Hook0 Rust SDK

Webhooks your users can trust, from the language that hates surprises

What is this?

The Rust SDK for Hook0, the open source Webhooks-as-a-Service platform for SaaS applications. It sends events, declares the event types your application uses, verifies the signature of a webhook you receive, and calls every operation the API declares through generated, documented types.

Both halves are feature-gated. producer sends events and declares event types, consumer verifies the signature of a webhook you received, and both are on by default, so an application that only receives can switch the other off and compile neither.

Features

• Send events - under an ID the client mints, so a retry cannot duplicate one
• Declare event types - upsert the ones your application emits, in one call
• Verify signatures - HMAC-SHA256 over a bilateral clock window
• The whole API, typed - one struct per schema, one variant per problem, one method per operation
• Bounded everywhere - attempts, backoff, timeouts, payload and answer, all yours to set
• Async throughout - built on reqwest and tokio, cancellable by the future that holds it

Quick Start

1. Install

cargo add hook0-client

Take only the half you use: hook0-client = { version = "1", default-features = false, features = ["consumer"] }.

2. Send an event

use hook0_client::{Event, Hook0Client};
use reqwest::Url;
use std::borrow::Cow;
use uuid::Uuid;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = Hook0Client::new(
        Url::parse("https://app.hook0.com/api/v1")?,
        Uuid::parse_str("0d0ea1e0-8b1f-4f4d-9c2a-1b5c9a3d7e21")?,
        &std::env::var("HOOK0_TOKEN")?,
    )?;

    let event_id = client
        .send_event(&Event {
            event_id: None,
            event_type: "billing.invoice.paid",
            payload: Cow::Borrowed(r#"{"invoice": "in_123"}"#),
            payload_content_type: "application/json",
            metadata: None,
            occurred_at: None,
            labels: vec![("environment".to_owned(), "production".to_owned())],
        })
        .await?;

    println!("ingested as {event_id}");
    Ok(())
}

3. Verify a webhook you receive

use hook0_client::verify_webhook_signature;
use std::time::Duration;

fn accept(
    signature: &str,
    body: &[u8],
    headers: &[(&str, &str)],
    subscription_secret: &str,
) -> bool {
    verify_webhook_signature(
        signature,
        body,
        headers,
        subscription_secret,
        Duration::from_secs(300),
    )
    .is_ok()
}

The clock window is bilateral, so a delivery dated too far ahead is refused exactly like one dated too far behind, because a window that only looked backwards is one a sender widens by dating its own delivery in the future. A header the signature covers but the request did not carry is refused before any code is computed.

Configuration

Every bound one send is held to is yours to set, and every one has a default.

Every default comes from clients/conformance/bounds.json, the corpus every Hook0 SDK reads. A number changed there fails every SDK still carrying the old one, so no two of them can bound different things.

The last three bound what the other end may cost you. A server that is broken or hostile can otherwise stream a head, a header or a body of any length into your process.

use hook0_client::{Hook0Client, RetryPolicy};
use std::time::Duration;

let client = Hook0Client::new(api_url, application_id, &token)?
    .with_retry_policy(RetryPolicy {
        max_attempts: 4,
        initial_backoff: Duration::from_millis(100),
        max_backoff: Duration::from_secs(2),
        max_total_delay: Duration::from_secs(5),
    })
    .with_request_timeout(Duration::from_secs(10))
    .with_max_payload_bytes(1024 * 1024)
    .with_max_response_bytes(8 * 1024 * 1024);

Usage

Sending is idempotent, and retried

send_event sends every event under an ID it knows, either the one set on the event or a UUIDv7 it mints when the event carries none. Passing no ID does not mean the ID comes from Hook0. The value comes from the client, travels with the request, and is what send_event returns.

That is what makes a retry safe. Hook0 keys events on their ID, so a request repeated after a network failure or a server error ingests the event once rather than twice. Without a client-chosen ID, the repeated request would create a second event and deliver it to every subscriber.

Retrying is limited to what could end differently. A request that got no answer, a server error and an instance saying it is being reached faster than it accepts are all retried. A 429 naming a spent quota is not, because a quota clears when a plan changes or a day turns, and no send can wait for that. A Retry-After the answer carries is honoured, clamped to what is left of the delay budget. A retried request Hook0 answers with EventAlreadyIngested reports success, since an earlier attempt of that same send reached the API. The same answer to a first attempt is a genuine conflict, and is reported as an error.

Declaring the event types you use

let created = client
    .upsert_event_types(&["billing.invoice.paid", "billing.invoice.voided"])
    .await?;

Only the ones your application does not declare yet are created, and those are what comes back.

Calling the rest of the API

Every operation the API declares is a method of a generated group, one group per entity.

use hook0_client::generated::{ApplicationsApi, ProblemId, RequestError};

let applications = ApplicationsApi::new(transport);

match applications.get(application_id).await {
    Ok(application) => println!("{}", application.name),
    Err(RequestError::Api(failure)) if failure.kind == Some(ProblemId::NotFound) => {
        // the API named a problem, and this is which one
    }
    Err(other) => return Err(other.into()),
}

Bringing your own transport

The generated groups are issued through a Transport, a trait with one method. Given a verb, a path, a query and a body, it answers the status and the bytes. The half of this crate that reaches the network implements it, and so can you, over whichever HTTP client your application already carries.

Development

clients/rust/src/generated/ is written by hook0-sdkgen from the OpenAPI snapshot the API commits, and is rewritten whole on every regeneration. A hand edit there is reverted the next time anyone regenerates, and the drift guard says so before that. Change the generator, then run:

UPDATE_SDK=rust cargo test -p hook0-sdkgen sdk_targets

Everything beside it, the transport, the retry loop and the signature verification, is hand-written and never regenerated, and so is tests/.

What a send retries, the bounds it is held to and how a signature is verified are dictated by the shared corpus at clients/conformance, which every SDK's suite reads, so a verdict changed there fails this client until it agrees again.

Every case runs against a real Hook0 over a loopback socket. Nothing here stands in for a part of the client.

cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings
cargo test --all-features

License

The Hook0 Rust SDK is free and open source, released under the MIT License. Use it, change it, ship it, in open source and in commercial work alike, as long as the copyright notice travels with it.

Hook0 itself is open source too. Read what Hook0 is, visit hook0.com, join the community, or write to support@hook0.com.

Maintained by David Sferruzza and François-Guillaume Ribreau.