Cache interceptor in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does NestJS CacheInterceptor affect response caching?

Consider a NestJS controller method decorated with @UseInterceptors(CacheInterceptor). What is the behavior of this interceptor when the method is called multiple times with the same parameters?

NestJS

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

@Controller('items')
@UseInterceptors(CacheInterceptor)
export class ItemsController {
  @Get()
  findAll() {
    console.log('Fetching items from DB');
    return ['item1', 'item2'];
  }
}

A The interceptor causes an error because caching requires manual cache setup in the controller.

B The first call logs 'Fetching items from DB' and returns the array; subsequent calls return cached data without logging.

C The interceptor caches the response but still logs 'Fetching items from DB' on every call.

D Every call logs 'Fetching items from DB' and returns fresh data; caching is disabled by default.

💡 Hint

Think about how caching avoids repeated work and what the console output tells you.

The CacheInterceptor caches the response of the method after the first call. So the console log runs only once. Later calls return cached data without executing the method body again.

📝 Syntax

intermediate

Which code correctly applies CacheInterceptor globally in NestJS?

You want to enable caching for all controllers in your NestJS app using CacheInterceptor. Which code snippet correctly sets this up in main.ts?

A app.useGlobalInterceptors(new CacheInterceptor());

B app.useGlobalInterceptors(new CacheInterceptor(app));

C app.useGlobalInterceptors(CacheInterceptor());

D app.useGlobalInterceptors(CacheInterceptor);

💡 Hint

Remember how to instantiate classes and pass them to global interceptors.

CacheInterceptor is a class and must be instantiated with new. Passing the class itself or calling it as a function causes errors.

🔧 Debug

advanced

Why does CacheInterceptor not cache responses for POST requests?

Given this controller method:

  @Post('create')
  @UseInterceptors(CacheInterceptor)
  createItem() {
    return { success: true };
  }

Why does the CacheInterceptor not cache the response for this POST request?

A CacheInterceptor caches all HTTP methods equally; the issue is elsewhere.

B POST requests require manual cache key setup; otherwise, caching is disabled.

C CacheInterceptor only caches GET requests by default; POST requests are excluded.

D The interceptor throws an error on POST requests and skips caching.

💡 Hint

Think about HTTP methods and typical caching rules.

CacheInterceptor caches only GET requests by default because POST requests usually change data and should not be cached.

🧠 Conceptual

advanced

What happens if CacheInterceptor is used without a cache manager provider?

You apply CacheInterceptor in a NestJS app but forget to import CacheModule. What will happen when a cached route is called?

A The CacheInterceptor caches responses but clears cache immediately after.

B The CacheInterceptor silently skips caching and returns fresh data every time.

C The app logs a warning but caches responses in memory by default.

D The app throws a runtime error because CacheInterceptor depends on CacheModule provider.

💡 Hint

Consider dependency injection and required modules in NestJS.

CacheInterceptor requires the cache manager provider from CacheModule. Without it, the app throws an error at runtime.

❓ state_output

expert

What is the cache key generated by CacheInterceptor for a GET request with query params?

Given this controller method:

  @Get('search')
  @UseInterceptors(CacheInterceptor)
  search(@Query('term') term: string) {
    return { results: [term] };
  }

When a client calls /search?term=book, what cache key does CacheInterceptor use internally?

A The full URL path including query string, e.g., '/search?term=book'

B Only the route path without query, e.g., '/search'

C A hash of the response body content

D The HTTP method plus route path, e.g., 'GET:/search'

💡 Hint

Think about how caching differentiates requests with different query parameters.

CacheInterceptor uses the full request URL including query parameters as the cache key to distinguish different queries.