Feature/prometheus metrics

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **Prometheus application metrics** — Replace JSON `/api/metrics` endpoint with Prometheus exposition format using Ktor MicrometerMetrics plugin; exposes `shoppimo_websocket_connections_total` (active WebSocket connections) and `shoppimo_shopping_lists_total` (total lists in database) alongside JVM and HTTP request metrics
+
 ## [6.1.0] - 2026-03-26
 
 ### Added

README.md

@@ -18,6 +18,7 @@ A real-time collaborative shopping list app. Create a list, share the link, and
 - **Dark Mode** — Light, dark, and system-preference theme toggle
 - **Smart Autocomplete** — Suggests items from your cross-list history as you type
 - **Push Notifications** — Opt-in Web Push alerts when list changes happen
+- **Prometheus Metrics** — Application metrics (online users, list count, JVM, HTTP requests) exposed for Prometheus scraping
 
 ## Tech Stack
 
@@ -126,6 +127,7 @@ For production deployment, see [DEPLOYMENT.md](DEPLOYMENT.md).
 | Method | Path | Description |
 |--------|------|-------------|
 | `GET` | `/api/health` | Health check |
+| `GET` | `/api/metrics` | Prometheus metrics |
 | `POST` | `/api/lists` | Create a new list |
 | `GET` | `/api/lists/{id}` | Get a list |
 | `POST` | `/api/lists/{id}/items` | Add an item |

backend/build.gradle.kts

@@ -57,6 +57,10 @@ dependencies {
     implementation("com.zaxxer:HikariCP:$hikaricp_version")
     implementation("org.postgresql:postgresql:$postgresql_version")
 
+    // Metrics
+    implementation("io.ktor:ktor-server-metrics-micrometer")
+    implementation("io.micrometer:micrometer-registry-prometheus:1.11.0")
+
     // Logging
     implementation("ch.qos.logback:logback-classic:$logback_version")
 

backend/src/main/kotlin/com/shoppinglist/Application.kt

@@ -20,6 +20,7 @@ fun Application.module() {
     DatabaseFactory.init()
     configureSerialization()
     configureCORS()
+    configureMetrics()
 
     val pushRepository = PushSubscriptionRepositoryImpl()
     val pushService = runCatching {

backend/src/main/kotlin/com/shoppinglist/models/HealthResponse.kt

@@ -14,21 +14,4 @@ data class HealthResponse(
 data class ErrorResponse(
     val status: String,
     val message: String
-)
-
-@Serializable
-data class MemoryInfo(
-    val total: Long,
-    val free: Long,
-    val used: Long,
-    val max: Long
-)
-
-@Serializable
-data class MetricsResponse(
-    val timestamp: Long,
-    val uptime: Long,
-    val memory: MemoryInfo,
-    val threads: Int,
-    val database: String
 )

backend/src/main/kotlin/com/shoppinglist/plugins/Metrics.kt

@@ -0,0 +1,62 @@
+package com.shoppinglist.plugins
+
+import com.shoppinglist.database.ShoppingLists
+import io.ktor.server.application.*
+import io.ktor.server.metrics.micrometer.*
+import io.micrometer.core.instrument.Gauge
+import io.micrometer.core.instrument.binder.jvm.JvmGcMetrics
+import io.micrometer.core.instrument.binder.jvm.JvmMemoryMetrics
+import io.micrometer.core.instrument.binder.jvm.JvmThreadMetrics
+import io.micrometer.core.instrument.binder.system.ProcessorMetrics
+import io.micrometer.prometheus.PrometheusConfig
+import io.micrometer.prometheus.PrometheusMeterRegistry
+import org.jetbrains.exposed.sql.selectAll
+import org.jetbrains.exposed.sql.transactions.transaction
+import java.util.concurrent.atomic.AtomicLong
+
+lateinit var appMicrometerRegistry: PrometheusMeterRegistry
+    private set
+
+// Falls back to last known value when the DB query fails inside the gauge supplier
+private val cachedListCount = AtomicLong(0)
+
+fun Application.configureMetrics() {
+    appMicrometerRegistry = PrometheusMeterRegistry(PrometheusConfig.DEFAULT)
+
+    install(MicrometerMetrics) {
+        registry = appMicrometerRegistry
+        meterBinders = listOf(
+            JvmMemoryMetrics(),
+            JvmGcMetrics(),
+            JvmThreadMetrics(),
+            ProcessorMetrics()
+        )
+    }
+
+    Gauge.builder("shoppimo_websocket_connections_total", this) { app ->
+        try {
+            val wsService = app.attributes.getOrNull(WebSocketServiceKey)
+            wsService?.getConnectionManager()
+                ?.getActiveListIds()
+                ?.sumOf { listId -> wsService.getConnectionManager().getConnectionCount(listId) }
+                ?.toDouble()
+                ?: 0.0
+        } catch (_: Exception) {
+            0.0
+        }
+    }
+        .description("Total number of active WebSocket connections across all lists")
+        .register(appMicrometerRegistry)
+
+    Gauge.builder("shoppimo_shopping_lists_total", cachedListCount) { cached ->
+        try {
+            val count = transaction { ShoppingLists.selectAll().count() }
+            cached.set(count)
+            count.toDouble()
+        } catch (_: Exception) {
+            cached.get().toDouble()
+        }
+    }
+        .description("Total number of shopping lists in the database")
+        .register(appMicrometerRegistry)
+}

backend/src/main/kotlin/com/shoppinglist/plugins/Routing.kt

@@ -8,8 +8,7 @@ import com.shoppinglist.routes.listRoutes
 import com.shoppinglist.routes.pushRoutes
 import com.shoppinglist.models.HealthResponse
 import com.shoppinglist.models.ErrorResponse
-import com.shoppinglist.models.MetricsResponse
-import com.shoppinglist.models.MemoryInfo
+
 import com.shoppinglist.repository.PushSubscriptionRepository
 import com.shoppinglist.services.PushNotificationService
 
@@ -52,38 +51,7 @@ fun Application.configureRouting(
             }
 
             get("/metrics") {
-                try {
-                    val runtime = Runtime.getRuntime()
-                    val memoryInfo = MemoryInfo(
-                        total = runtime.totalMemory(),
-                        free = runtime.freeMemory(),
-                        used = runtime.totalMemory() - runtime.freeMemory(),
-                        max = runtime.maxMemory()
-                    )
-
-                    val metrics = MetricsResponse(
-                        timestamp = System.currentTimeMillis(),
-                        uptime = java.lang.management.ManagementFactory.getRuntimeMXBean().uptime,
-                        memory = memoryInfo,
-                        threads = java.lang.management.ManagementFactory.getThreadMXBean().threadCount,
-                        database = try {
-                            com.shoppinglist.database.DatabaseFactory.testConnection()
-                            "connected"
-                        } catch (e: Exception) {
-                            "disconnected"
-                        }
-                    )
-
-                    call.respond(metrics)
-                } catch (e: Exception) {
-                    call.respond(
-                        status = HttpStatusCode.InternalServerError,
-                        message = ErrorResponse(
-                            status = "ERROR",
-                            message = e.message ?: "Unknown error"
-                        )
-                    )
-                }
+                call.respond(appMicrometerRegistry.scrape())
             }
 
             if (pushRepository != null) {