From d97992e96798c49e14aa496f0d23af82b3a73d36 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ullrich=20Sch=C3=A4fer?= <hi+github@ullrich.is>
Date: Fri, 3 Apr 2026 10:02:21 +0100
Subject: [PATCH] Guard all metric registrations against re-evaluation

Vite's dev server can re-evaluate modules, causing prom-client
"already registered" errors for all metrics, not just default ones.
Use getOrCreate pattern to reuse existing metrics from the registry.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 apps/planner/app/lib/metrics.server.ts | 54 ++++++++++++++++----------
 1 file changed, 33 insertions(+), 21 deletions(-)

diff --git a/apps/planner/app/lib/metrics.server.ts b/apps/planner/app/lib/metrics.server.ts
index 059e5ab..46d2060 100644
--- a/apps/planner/app/lib/metrics.server.ts
+++ b/apps/planner/app/lib/metrics.server.ts
@@ -1,32 +1,44 @@
 import client from "prom-client";
 
-// Collect default Node.js metrics (event loop, heap, GC)
-// Guard against duplicate registration during HMR
+// Guard all metric registration — Vite's dev server can re-evaluate
+// this module, causing "already registered" errors.
+function getOrCreate<T>(name: string, create: () => T): T {
+  return (client.register.getSingleMetric(name) as T) ?? create();
+}
+
 if (!client.register.getSingleMetric("process_cpu_user_seconds_total")) {
   client.collectDefaultMetrics();
 }
 
-export const httpRequestDuration = new client.Histogram({
-  name: "http_request_duration_seconds",
-  help: "Duration of HTTP requests in seconds",
-  labelNames: ["method", "route", "status"] as const,
-  buckets: [0.01, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5],
-});
+export const httpRequestDuration = getOrCreate("http_request_duration_seconds", () =>
+  new client.Histogram({
+    name: "http_request_duration_seconds",
+    help: "Duration of HTTP requests in seconds",
+    labelNames: ["method", "route", "status"] as const,
+    buckets: [0.01, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5],
+  }),
+);
 
-export const plannerActiveSessions = new client.Gauge({
-  name: "planner_active_sessions",
-  help: "Number of active planner sessions",
-});
+export const plannerActiveSessions = getOrCreate("planner_active_sessions", () =>
+  new client.Gauge({
+    name: "planner_active_sessions",
+    help: "Number of active planner sessions",
+  }),
+);
 
-export const plannerConnectedClients = new client.Gauge({
-  name: "planner_connected_clients",
-  help: "Number of connected WebSocket clients",
-});
+export const plannerConnectedClients = getOrCreate("planner_connected_clients", () =>
+  new client.Gauge({
+    name: "planner_connected_clients",
+    help: "Number of connected WebSocket clients",
+  }),
+);
 
-export const brouterRequestDuration = new client.Histogram({
-  name: "brouter_request_duration_seconds",
-  help: "Duration of BRouter API requests in seconds",
-  buckets: [0.1, 0.25, 0.5, 1, 2, 5, 10],
-});
+export const brouterRequestDuration = getOrCreate("brouter_request_duration_seconds", () =>
+  new client.Histogram({
+    name: "brouter_request_duration_seconds",
+    help: "Duration of BRouter API requests in seconds",
+    buckets: [0.1, 0.25, 0.5, 1, 2, 5, 10],
+  }),
+);
 
 export const registry = client.register;