Why caching reduces response latency in NestJS - Visual Breakdown

Concept Flow - Why caching reduces response latency

Client sends request

↓

Check cache for data

↓

Return cached

↓

response fast

↓

Store data in cache

↓

Return response

The system first checks if the requested data is in cache. If yes, it returns it immediately, skipping slower database or API calls, thus reducing response time.

Execution Sample

NestJS

async getUser(id: string) {
  const cached = await this.cacheManager.get(id);
  if (cached) return cached;
  const user = await this.userService.findById(id);
  await this.cacheManager.set(id, user);
  return user;
}

This function tries to get user data from cache first; if missing, it fetches from the database, caches it, then returns.

Execution Table

Step	Action	Cache Check Result	Data Source	Cache Update	Response
1	Receive request for user id '123'	N/A	N/A	N/A	N/A
2	Check cache for '123'	Miss	N/A	N/A	N/A
3	Fetch user from DB	N/A	Database	N/A	N/A
4	Store user data in cache	N/A	N/A	Cache set '123'	N/A
5	Return user data	N/A	N/A	N/A	User data from DB
6	Receive request for user id '123' again	N/A	N/A	N/A	N/A
7	Check cache for '123'	Hit	Cache	N/A	User data from cache
8	Return cached user data	N/A	N/A	N/A	User data from cache

💡 Execution stops after returning user data from cache or DB.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 7	Final
cacheManager.get('123')	undefined	undefined (miss)	user object cached	user object cached (hit)	user object cached
user	undefined	undefined	user object from DB	user object from DB	user object returned

Key Moments - 2 Insights

Why does the function return immediately when cache has data?

Because the cache hit at step 7 means data is ready locally, so no need to wait for slower DB fetch (see execution_table row 7).

What happens when cache misses the data?

The function fetches data from the database (step 3), then stores it in cache (step 4) before returning (step 5), as shown in execution_table rows 3-5.

Visual Quiz - 3 Questions

Look at the execution table, what is the data source at step 3?

A Database

B API

C Cache

D Undefined

💡 Hint

Check the 'Data Source' column at step 3 in the execution_table.

At which step does the cache get updated with new data?

A Step 7

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Cache Update' column in the execution_table.

If the cache always hits, which steps would be skipped?

A Steps 2 and 7

B Steps 3 and 4

C Steps 5 and 8

D Steps 1 and 6

💡 Hint

Refer to execution_table rows where cache miss triggers DB fetch and cache set.

Concept Snapshot

Caching stores data temporarily to serve future requests faster.
Check cache first: if data found, return immediately.
If not found, fetch from DB/API, store in cache, then return.
Reduces response latency by avoiding slow data fetches repeatedly.
Common in NestJS using cacheManager with async get/set methods.

Full Transcript

When a client requests data, the system first checks if the data is in cache. If it is, the cached data is returned immediately, making the response very fast. If the data is not in cache, the system fetches it from the database or an API, then stores it in cache for future requests before returning it. This process reduces response latency by avoiding repeated slow data fetches. In NestJS, this is done using the cacheManager's get and set methods asynchronously.