Health and Metrics

Service Health

The /health endpoint has nothing to check, because the reference implementation does not need to connect to any other services. Therefore, once the reference implementation is running, it can always report a healthy status:

async fn get_health() -> StatusCode {
    StatusCode::OK
}

In practice, a connector should make sure that any upstream services can be successfully contacted, and respond accordingly.

Metrics

The reference implementation maintains some generic access metrics in its application state:

• metrics.total_requests counts the number of requests ever served, and
• metrics.active_requests counts the number of requests currently being served.

The metrics endpoint reports these metrics using the Rust prometheus crate:

async fn get_metrics(State(state): State<Arc<Mutex<AppState>>>) -> Result<String> {
    let state = state.lock().await;
    state.metrics.as_text().ok_or((
        StatusCode::INTERNAL_SERVER_ERROR,
        Json(models::ErrorResponse {
            message: "cannot encode metrics".into(),
            details: serde_json::Value::Null,
        }),
    ))
}

To maintain these metrics, it uses a simple metrics middleware:

async fn metrics_middleware<T>(
    state: State<Arc<Mutex<AppState>>>,
    request: axum::http::Request<T>,
    next: axum::middleware::Next<T>,
) -> axum::response::Response {
    // Don't hold the lock to update metrics, since the
    // lock doesn't protect the metrics anyway.
    let metrics = {
        let state = state.lock().await;
        state.metrics.clone()
    };

    metrics.total_requests.inc();
    metrics.active_requests.inc();
    let response = next.run(request).await;
    metrics.active_requests.dec();
    response
}