feat(getOrFetchUser): redis cache; use debug counters

Author: Dragory

backend/src/utils/getOrFetchUser.ts

@@ -1,23 +1,65 @@
 import { Client, User } from "discord.js";
+import { redis } from "../data/redis.js";
+import { incrementDebugCounter } from "../debugCounters.js";
 
 const getOrFetchUserPromises: Map<string, Promise<User | undefined>> = new Map();
 
+const UNKNOWN_KEY = "__UNKNOWN__";
+
+const baseCacheTimeSeconds = 60 * 60; // 1 hour
+const cacheTimeJitterSeconds = 5 * 60; // 5 minutes
+
+// Use jitter on cache time to avoid tons of keys expiring at the same time
+const generateCacheTime = () => {
+  const jitter = Math.floor(Math.random() * cacheTimeJitterSeconds);
+  return baseCacheTimeSeconds + jitter;
+};
+
 /**
  * Gets a user from cache or fetches it from the API if not cached.
  * Concurrent requests are merged.
  */
 export async function getOrFetchUser(bot: Client, userId: string): Promise<User | undefined> {
+  // 1. Check Discord.js cache
   const cachedUser = bot.users.cache.get(userId);
   if (cachedUser) {
+    incrementDebugCounter("getOrFetchUser:djsCache");
     return cachedUser;
   }
 
+  // 2. Check Redis
+  const redisCacheKey = `cache:user:${userId}`;
+  const userData = await redis.get(redisCacheKey);
+  if (userData) {
+    incrementDebugCounter("getOrFetchUser:redisCache");
+    if (userData === UNKNOWN_KEY) {
+      console.log("Found unknown user in Redis cache");
+      return undefined;
+    }
+    console.log("Found user in Redis cache");
+    // @ts-expect-error Replace with a proper solution once that exists
+    return new User(bot, JSON.parse(userData));
+  }
+
   if (!getOrFetchUserPromises.has(userId)) {
+    incrementDebugCounter("getOrFetchUser:fresh");
     getOrFetchUserPromises.set(
       userId,
       bot.users
         .fetch(userId)
-        .catch(() => undefined)
+        .catch(async () => {
+          return undefined;
+        })
+        .then(async (user) => {
+          const cacheValue = user ? JSON.stringify(user.toJSON()) : UNKNOWN_KEY;
+          await redis.set(redisCacheKey, cacheValue, {
+            expiration: {
+              type: "EX",
+              value: generateCacheTime(),
+            },
+          });
+          return user;
+        })
         .finally(() => {
           getOrFetchUserPromises.delete(userId);
         }),