Cache interceptor in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Cache Interceptor in NestJS?

A Cache Interceptor in NestJS is a tool that automatically stores the response of a request so that future requests can get the stored response quickly without running the full logic again.

intermediate

How do you enable caching globally in a NestJS application?

You enable caching globally by importing and configuring CacheModule in the root module's imports array, and providing { provide: APP_INTERCEPTOR, useClass: CacheInterceptor } in the providers array.

intermediate

Which decorator is used to disable caching on a specific route in NestJS?

The @CacheKey() decorator sets a custom cache key, but to disable caching on a route, you use the @CacheTTL(0) decorator or avoid applying the CacheInterceptor on that route.

beginner

What is the role of CacheModule in NestJS caching?

CacheModule provides the caching functionality and configuration, like setting the store type (memory, redis), default TTL (time to live), and other options for caching in NestJS.

intermediate

How does CacheInterceptor decide when to return cached data?

CacheInterceptor checks if a cached response exists for the request key. If yes, it returns the cached data immediately. If not, it runs the handler, caches the response, then returns it.

What does CacheInterceptor do in NestJS?

A Stores and returns cached responses automatically

B Handles user authentication

C Manages database connections

D Formats HTTP responses

CacheInterceptor automatically caches responses to speed up repeated requests.

Which module must be imported to use caching in NestJS?

A HttpModule

B CacheModule

C ConfigModule

D AuthModule

CacheModule provides caching features and configuration in NestJS.

How do you apply CacheInterceptor globally in NestJS?

A Use @GlobalCache() decorator

B Add CacheInterceptor in controller constructor

C Set cache: true in main.ts

D Provide { provide: APP_INTERCEPTOR, useClass: CacheInterceptor } in AppModule providers

You apply CacheInterceptor globally by providing it with the APP_INTERCEPTOR token in the root module's providers array.

What happens if CacheInterceptor finds no cached data for a request?

A Returns an error

B Skips the request

C Executes the handler and caches the response

D Returns null

If no cached data exists, CacheInterceptor runs the handler, caches the result, then returns it.

Which decorator can set a custom cache key in NestJS?

A @CacheKey()

B @CacheTTL()

C @CacheDisable()

D @CacheControl()

@CacheKey() sets a custom key for caching responses.

Explain how CacheInterceptor works in NestJS and how it improves application performance.

Think about how saving answers helps you avoid repeating work.

Describe the steps to enable caching globally in a NestJS app using CacheModule and CacheInterceptor.

Remember the setup involves both module import and interceptor application.

Practice

1. What is the main purpose of the CacheInterceptor in NestJS?

easy

A. To handle authentication and authorization

B. To store responses and speed up repeated requests

C. To log all incoming requests

D. To format response data before sending

Solution

1. Step 1: Understand the role of CacheInterceptor

   The CacheInterceptor caches HTTP responses to avoid recomputing them on repeated requests.

2. Step 2: Compare with other options

   Logging, authentication, and formatting are handled by other interceptors or middleware, not CacheInterceptor.

3. Final Answer:

   To store responses and speed up repeated requests -> Option B

4. Quick Check:

   CacheInterceptor caches responses = C [OK]

Hint: CacheInterceptor caches responses to speed up requests [OK]

Common Mistakes:

• Confusing CacheInterceptor with logging interceptors
• Thinking it handles security tasks
• Assuming it formats data

2. Which of the following is the correct way to apply the CacheInterceptor to a controller method in NestJS?

easy

A. Add CacheInterceptor in the constructor of the controller

B. @Cacheable() decorator on the method

C. @UseInterceptors(CacheInterceptor) above the method

D. Use @CacheInterceptor() as a method parameter

Solution

1. Step 1: Recall how to apply interceptors in NestJS

   Interceptors are applied using the @UseInterceptors() decorator above methods or controllers.

2. Step 2: Check each option

   Only @UseInterceptors(CacheInterceptor) above the method uses the correct decorator syntax. Options B, C, and D are invalid or do not exist in NestJS.

3. Final Answer:

   @UseInterceptors(CacheInterceptor) above the method -> Option C

4. Quick Check:

   UseInterceptors decorator = A [OK]

Hint: Use @UseInterceptors(CacheInterceptor) to apply caching [OK]

Common Mistakes:

• Using non-existent @Cacheable decorator
• Trying to inject interceptor in constructor
• Passing interceptor as method parameter

3. Given this code snippet, what will be the output on the second request to /hello?

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

@Controller()
export class HelloController {
  private count = 0;

  @UseInterceptors(CacheInterceptor)
  @Get('hello')
  getHello() {
    this.count++;
    return `Hello count: ${this.count}`;
  }
}

medium

A. "Hello count: 1" first request, "Hello count: 2" second request

B. Error because count is private

C. "Hello count: 0" on both requests

D. "Hello count: 1" on first and second request

Solution

1. Step 1: Understand CacheInterceptor behavior

   CacheInterceptor caches the response of the first request and returns it for subsequent requests without calling the method again.

2. Step 2: Analyze the count variable increment

   On the first request, count increments to 1 and returns "Hello count: 1". On the second request, the cached response is returned, so count does not increment again.

3. Final Answer:

   "Hello count: 1" on first and second request -> Option D

4. Quick Check:

   Cache returns cached response = B [OK]

Hint: CacheInterceptor returns cached response, method not re-executed [OK]

Common Mistakes:

• Assuming count increments on every request
• Thinking private variables cause errors here
• Ignoring caching effect on method execution

4. Identify the error in this NestJS CacheInterceptor usage:

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

@Controller()
export class TestController {
  @UseInterceptors(CacheInterceptor())
  @Get('test')
  test() {
    return 'Test response';
  }
}

medium

A. CacheInterceptor should not be called as a function

B. Missing CacheModule import in the controller

C. Method test must be async to use CacheInterceptor

D. CacheInterceptor must be applied globally, not per method

Solution

1. Step 1: Check how CacheInterceptor is applied

   CacheInterceptor is a class, not a function, so it should be passed without parentheses to @UseInterceptors().

2. Step 2: Verify other options

   CacheModule import is required but not shown here (not an error in this snippet). Method does not need to be async. CacheInterceptor can be applied per method.

3. Final Answer:

   CacheInterceptor should not be called as a function -> Option A

4. Quick Check:

   Use class name without () = A [OK]

Hint: Pass CacheInterceptor class, not call it as function [OK]

Common Mistakes:

• Calling CacheInterceptor with parentheses
• Assuming method must be async
• Thinking CacheModule import error here

5. You want to customize the cache key for a route in NestJS using CacheInterceptor. Which approach correctly achieves this?

hard

A. Extend CacheInterceptor and override the trackBy method to return a custom key

B. Pass a custom key string directly to @UseInterceptors(CacheInterceptor)

C. Set a custom key in the controller constructor

D. Use @CacheKey('customKey') decorator on the method

Solution

1. Step 1: Understand how to customize cache keys

   CacheInterceptor allows customization by extending it and overriding the trackBy method to define how keys are generated.

2. Step 2: Evaluate other options

   Passing a string to @UseInterceptors() is invalid. Setting keys in constructor or using @CacheKey decorator does not exist in NestJS.

3. Final Answer:

   Extend CacheInterceptor and override the trackBy method to return a custom key -> Option A

4. Quick Check:

   Override trackBy method = D [OK]

Hint: Override trackBy method in extended CacheInterceptor [OK]

Common Mistakes:

• Trying to pass key directly to decorator
• Using non-existent @CacheKey decorator
• Setting keys in constructor instead of method