truelayer-interview/src/app.rs

use crate::{
    config::AppConfig, error::AppError, pokemon::PokeApiClient, rate_limit::RateLimiter,
    service::PokedexService, telemetry::Telemetry, translation::TranslationClient,
};
use axum::{
    Json, Router,
    extract::{MatchedPath, Path, Request, State},
    http::{StatusCode, header},
    middleware,
    response::{IntoResponse, Response},
    routing::get,
};
use serde::Serialize;
use std::{sync::Arc, time::Instant};
use tower::ServiceBuilder;
use tower_http::{
    ServiceBuilderExt,
    request_id::MakeRequestUuid,
    trace::{DefaultMakeSpan, DefaultOnResponse, TraceLayer},
};

#[derive(Clone)]
pub struct AppState {
    service: Arc<PokedexService>,
    telemetry: Arc<Telemetry>,
    rate_limiter: Arc<RateLimiter>,
}

impl AppState {
    #[must_use]
    pub fn new(service: PokedexService, telemetry: Telemetry, rate_limiter: RateLimiter) -> Self {
        Self {
            service: Arc::new(service),
            telemetry: Arc::new(telemetry),
            rate_limiter: Arc::new(rate_limiter),
        }
    }

    pub fn from_config(config: &AppConfig) -> Result<Self, AppError> {
        let telemetry = Telemetry::new(&config.service_name);
        let pokemon = PokeApiClient::new(&config.pokeapi_base_url, config.request_timeout)?;
        let translations =
            TranslationClient::new(&config.translations_base_url, config.request_timeout)?;
        let service = PokedexService::new(pokemon, translations, telemetry.metrics());
        Ok(Self::new(
            service,
            telemetry,
            RateLimiter::new(config.rate_limit_per_second, config.rate_limit_burst)
                .map_err(|_| AppError::Internal)?,
        ))
    }
}

pub fn create_app(state: AppState) -> Router {
    let tracing_layers = ServiceBuilder::new()
        .set_x_request_id(MakeRequestUuid)
        .layer(
            TraceLayer::new_for_http()
                .make_span_with(DefaultMakeSpan::new().include_headers(true))
                .on_response(DefaultOnResponse::new().include_headers(true)),
        )
        .propagate_x_request_id();

    Router::new()
        .route("/health", get(health))
        .route("/metrics", get(metrics))
        .route("/pokemon/{name}", get(get_pokemon))
        .route("/pokemon/translated/{name}", get(get_translated_pokemon))
        .with_state(state.clone())
        .layer(middleware::from_fn_with_state(
            state.clone(),
            enforce_rate_limit,
        ))
        .layer(middleware::from_fn_with_state(state, record_http_metrics))
        .layer(tracing_layers)
}

async fn health() -> Json<HealthResponse> {
    Json(HealthResponse { status: "ok" })
}

async fn metrics(State(state): State<AppState>) -> Result<impl IntoResponse, AppError> {
    let body = state.telemetry.render_prometheus()?;
    Ok(([(header::CONTENT_TYPE, "text/plain; version=0.0.4")], body))
}

async fn enforce_rate_limit(
    State(state): State<AppState>,
    req: Request,
    next: middleware::Next,
) -> Response {
    if state.rate_limiter.try_acquire() {
        next.run(req).await
    } else {
        (
            StatusCode::TOO_MANY_REQUESTS,
            Json(serde_json::json!({ "error": "rate limit exceeded" })),
        )
            .into_response()
    }
}

async fn record_http_metrics(
    State(state): State<AppState>,
    req: Request,
    next: middleware::Next,
) -> Response {
    let start = Instant::now();
    let method = req.method().clone();
    let path = req.extensions().get::<MatchedPath>().map_or_else(
        || req.uri().path().to_owned(),
        |path| path.as_str().to_owned(),
    );

    let response = next.run(req).await;
    let status = response.status();
    state.telemetry.metrics().record_http_request(
        method.as_str(),
        &path,
        status.as_u16(),
        start.elapsed(),
    );

    response
}

async fn get_pokemon(
    State(state): State<AppState>,
    Path(name): Path<String>,
) -> Result<Json<crate::domain::PokemonInfo>, AppError> {
    state.service.get_basic(&name).await.map(Json)
}

async fn get_translated_pokemon(
    State(state): State<AppState>,
    Path(name): Path<String>,
) -> Result<Json<crate::domain::PokemonInfo>, AppError> {
    state.service.get_translated(&name).await.map(Json)
}

