diff --git a/packages/hydrooj/package.json b/packages/hydrooj/package.json
index 0d6a90585..08e6ed169 100644
--- a/packages/hydrooj/package.json
+++ b/packages/hydrooj/package.json
@@ -1,6 +1,6 @@
 {
     "name": "hydrooj",
-    "version": "3.15.14",
+    "version": "3.15.15",
     "bin": "bin/hydrooj.js",
     "main": "src/loader",
     "module": "src/loader",
diff --git a/packages/hydrooj/src/service/server.ts b/packages/hydrooj/src/service/server.ts
index 1c7bc20d8..407c18c21 100644
--- a/packages/hydrooj/src/service/server.ts
+++ b/packages/hydrooj/src/service/server.ts
@@ -391,6 +391,7 @@ export function Connection(
             args, request, response, user, domain, UiContext,
         } = ctx.HydroContext;
         const h = new RouteConnHandler(ctx, args, request, response, user, domain, UiContext);
+        await bus.emit('connection/create', h);
         ctx.handler = h;
         h.conn = conn;
         try {
@@ -402,7 +403,10 @@ export function Connection(
                     h.message(JSON.parse(e.data.toString()));
                 };
             }
-            conn.onclose = () => h.cleanup?.(args);
+            conn.onclose = () => {
+                h.cleanup?.(args);
+                bus.emit('connection/close', h);
+            };
         } catch (e) {
             await h.onerror(e);
         }
diff --git a/packages/prom-client/metrics.ts b/packages/prom-client/metrics.ts
index 7e448846e..a66ff8161 100644
--- a/packages/prom-client/metrics.ts
+++ b/packages/prom-client/metrics.ts
@@ -35,6 +35,16 @@ bus.on('handler/after/ProblemSubmit', (that) => {
     submissionCounter.inc({ lang: that.args.lang, domainId: that.args.domainId });
 });
 
+const connectionGauge = createMetric(Gauge, 'hydro_connection', 'connectioncount', {
+    labelNames: ['domainId'],
+});
+bus.on('connection/create', (h) => {
+    connectionGauge.inc({ domainId: h.args.domainId });
+});
+bus.on('connection/close', (h) => {
+    connectionGauge.inc({ domainId: h.args.domainId }, -1);
+});
+
 const taskColl = db.collection('task');
 createMetric(Gauge, 'hydro_task', 'taskcount', {
     async collect() {
diff --git a/packages/prom-client/package.json b/packages/prom-client/package.json
index 783c8baab..819400e33 100644
--- a/packages/prom-client/package.json
+++ b/packages/prom-client/package.json
@@ -1,6 +1,6 @@
 {
     "name": "@hydrooj/prom-client",
-    "version": "0.0.1",
+    "version": "0.0.2",
     "dependencies": {
         "prom-client": "^14.0.1"
     }