added LRU cache to avoid asking firebase to decode the tokens

added prometheus metrics for the token cache

backend/src/utils/auth.ts

@@ -1,9 +1,44 @@
 import admin from "firebase-admin";
 import { UserRecord } from "firebase-admin/lib/auth/user-record";
 import { DecodedIdToken } from "firebase-admin/lib/auth/token-verifier";
+import LRUCache from "lru-cache";
+import {
+  recordTokenCacheAccess,
+  setTokenCacheLength,
+  setTokenCacheSize,
+} from "./prometheus";
+
+const tokenCache = new LRUCache<string, DecodedIdToken>({
+  max: 20000,
+  maxSize: 20000000, // 20MB
+  sizeCalculation: (token, key): number =>
+    JSON.stringify(token).length + key.length, //sizeInBytes
+});
+
+const TOKEN_CACHE_BUFFER = 1000 * 60 * 5; // 5 minutes
 
 export async function verifyIdToken(idToken: string): Promise<DecodedIdToken> {
-  return await admin.auth().verifyIdToken(idToken, true);
+  setTokenCacheLength(tokenCache.size);
+  setTokenCacheSize(tokenCache.calculatedSize ?? 0);
+
+  const cached = tokenCache.get(idToken);
+
+  if (cached) {
+    const expirationDate = (cached.exp - TOKEN_CACHE_BUFFER) * 1000;
+
+    if (expirationDate > Date.now()) {
+      recordTokenCacheAccess("hit_expired");
+      tokenCache.delete(idToken);
+    } else {
+      recordTokenCacheAccess("hit");
+      return cached;
+    }
+  }
+  recordTokenCacheAccess("miss");
+
+  const decoded = await admin.auth().verifyIdToken(idToken, true);
+  tokenCache.set(idToken, decoded);
+  return decoded;
 }
 
 export async function updateUserEmail(

backend/src/utils/prometheus.ts

@@ -227,3 +227,33 @@ export function recordRequestCountry(
 
   requestCountry.inc({ path: pathNoGet, country });
 }
+
+const tokenCacheAccess = new Counter({
+  name: "api_token_cache_access",
+  help: "Token cache access",
+  labelNames: ["status"],
+});
+
+export function recordTokenCacheAccess(
+  status: "hit" | "miss" | "hit_expired"
+): void {
+  tokenCacheAccess.inc({ status });
+}
+
+const tokenCacheSize = new Gauge({
+  name: "api_token_cache_size",
+  help: "Token cache size",
+});
+
+export function setTokenCacheSize(size: number): void {
+  tokenCacheSize.set(size);
+}
+
+const tokenCacheLength = new Gauge({
+  name: "api_token_cache_length",
+  help: "Token cache length",
+});
+
+export function setTokenCacheLength(length: number): void {
+  tokenCacheLength.set(length);
+}