Instrument catalog with prometheus

Signed-off-by: Johan Haals <johan.haals@gmail.com>

packages/backend/package.json

@@ -58,9 +58,11 @@
     "example-app": "^0.2.41",
     "express": "^4.17.1",
     "express-promise-router": "^4.1.0",
+    "express-prom-bundle": "^6.3.6",
     "knex": "^0.95.1",
     "pg": "^8.3.0",
     "pg-connection-string": "^2.3.0",
+    "prom-client": "^13.2.0",
     "sqlite3": "^5.0.1",
     "winston": "^3.2.1"
   },

packages/backend/src/index.ts

@@ -36,6 +36,7 @@ import {
 } from '@backstage/backend-common';
 import { Config } from '@backstage/config';
 import healthcheck from './plugins/healthcheck';
+import { metricsInit, metricsHandler } from './metrics';
 import auth from './plugins/auth';
 import catalog from './plugins/catalog';
 import codeCoverage from './plugins/codecoverage';
@@ -72,6 +73,7 @@ function makeCreateEnv(config: Config) {
 }
 
 async function main() {
+  metricsInit();
   const logger = getRootLogger();
 
   logger.info(
@@ -124,6 +126,7 @@ async function main() {
   const service = createServiceBuilder(module)
     .loadConfig(config)
     .addRouter('', await healthcheck(healthcheckEnv))
+    .addRouter('', metricsHandler())
     .addRouter('/api', apiRouter)
     .addRouter('', await app(appEnv));
 

packages/backend/src/metrics.ts

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2021 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { useHotCleanup } from '@backstage/backend-common';
+import { RequestHandler, Request } from 'express';
+import promBundle from 'express-prom-bundle';
+import prom from 'prom-client';
+import * as url from 'url';
+
+const rootRegEx = new RegExp('^/([^/]*)/.*');
+const apiRegEx = new RegExp('^/api/([^/]*)/.*');
+
+function normalizePath(req: Request): string {
+  const path = url.parse(req.originalUrl || req.url).pathname || '/';
+
+  // Capture /api/ and the plugin name
+  if (apiRegEx.test(path)) {
+    return path.replace(apiRegEx, '/api/$1');
+  }
+
+  // Only the first path segment at root level
+  return path.replace(rootRegEx, '/$1');
+}
+
+export function metricsInit(): void {
+  prom.collectDefaultMetrics({ prefix: 'backstage_' });
+}
+
+/**
+ * Adds a /metrics endpoint, register default runtime metrics and instrument the router.
+ */
+export function metricsHandler(): RequestHandler {
+  // We can only initialize the metrics once and have to clean them up between hot reloads
+  useHotCleanup(module, () => prom.register.clear());
+
+  return promBundle({
+    includeMethod: true,
+    includePath: true,
+    // Using includePath alone is problematic, as it will include path labels with high
+    // cardinality (e.g. path params). Instead we would have to template them. However, this
+    // is difficult, as every backend plugin might use different routes. Instead we only take
+    // the first directory of the path, to have at least an idea how each plugin performs:
+    normalizePath,
+    promClient: { collectDefaultMetrics: {} },
+  });
+}