feat(cache): add caching to statistics

src/app.ts

@@ -3,12 +3,15 @@ import express from "express";
 import { flatten } from "lodash/fp";
 
 import packageJson from "../package.json";
+import { createCache } from "./caching/caching-service";
 import { corsOrigins, port } from "./config";
 import { getGitHubData } from "./github/github-service";
 import { matomoConfig } from "./matomo/matomo-config";
 import { getMultiSiteMatomoData } from "./matomo/matomo-service";
 import { getYoutubeData } from "./youtube/youtube-service";
 
+const CACHE_TTL = 10 * 60 * 1000;
+
 const app = express();
 
 app.use(
@@ -25,14 +28,28 @@ app.get("/healthz", (req, res) => {
   res.send("OK");
 });
 
+const statsCache = createCache(
+  async () =>
+    Promise.all([
+      getMultiSiteMatomoData(matomoConfig),
+      getYoutubeData(),
+      getGitHubData(),
+    ]).then(flatten),
+  CACHE_TTL
+);
+
 app.get("/statistics", (req, res) => {
-  void Promise.all([
-    getMultiSiteMatomoData(matomoConfig),
-    getYoutubeData(),
-    getGitHubData(),
-  ])
-    .then(flatten)
-    .then((data) => res.json(data));
+  res.json({
+    lastFetchTimestamp: statsCache.getLastFetchTimestamp(),
+    result: statsCache.read(),
+  });
+});
+
+app.post("/refresh", (req, res) => {
+  void statsCache
+    .refresh()
+    .then(() => res.status(204).send())
+    .catch(() => res.status(500).send("Error refreshing cache"));
 });
 
 app.listen(port, () => {

src/caching/caching-service.test.ts

@@ -0,0 +1,14 @@
+import { createCache } from "./caching-service";
+
+describe("caching-utils", () => {
+  describe("createCache", () => {
+    it("reads cache with fetch data value", async () => {
+      const CACHE_TTL = 10 * 60 * 1000;
+      const cacheValue = ["foo", "bar"];
+
+      const { read, refresh } = createCache(() => cacheValue, CACHE_TTL);
+      await refresh();
+      expect(read()).toEqual(cacheValue);
+    });
+  });
+});

src/caching/caching-service.ts

@@ -0,0 +1,28 @@
+type CacheObject<T> = {
+  read: () => T | null;
+  refresh: () => Promise<void>;
+  getLastFetchTimestamp: () => number | null;
+};
+
+type FetchData<T> = () => Promise<T> | T;
+
+export const createCache = <T>(
+  fetchData: FetchData<T>,
+  ttl: number
+): CacheObject<T> => {
+  let cacheData: null | T = null;
+  let lastFetchTimestamp = 0;
+  let refreshTimeout: NodeJS.Timeout | null = null;
+  const refresh = async () => {
+    refreshTimeout?.unref();
+    cacheData = await fetchData();
+    lastFetchTimestamp = Date.now();
+    refreshTimeout = setTimeout(() => void refresh(), ttl);
+  };
+  void refresh();
+  return {
+    getLastFetchTimestamp: () => lastFetchTimestamp,
+    read: () => cacheData,
+    refresh,
+  };
+};