Redis Edge Cache

Shielding MongoDB from the Thundering Herd with a transparent Redis proxy.

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1. The Problem: Thundering Herd

The product catalog is read‑heavy. During flash sales, thousands of users may hit GET /api/v1/products?page=1 simultaneously. Without caching, every request would trigger a MongoDB query, potentially overwhelming the database.

Reshma‑Core solves this with a transparent Redis edge cache that serves cached JSON directly from RAM. Cache misses are handled by the controller, and the response is automatically stored for subsequent requests.

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2. The Proxy Pattern (cacheMiddleware)

The cacheMiddleware (from cache.middleware.ts) intercepts all GET requests and acts as a proxy between the client and the controller.

How It Works

1. Generate a cache key – based on the full URL (including query parameters).
   Example: reshma:cache:/api/v1/products?page=1&limit=20&itemType=BANGLE

2. Check Redis for a cache hit:

   • If found → serve the JSON directly without calling the controller.
     Response header: X-Cache: HIT
   • If not found → call the controller, then hijack res.json to store a copy in Redis before sending it to the client.
     Response header: X-Cache: MISS

3. Cache duration – configurable per route (e.g., 300 seconds for product catalog).

Code excerpt (cache.middleware.ts):

const key = `reshma:cache:${req.originalUrl}`;
const cachedData = await redisClient.get(key);
if (cachedData) {
  res.setHeader("X-Cache", "HIT");
  return res.status(200).json(JSON.parse(cachedData));
}

res.setHeader("X-Cache", "MISS");
const originalJson = res.json.bind(res);
res.json = (body) => {
  if (res.statusCode >= 200 && res.statusCode < 300) {
    redisClient.setEx(key, durationInSeconds, JSON.stringify(body));
  }
  return originalJson(body);
};
next();

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3. Cache Invalidation

Cached data becomes stale when an admin updates a product. Instead of waiting for the TTL to expire, the CacheManager instantly invalidates all keys matching a pattern.

From CacheManager.invalidateCachePattern() (cache.utils.ts):

const searchPattern = `reshma:cache:${pattern}*`;
const keys = await redisClient.keys(searchPattern);
if (keys.length > 0) await redisClient.del(keys);

Where Invalidation Is Triggered

Inside AdminProductController, after every create, update, or soft‑delete:

CacheManager.invalidateCachePattern("/api/v1/products").catch((err) =>
  logger.error(`Background cache invalidation failed: ${err.message}`)
);

The invalidation is fire‑and‑forget – it does not block the admin response.

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4. Route Configuration

The cache middleware is applied only to public read‑only endpoints:

router.get(
  "/",
  standardLimiter,
  cacheMiddleware(300),   // 5 minutes TTL
  validate(GetProductsQuerySchema),
  PublicProductController.getProducts,
);

router.get(
  "/:id",
  standardLimiter,
  cacheMiddleware(300),
  validate(GetProductByIdSchema),
  PublicProductController.getProductById,
);

• TTL = 300 seconds – balances freshness with performance.
• Never cache POST, PUT, PATCH, DELETE – the middleware checks req.method !== "GET" and bypasses caching.

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5. Resilience & Failure Handling

• Redis offline? The middleware checks redisClient.isOpen. If Redis is down, it logs a warning and bypasses the cache – the database is still reachable.
• Cache write failures – logged but do not affect the response.
• No cache stampede protection – if the cache expires while many requests arrive simultaneously, all will hit the database. However, the 5‑minute TTL makes this unlikely; you can add a mutex lock if needed.

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6. Performance Impact

Scenario	Response Time	Database Load
Cache hit	~2 ms (Redis RAM)	0 queries
Cache miss + store	Controller + MongoDB (e.g., 50 ms) + Redis write	1 query
Subsequent hits	~2 ms	0 queries

With a 5‑minute TTL, 10,000 concurrent requests to the same endpoint result in only 1 MongoDB query.

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7. Related Files

File	Purpose
src/shared/middlewares/cache.middleware.ts	Edge cache middleware (proxy pattern).
src/shared/utils/cache.utils.ts	CacheManager for pattern‑based invalidation.
src/modules/products/product.admin.controller.ts	Triggers invalidation on admin mutations.
src/modules/products/product.routes.ts	Applies cache middleware to public GET routes.

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Next Steps

• See Background Jobs & Cron for asynchronous workers (email, PDF, exports).
• Explore Media & Storage for Cloudinary uploads.
• Understand Security Hardening for rate limiting and DoS protection.

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The Reshma-Core Team