#[derive(Serialize)]
struct HealthResponse {
    status: &'static str,
}

#[cfg(test)]
mod tests {
    use super::{AppState, create_app};
    use crate::{
        pokemon::PokeApiClient, rate_limit::RateLimiter, service::PokedexService,
        telemetry::Telemetry, translation::TranslationClient,
    };
    use axum::{
        body::{Body, to_bytes},
        http::{Request, StatusCode, header::HeaderName},
    };
    use serde_json::Value;
    use std::time::Duration;
    use tower::ServiceExt;
    use wiremock::{Mock, MockServer, ResponseTemplate, matchers};

    #[tokio::test]
    async fn health_endpoint_reports_ok() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        let app = create_app(state_for(&pokeapi, &translations));

        let response = app
            .oneshot(request("/health"))
            .await
            .expect("request should be handled");

        assert_eq!(response.status(), StatusCode::OK);
        assert_eq!(
            json_body(response).await,
            serde_json::json!({ "status": "ok" })
        );
    }

    #[tokio::test]
    async fn pokemon_endpoint_returns_basic_pokemon_information() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        mount_species(&pokeapi, "mewtwo", true, "rare", "Created by science.").await;
        let app = create_app(state_for(&pokeapi, &translations));

        let response = app
            .oneshot(request("/pokemon/mewtwo"))
            .await
            .expect("request should be handled");

        assert_eq!(response.status(), StatusCode::OK);
        assert_eq!(
            json_body(response).await,
            serde_json::json!({
                "name": "mewtwo",
                "description": "Created by science.",
                "habitat": "rare",
                "isLegendary": true
            })
        );
    }

    #[tokio::test]
    async fn translated_endpoint_returns_translated_description() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        mount_species(&pokeapi, "pikachu", false, "forest", "Electric cheeks.").await;
        Mock::given(matchers::method("POST"))
            .and(matchers::path("/translate/shakespeare.json"))
            .respond_with(ResponseTemplate::new(200).set_body_json(serde_json::json!({
                "contents": { "translated": "Electric cheeks, prithee." }
            })))
            .mount(&translations)
            .await;
        let app = create_app(state_for(&pokeapi, &translations));

        let response = app
            .oneshot(request("/pokemon/translated/pikachu"))
            .await
            .expect("request should be handled");

        assert_eq!(response.status(), StatusCode::OK);
        assert_eq!(
            json_body(response).await["description"],
            "Electric cheeks, prithee."
        );
    }

    #[tokio::test]
    async fn every_response_has_a_request_id() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        let app = create_app(state_for(&pokeapi, &translations));

        let response = app
            .oneshot(request("/health"))
            .await
            .expect("request should be handled");

        assert!(response.headers().contains_key("x-request-id"));
    }

    #[tokio::test]
    async fn keeps_incoming_request_id_when_present() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        let app = create_app(state_for(&pokeapi, &translations));
        let request = Request::builder()
            .uri("/health")
            .header(HeaderName::from_static("x-request-id"), "external-id")
            .body(Body::empty())
            .expect("request should build");

        let response = app
            .oneshot(request)
            .await
            .expect("request should be handled");

        assert_eq!(response.headers()["x-request-id"], "external-id");
    }

    #[tokio::test]
    async fn metrics_endpoint_exposes_http_and_upstream_otel_metrics() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        mount_species(&pokeapi, "mewtwo", true, "rare", "Created by science.").await;
        let app = create_app(state_for(&pokeapi, &translations));
        let response = app
            .clone()
            .oneshot(request("/pokemon/mewtwo"))
            .await
            .expect("request should be handled");
        assert_eq!(response.status(), StatusCode::OK);

        let response = app
            .oneshot(request("/metrics"))
            .await
            .expect("request should be handled");
        let body = text_body(response).await;

        assert!(body.contains("http_server_requests"));
        assert!(body.contains("upstream_client_requests"));
    }

