From ee0b71a4accfc42929fdfaf1664c0fb96b62b24a Mon Sep 17 00:00:00 2001
From: Valentin Brandl <mail+github@vbrandl.net>
Date: Sat, 8 Oct 2022 01:28:32 +0200
Subject: [PATCH] Start second server on different port for metrics endpoint
 (#1444)

* Start second server on different port for metrics endpoint

* Don't track metrics on metrics server
---
 examples/prometheus-metrics/src/main.rs | 63 +++++++++++++++++--------
 1 file changed, 44 insertions(+), 19 deletions(-)

diff --git a/examples/prometheus-metrics/src/main.rs b/examples/prometheus-metrics/src/main.rs
index 1152b3c7..5ab3fbdd 100644
--- a/examples/prometheus-metrics/src/main.rs
+++ b/examples/prometheus-metrics/src/main.rs
@@ -23,6 +23,46 @@ use std::{
 };
 use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
+fn metrics_app() -> Router {
+    let recorder_handle = setup_metrics_recorder();
+    Router::new().route("/metrics", get(move || ready(recorder_handle.render())))
+}
+
+fn main_app() -> Router {
+    Router::new()
+        .route("/fast", get(|| async {}))
+        .route(
+            "/slow",
+            get(|| async {
+                tokio::time::sleep(Duration::from_secs(1)).await;
+            }),
+        )
+        .route_layer(middleware::from_fn(track_metrics))
+}
+
+async fn start_main_server() {
+    let app = main_app();
+
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap()
+}
+
+async fn start_metrics_server() {
+    let app = metrics_app();
+
+    // NOTE: expose metrics enpoint on a different port
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3001));
+    tracing::debug!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap()
+}
+
 #[tokio::main]
 async fn main() {
     tracing_subscriber::registry()
@@ -33,25 +73,10 @@ async fn main() {
         .with(tracing_subscriber::fmt::layer())
         .init();
 
-    let recorder_handle = setup_metrics_recorder();
-
-    let app = Router::new()
-        .route("/fast", get(|| async {}))
-        .route(
-            "/slow",
-            get(|| async {
-                tokio::time::sleep(Duration::from_secs(1)).await;
-            }),
-        )
-        .route("/metrics", get(move || ready(recorder_handle.render())))
-        .route_layer(middleware::from_fn(track_metrics));
-
-    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
-    tracing::debug!("listening on {}", addr);
-    axum::Server::bind(&addr)
-        .serve(app.into_make_service())
-        .await
-        .unwrap();
+    // The `/metrics` endpoint should not be publicly available. If behind a reverse proxy, this
+    // can be achieved by rejecting requests to `/metrics`. In this example, a second server is
+    // started on another port to expose `/metrics`.
+    let (_main_server, _metrics_server) = tokio::join!(start_main_server(), start_metrics_server());
 }
 
 fn setup_metrics_recorder() -> PrometheusHandle {