Cache middleware pattern in Express - Step-by-Step Execution

Concept Flow - Cache middleware pattern

Incoming Request

↓

Check Cache for Data

↓

Return Cached Data

↓

Send Response

↓

Store Data in Cache

↓

Send Response

The middleware checks if data is in cache. If yes, it returns cached data. If no, it fetches fresh data, stores it in cache, then sends response.

Execution Sample

Express

const cache = {};

function cacheMiddleware(req, res, next) {
  const key = req.url;
  if (cache[key]) {
    return res.send(cache[key]);
  }
  res.sendResponse = res.send.bind(res);
  res.send = (body) => {
    cache[key] = body;
    res.sendResponse(body);
  };
  next();
}

This middleware checks if the response for the request URL is cached. If yes, it sends cached data. Otherwise, it overrides res.send to cache the response before sending.

Execution Table

Step	Action	Cache State	Response Sent	Next Called
1	Request arrives for /data	{}	No	Yes
2	Check cache for /data	{}	No	Yes
3	Cache miss, override res.send	{}	No	Yes
4	Call next middleware to fetch data	{}	No	Yes
5	Data fetched: 'Hello World'	{}	No	Yes
6	res.send('Hello World') called	{'/data': 'Hello World'}	Yes	No
7	Response sent with 'Hello World'	{'/data': 'Hello World'}	Yes	No
8	Next request arrives for /data	{'/data': 'Hello World'}	No	No
9	Check cache for /data	{'/data': 'Hello World'}	No	No
10	Cache hit, send cached response	{'/data': 'Hello World'}	Yes	No
11	Response sent with cached 'Hello World'	{'/data': 'Hello World'}	Yes	No

💡 Execution stops after response is sent either from cache or fresh data.

Variable Tracker

Variable	Start	After Step 6	After Step 10	Final
cache	{}	{'/data': 'Hello World'}	{'/data': 'Hello World'}	{'/data': 'Hello World'}
res.sendResponse	undefined	function	undefined	undefined
res.send	function	overridden function	function	function

Key Moments - 3 Insights

Why do we override res.send inside the middleware?

We override res.send to capture the response body so we can store it in the cache before sending it to the client, as shown in step 6 of the execution_table.

What happens if the cache has the data?

If cache has data (cache hit), the middleware sends the cached response immediately without calling next(), as shown in steps 9 and 10.

Why do we call next() only on cache miss?

Calling next() passes control to the next middleware or handler to fetch fresh data only when cache misses, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the cache content after step 6?

A undefined

B {}

C {"/data": "Hello World"}

D {"/data": null}

💡 Hint

Check the 'Cache State' column at step 6 in the execution_table.

At which step does the middleware send cached data without calling next()?

A Step 4

B Step 10

C Step 3

D Step 1

💡 Hint

Look for 'Cache hit' and 'Next Called' = No in the execution_table.

If we remove the override of res.send, what will happen on cache miss?

A Response will not be cached

B Cache will store empty data

C Middleware will send cached data anyway

D Next middleware will not be called

💡 Hint

Refer to how caching happens at step 6 by overriding res.send.

Concept Snapshot

Cache Middleware Pattern in Express:
- Middleware checks if response for req.url is cached.
- If cached, sends cached response immediately.
- If not, overrides res.send to cache response body.
- Calls next() to fetch fresh data.
- Stores fresh response in cache before sending.
- Improves performance by avoiding repeated data fetch.

Full Transcript

This visual execution trace shows how cache middleware works in Express. When a request arrives, the middleware checks if the response is already cached for the request URL. If yes, it sends the cached response immediately without calling the next middleware. If no, it overrides the res.send method to capture the response body when the next middleware sends it. Then it calls next() to fetch fresh data. When the response is sent, the overridden res.send caches the response body before sending it to the client. This process speeds up repeated requests by serving cached data quickly. The variable tracker shows how the cache object updates after the first response. Key moments clarify why res.send is overridden and when next() is called. The quiz tests understanding of cache state and middleware flow.