From 65304798de612b66d40b9e3e36dea79d6bae4895 Mon Sep 17 00:00:00 2001
From: Lars Strojny <lars@strojny.net>
Date: Thu, 24 Nov 2022 19:35:02 +0100
Subject: [PATCH] Memoized renderering (#37)

Pre-render metrics on discovery once and then reuse rendered response.
---
 src/adapters/http/api.ts            |  2 +-
 src/platform.ts                     |  2 +-
 src/prometheus.ts                   | 23 ++++++++++++-----------
 tests/adapters/http/fastify.test.ts | 10 +++++-----
 4 files changed, 19 insertions(+), 18 deletions(-)

diff --git a/src/adapters/http/api.ts b/src/adapters/http/api.ts
index 2b9abfb..1424b93 100644
--- a/src/adapters/http/api.ts
+++ b/src/adapters/http/api.ts
@@ -28,5 +28,5 @@ export interface HttpServer {
     onMetrics(): HttpResponse
     onNotFound(): HttpResponse
     onError(error: Error): HttpResponse
-    updateMetrics(metrics: Metric[]): void
+    onMetricsDiscovery(metrics: Metric[]): void
 }
diff --git a/src/platform.ts b/src/platform.ts
index 0087638..4bc6b06 100644
--- a/src/platform.ts
+++ b/src/platform.ts
@@ -48,7 +48,7 @@ export class PrometheusExporterPlatform implements IndependentPlatformPlugin {
         discover({ log: this.log, config: this.config })
             .then((devices) => {
                 const metrics = aggregate(devices, new Date())
-                this.httpServer.updateMetrics(metrics)
+                this.httpServer.onMetricsDiscovery(metrics)
                 this.log.debug('HAP discovery completed, %d metrics discovered', metrics.length)
                 this.startHapDiscovery()
             })
diff --git a/src/prometheus.ts b/src/prometheus.ts
index 60d9d6c..7ffdf7a 100644
--- a/src/prometheus.ts
+++ b/src/prometheus.ts
@@ -44,13 +44,17 @@ function headers(contentType: string, headers: Record<string, string> = {}): Rec
 }
 
 export class PrometheusServer implements HttpServer {
-    private metricsInitialized = false
-    private metrics: Metric[] = []
+    private metricsDiscovered = false
+    private metricsResponse = ''
 
-    constructor(public readonly config: HttpConfig, public readonly log: Logger | undefined = undefined) {}
+    constructor(
+        public readonly config: HttpConfig,
+        public readonly log: Logger | undefined = undefined,
+        private readonly renderer: MetricsRenderer = new MetricsRenderer(config.prefix),
+    ) {}
 
     onRequest(): HttpResponse | undefined {
-        if (!this.metricsInitialized) {
+        if (!this.metricsDiscovered) {
             return {
                 statusCode: 503,
                 headers: headers(textContentType, { 'Retry-After': String(retryAfterWhileDiscovery) }),
@@ -60,13 +64,10 @@ export class PrometheusServer implements HttpServer {
     }
 
     onMetrics(): HttpResponse {
-        const renderer = new MetricsRenderer(this.config.prefix)
-        const metrics = this.metrics.map((metric) => renderer.render(metric)).join('\n')
-
         return {
             statusCode: 200,
             headers: headers(metricsContentType),
-            body: metrics,
+            body: this.metricsResponse,
         }
     }
 
@@ -86,9 +87,9 @@ export class PrometheusServer implements HttpServer {
         }
     }
 
-    updateMetrics(metrics: Metric[]): void {
-        this.metrics = metrics
-        this.metricsInitialized = true
+    onMetricsDiscovery(metrics: Metric[]): void {
+        this.metricsResponse = metrics.map((metric) => this.renderer.render(metric)).join('\n')
+        this.metricsDiscovered = true
     }
 }
 
diff --git a/tests/adapters/http/fastify.test.ts b/tests/adapters/http/fastify.test.ts
index 470a6e1..dddaaa8 100644
--- a/tests/adapters/http/fastify.test.ts
+++ b/tests/adapters/http/fastify.test.ts
@@ -46,7 +46,7 @@ describe('Fastify HTTP adapter', () => {
 
     test('Serves 404 on / when metrics are available', () => {
         const testServer = createTestServer()
-        testServer.prometheus.updateMetrics([])
+        testServer.prometheus.onMetricsDiscovery([])
 
         return request(testServer.http)
             .get('/')
@@ -58,7 +58,7 @@ describe('Fastify HTTP adapter', () => {
     test('Serves metrics', () => {
         const testServer = createTestServer()
         const timestamp = new Date('2020-01-01 00:00:00 UTC')
-        testServer.prometheus.updateMetrics([
+        testServer.prometheus.onMetricsDiscovery([
             new Metric('metric', 0.1, timestamp, { name: 'metric' }),
             new Metric('total_something', 100, timestamp, { name: 'counter' }),
         ])
@@ -79,7 +79,7 @@ describe('Fastify HTTP adapter', () => {
 
     test('Basic auth denied without user', () => {
         const testServer = createTestServerWithBasicAuth({ joanna: secretAsBcrypt })
-        testServer.prometheus.updateMetrics([])
+        testServer.prometheus.onMetricsDiscovery([])
 
         return request(testServer.http)
             .get('/metrics')
@@ -90,7 +90,7 @@ describe('Fastify HTTP adapter', () => {
 
     test('Basic auth denied with incorrect user', () => {
         const testServer = createTestServerWithBasicAuth({ joanna: secretAsBcrypt })
-        testServer.prometheus.updateMetrics([])
+        testServer.prometheus.onMetricsDiscovery([])
 
         return request(testServer.http)
             .get('/metrics')
@@ -103,7 +103,7 @@ describe('Fastify HTTP adapter', () => {
     test('Basic auth grants access', () => {
         const testServer = createTestServerWithBasicAuth({ joanna: secretAsBcrypt })
         const timestamp = new Date('2020-01-01 00:00:00 UTC')
-        testServer.prometheus.updateMetrics([
+        testServer.prometheus.onMetricsDiscovery([
             new Metric('metric', 0.1, timestamp, { name: 'metric' }),
             new Metric('total_something', 100, timestamp, { name: 'counter' }),
         ])