------------------+

Note: The Rust templates are in a repo that contains several other languages; they will all be installed together.

Install the Tools

To build Spin components, you’ll need the wasm32-wasi target for Rust.

$ rustup target add wasm32-wasi

If you get a lot of strange errors when you try to build your first Rust component, check that you have this target installed by running rustup target list --installed. This is the most common source of problems when starting out with Rust in Spin!

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.

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:

use anyhow::Result;
use bytes::Bytes;
use spin_sdk::redis_component;

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

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

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 animal facts and inserts a custom header into the response before returning:

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

/// Send an HTTP request and return the response.
#[http_component]
fn send_outbound(_req: Request) -> Result<Response> {
    let mut res = spin_sdk::outbound_http::send_request(
        http::Request::builder()
            .method("GET")
            .uri("https://random-data-api.fermyon.app/animals/json")
            .body(None)?,
    )?;
    res.headers_mut()
        .insert("spin-component", "rust-outbound-http".try_into()?);
    println!("{:?}", res);
    Ok(res)
}

The http::Request::builder() method is provided by the Rust http crate. The http crate is already added to projects using the Spin http-rust template. If you create a project without using this template, you’ll need to add the http crate yourself via cargo add http.

Before we can execute this component, we need to add the random-data-api.fermyon.app 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_manifest_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 = ["random-data-api.fermyon.app"]
[component.trigger]
route = "/outbound"

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 /outbound:

$ curl -i localhost:3000/outbound
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

{"timestamp":1684299253331,"fact":"Reindeer grow new antlers every year"}

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.

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:

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

// The environment variable set in `spin.toml` that points to the
// address of the Redis server that the component will publish
// a message to.
const REDIS_ADDRESS_ENV: &str = "REDIS_ADDRESS";

// The environment variable set in `spin.toml` that specifies
// the Redis channel that the component will publish to.
const REDIS_CHANNEL_ENV: &str = "REDIS_CHANNEL";

/// 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`.
#[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 = redis::get(&address, "mykey").map_err(|_| anyhow!("Error querying Redis"))?;

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

    // Set the Redis key "int-key" to value 0
    redis::set(&address, "int-key", format!("{:x}", 0).as_bytes())
        .map_err(|_| anyhow!("Error executing Redis set command"))?;
    let int_value = redis::incr(&address, "int-key")
        .map_err(|_| anyhow!("Error executing Redis incr command",))?;
    assert_eq!(int_value, 1);

    // Publish to Redis
    match 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.

Storing Data in the Spin Key-Value Store

Spin has a key-value store built in. For information about using it from Rust, see the key-value store tutorial.

Serializing Objects to the Key-Value Store

The Spin key-value API stores and retrieves only lists of bytes. The Rust SDK provides helper functions that allow you to store and retrieve Serde serializable values in a typed way. The underlying storage format is JSON (and is accessed via the get_json and set_json helpers).

To use the get_json and set_json helpers, you must enable the Spin SDK’s optional json feature in your Cargo.toml file. To make your objects serializable, you will also need a reference to serde. The relevant Cargo.toml entries look like this:

[dependencies]
// --snip --
serde = {version = "1.0.163", features = ["derive"]}
spin-sdk = {git = "https://github.com/fermyon/spin", version = "1.2.0", features = ["json"]}
// --snip --

The Rust code below shows how to store and retrieve serializable objects from the key-value store (note how the example below implements Serde’s derive feature):

use anyhow::Result;
use serde::{Deserialize, Serialize};
use spin_sdk::{
    http::{Request, Response},
    http_component,
    key_value::Store,
};

// Define a serializable User type
#[derive(Serialize, Deserialize)]
struct User {
    fingerprint: String,
    location: String,
}

#[http_component]
fn handle_request(_req: Request) -> Result<Response> {
    // Open the default key-value store
    let store = Store::open_default()?;

    // Create an instance of a User object and populate the values
    let user = User {
        fingerprint: "0x1234".to_owned(),
        location: "Brisbane".to_owned(),
    };
    // Store the User object using the "my_json" key
    store.set_json("my_json", &user)?;
    // Retrieve the user object from the key-value store, using the "my_json" key
    let retrieved_user: User = store.get_json("my_json")?;
    // Return the user's fingerprint as the response body
    Ok(http::Response::builder()
        .status(200)
        .body(Some(retrieved_user.fingerprint.into()))?)
}

Once built and running (using spin build and spin up) you can test the above example in your browser (by visiting localhost:3000) or via curl, as shown below:

$ curl localhost:3000
HTTP/1.1 200 OK

0x1234

Storing Data in Relational Databases

Spin provides clients for MySQL and PostgreSQL. For information about using them from Rust, see Relational Databases.

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

If you bump into issues building and running your Rust component, here are some common causes of problems:

  • Make sure cargo is present in your path
  • Make sure the Rust version is recent.
    • To check: run cargo --version. The Spin SDK needs Rust 1.64 or above.
    • To update: run rustup update.
  • Make sure the wasm32-wasi compiler target is installed.
    • To check: run rustup target list --installed and check that wasm32-wasi is on the list.
    • To install: run rustup target add wasm32-wasi.
  • Make sure you are building in release mode. Spin manifests refer to your Wasm file by a path, and the default path corresponds to release builds.
    • To build manually: run cargo build --release --target wasm32-wasi.
    • If you’re using spin build and the templates, this should be set up correctly for you.
  • Make sure that the source field in the component manifest match the path and name of the Wasm file in target/wasm32-wasi/release. These could get out of sync if you renamed the Rust package in its Cargo.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 project 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" }

wit-bindgen evolves rapidly to track draft standards. Very recent versions of wit-bindgen are unlikely to work correctly with Spin. So the dependency must be pinned to a specific rev. Over time, Spin expects to track “peninsulas of stability” in the evolving standards.

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