Cache interceptor in NestJS - Step-by-Step Execution

Concept Flow - Cache interceptor

Request received

↓

Check cache for response

↓

Return cached

↓

Send cached

↓

Send response

The cache interceptor checks if a response is cached for a request. If yes, it returns cached data. If no, it calls the handler, caches the result, then returns it.

Execution Sample

NestJS

import { CacheInterceptor, Controller, Get, UseInterceptors } from '@nestjs/common';

@Controller('items')
@UseInterceptors(CacheInterceptor)
export class ItemsController {
  @Get()
  findAll() { return ['item1', 'item2']; }
}

This code uses CacheInterceptor to cache the response of the findAll method in ItemsController.

Execution Table

Step	Action	Cache Check Result	Handler Called	Cache Updated	Response Sent
1	Request to GET /items	No cache found	Yes	Cache stores ['item1', 'item2']	['item1', 'item2'] sent
2	Request to GET /items again	Cache hit	No	No change	Cached ['item1', 'item2'] sent
3	Request to GET /items after cache expiry	No cache found	Yes	Cache stores ['item1', 'item2']	['item1', 'item2'] sent

💡 Execution stops after sending response to client.

Variable Tracker

Variable	Start	After 1	After 2	After 3
cache	empty	['item1', 'item2']	['item1', 'item2']	['item1', 'item2'] (refreshed)
handlerCalled	false	true	false	true
responseSent	none	['item1', 'item2']	['item1', 'item2']	['item1', 'item2']

Key Moments - 2 Insights

Why does the handler not run on the second request?

Because the cache check found a stored response (see execution_table step 2), so the interceptor returns cached data without calling the handler.

What happens when the cache expires?

The cache is empty again (step 3), so the handler runs to get fresh data, which is then stored in cache for future requests.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'handlerCalled' at step 2?

A true

B undefined

C false

D null

💡 Hint

Check the 'handlerCalled' row in variable_tracker at 'After 2' column.

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

A Step 1

B Step 2

C Step 3

D No step updates cache

💡 Hint

Look at the 'Cache Updated' column in execution_table.

If the cache never expires, which step would the handler never be called?

A Step 1

B Step 3

C Step 2

D Handler always called

💡 Hint

Refer to 'handlerCalled' in variable_tracker and cache expiry explanation in key_moments.

Concept Snapshot

Cache Interceptor in NestJS:
- Checks cache before calling handler
- Returns cached response if available
- Calls handler and caches result if not
- Improves performance by avoiding repeated work
- Use @UseInterceptors(CacheInterceptor) on controllers or routes

Full Transcript

The Cache Interceptor in NestJS works by intercepting incoming requests. When a request arrives, it first checks if a cached response exists. If yes, it returns that cached response immediately without calling the handler method. If no cached response is found, it calls the handler to get fresh data, stores this data in the cache, and then sends it back to the client. This process repeats for each request. When the cache expires, the interceptor calls the handler again to refresh the cache. This mechanism helps improve performance by reducing repeated processing for the same requests.