fix: cache GitHub App ID to reduce SSM calls (#4994)

Cache the GitHub App ID at module level to avoid repeated SSM parameter
fetches on every rate limit metric update. The value is now fetched once
per Lambda execution context and reused across invocations.

Especially when multiple events are badged, repeatedly calling the SSM
Parameter API feels wasteful and adds a non-negligible amount of latency
to several functions in the scale-up Lambda.

Author: Brend-Smits

lambdas/functions/control-plane/src/github/rate-limit.test.ts

@@ -3,6 +3,7 @@ import { createSingleMetric } from '@aws-github-runner/aws-powertools-util';
 import { MetricUnit } from '@aws-lambda-powertools/metrics';
 import { metricGitHubAppRateLimit } from './rate-limit';
 import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { getParameter } from '@aws-github-runner/aws-ssm-util';
 
 process.env.PARAMETER_GITHUB_APP_ID_NAME = 'test';
 vi.mock('@aws-github-runner/aws-ssm-util', async () => {
@@ -78,4 +79,27 @@ describe('metricGitHubAppRateLimit', () => {
 
     expect(createSingleMetric).not.toHaveBeenCalled();
   });
+
+  it('should cache GitHub App ID and only call getParameter once', async () => {
+    // Reset modules to clear the appIdPromise cache
+    vi.resetModules();
+    const { metricGitHubAppRateLimit: freshMetricFunction } = await import('./rate-limit');
+
+    process.env.ENABLE_METRIC_GITHUB_APP_RATE_LIMIT = 'true';
+    const headers: ResponseHeaders = {
+      'x-ratelimit-remaining': '10',
+      'x-ratelimit-limit': '60',
+    };
+
+    const mockGetParameter = vi.mocked(getParameter);
+    mockGetParameter.mockClear();
+
+    await freshMetricFunction(headers);
+    await freshMetricFunction(headers);
+    await freshMetricFunction(headers);
+
+    // getParameter should only be called once due to caching
+    expect(mockGetParameter).toHaveBeenCalledTimes(1);
+    expect(mockGetParameter).toHaveBeenCalledWith(process.env.PARAMETER_GITHUB_APP_ID_NAME);
+  });
 });

lambdas/functions/control-plane/src/github/rate-limit.ts

@@ -4,6 +4,16 @@ import { MetricUnit } from '@aws-lambda-powertools/metrics';
 import yn from 'yn';
 import { getParameter } from '@aws-github-runner/aws-ssm-util';
 
+// Cache the app ID to avoid repeated SSM calls across Lambda invocations
+let appIdPromise: Promise<string> | null = null;
+
+async function getAppId(): Promise<string> {
+  if (!appIdPromise) {
+    appIdPromise = getParameter(process.env.PARAMETER_GITHUB_APP_ID_NAME);
+  }
+  return appIdPromise;
+}
+
 export async function metricGitHubAppRateLimit(headers: ResponseHeaders): Promise<void> {
   try {
     const remaining = parseInt(headers['x-ratelimit-remaining'] as string);
@@ -13,7 +23,7 @@ export async function metricGitHubAppRateLimit(headers: ResponseHeaders): Promis
 
     const updateMetric = yn(process.env.ENABLE_METRIC_GITHUB_APP_RATE_LIMIT);
     if (updateMetric) {
-      const appId = await getParameter(process.env.PARAMETER_GITHUB_APP_ID_NAME);
+      const appId = await getAppId();
       const metric = createSingleMetric('GitHubAppRateLimitRemaining', MetricUnit.Count, remaining, {
         AppId: appId,
       });