    #[tokio::test]
    async fn rate_limit_rejects_requests_over_the_configured_burst() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        let telemetry = Telemetry::new("test-service");
        let app = create_app(AppState::new(
            PokedexService::new(
                PokeApiClient::new(pokeapi.uri(), Duration::from_secs(1))
                    .expect("client should build"),
                TranslationClient::new(translations.uri(), Duration::from_secs(1))
                    .expect("client should build"),
                telemetry.metrics(),
            ),
            telemetry,
            RateLimiter::new(0.01, 1).expect("rate limiter should build"),
        ));

        let first = app
            .clone()
            .oneshot(request("/health"))
            .await
            .expect("request should be handled");
        let second = app
            .oneshot(request("/health"))
            .await
            .expect("request should be handled");

        assert_eq!(first.status(), StatusCode::OK);
        assert_eq!(second.status(), StatusCode::TOO_MANY_REQUESTS);
        assert_eq!(json_body(second).await["error"], "rate limit exceeded");
    }

    #[tokio::test]
    async fn pokemon_endpoint_maps_missing_pokemon_to_404() {
        let pokeapi = MockServer::start().await;
        let translations = MockServer::start().await;
        Mock::given(matchers::method("GET"))
            .and(matchers::path("/pokemon-species/nope"))
            .respond_with(ResponseTemplate::new(404))
            .mount(&pokeapi)
            .await;
        let app = create_app(state_for(&pokeapi, &translations));

        let response = app
            .oneshot(request("/pokemon/nope"))
            .await
            .expect("request should be handled");

        assert_eq!(response.status(), StatusCode::NOT_FOUND);
        assert_eq!(json_body(response).await["error"], "pokemon not found");
    }

    async fn mount_species(
        server: &MockServer,
        name: &str,
        is_legendary: bool,
        habitat: &str,
        description: &str,
    ) {
        Mock::given(matchers::method("GET"))
            .and(matchers::path(format!("/pokemon-species/{name}")))
            .respond_with(ResponseTemplate::new(200).set_body_json(serde_json::json!({
                "name": name,
                "is_legendary": is_legendary,
                "habitat": { "name": habitat },
                "flavor_text_entries": [
                    { "flavor_text": description, "language": { "name": "en" } }
                ]
            })))
            .mount(server)
            .await;
    }

    fn state_for(pokeapi: &MockServer, translations: &MockServer) -> AppState {
        let telemetry = Telemetry::new("test-service");
        AppState::new(
            PokedexService::new(
                PokeApiClient::new(pokeapi.uri(), Duration::from_secs(1))
                    .expect("client should build"),
                TranslationClient::new(translations.uri(), Duration::from_secs(1))
                    .expect("client should build"),
                telemetry.metrics(),
            ),
            telemetry,
            RateLimiter::new(100.0, 100).expect("rate limiter should build"),
        )
    }

    fn request(path: &str) -> Request<Body> {
        Request::builder()
            .uri(path)
            .body(Body::empty())
            .expect("request should build")
    }

    async fn json_body(response: axum::response::Response) -> Value {
        let bytes = body_bytes(response).await;
        serde_json::from_slice(&bytes).expect("body should be valid JSON")
    }

    async fn text_body(response: axum::response::Response) -> String {
        String::from_utf8(body_bytes(response).await.to_vec()).expect("body should be valid UTF-8")
    }

    async fn body_bytes(response: axum::response::Response) -> axum::body::Bytes {
        to_bytes(response.into_body(), 1024 * 1024)
            .await
            .expect("body should be readable")
    }
}