fix: use singleton PostHog client instead of per-request instantiation

apps/backend/middleware/legacy/mirror.middleware.ts

@@ -3,6 +3,16 @@ import { MirrorCommandQueue } from './commandqueue.mirror';
 
 import { PostHog } from 'posthog-node';
 
+let postHogClient: PostHog | null = null;
+function getPostHogClient(): PostHog {
+  if (!postHogClient) {
+    postHogClient = new PostHog(process.env.POSTHOG_API_KEY ?? '', {
+      host: 'https://us.i.posthog.com',
+    });
+  }
+  return postHogClient;
+}
+
 export const createBackendMirrorMiddleware =
   <T>(createCommand: (req: Request, data: T) => Promise<string[]>) =>
   async (req: Request, res: Response, next: NextFunction) => {
@@ -12,9 +22,7 @@ export const createBackendMirrorMiddleware =
     res.once('finish', () => {
       void (async () => {
         try {
-          const postHogClient = new PostHog(process.env.POSTHOG_API_KEY ?? '', {
-            host: 'https://us.i.posthog.com',
-          });
+          const client = getPostHogClient();
 
           console.log('Response finished, checking for mirror work...');
           const ok = res.statusCode >= 200 && res.statusCode < 305;
@@ -23,7 +31,7 @@ export const createBackendMirrorMiddleware =
           console.log('Response status:', res.statusCode, 'ok?', ok);
 
           const distinctId = (req as any).user?.id ?? req.ip ?? 'anonymous';
-          let mirrorOn = await postHogClient.isFeatureEnabled('backend-mirror', distinctId);
+          let mirrorOn = await client.isFeatureEnabled('backend-mirror', distinctId);
           mirrorOn ??= false;
 
           if (!ok || data == null || !mirrorOn) return;
@@ -32,8 +40,6 @@ export const createBackendMirrorMiddleware =
 
           const queue = MirrorCommandQueue.instance();
           queue.enqueue(await createCommand(req, data));
-
-          await postHogClient.shutdown();
         } catch (e) {
           console.error('Error in mirror middleware:', e);
         }

tests/unit/middleware/legacy/mirror.posthog.test.ts

@@ -0,0 +1,68 @@
+const mockPostHogInstance = {
+  isFeatureEnabled: jest.fn().mockResolvedValue(true),
+  shutdown: jest.fn().mockResolvedValue(undefined),
+};
+
+const PostHogConstructor = jest.fn(() => mockPostHogInstance);
+
+jest.mock('posthog-node', () => ({
+  PostHog: PostHogConstructor,
+}));
+
+jest.mock('../../../../apps/backend/middleware/legacy/commandqueue.mirror', () => ({
+  MirrorCommandQueue: {
+    instance: jest.fn(() => ({
+      enqueue: jest.fn(),
+    })),
+  },
+}));
+
+import { createBackendMirrorMiddleware } from '../../../../apps/backend/middleware/legacy/mirror.middleware';
+import { Request, Response } from 'express';
+import { EventEmitter } from 'events';
+
+function createMockReqRes() {
+  const req = { user: { id: 'user-1' }, ip: '127.0.0.1' } as unknown as Request;
+
+  const res = new EventEmitter() as Response & EventEmitter;
+  res.statusCode = 200;
+  (res as any).locals = {};
+  res.getHeader = jest.fn().mockReturnValue('application/json');
+  const origSend = jest.fn().mockReturnThis();
+  res.send = origSend;
+
+  return { req, res };
+}
+
+describe('PostHog client instantiation', () => {
+  beforeEach(() => {
+    PostHogConstructor.mockClear();
+    mockPostHogInstance.isFeatureEnabled.mockClear();
+    mockPostHogInstance.shutdown.mockClear();
+  });
+
+  it('creates PostHog at most once across multiple requests', async () => {
+    const createCommand = jest.fn().mockResolvedValue(['SQL1']);
+    const middleware = createBackendMirrorMiddleware(createCommand);
+
+    const { req: req1, res: res1 } = createMockReqRes();
+    const { req: req2, res: res2 } = createMockReqRes();
+    const next = jest.fn();
+
+    await middleware(req1, res1, next);
+    await middleware(req2, res2, next);
+
+    // Send responses to populate mirrorData
+    res1.send(JSON.stringify({ ok: true }));
+    res2.send(JSON.stringify({ ok: true }));
+
+    // Trigger finish events
+    res1.emit('finish');
+    res2.emit('finish');
+
+    // Allow async callbacks to settle
+    await new Promise((r) => setTimeout(r, 50));
+
+    expect(PostHogConstructor).toHaveBeenCalledTimes(1);
+  });
+});