Custom cache keys in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the CacheInterceptor from NestJS.

NestJS

import { [1] } from '@nestjs/common';

Drag options to blanks, or click blank then click option'

ACacheInterceptor

BCacheModule

CCacheService

DCacheKey

Attempts:

3 left

💡 Hint

Common Mistakes

Importing CacheModule instead of CacheInterceptor

Using CacheService which is not a valid import here

The CacheInterceptor is imported from @nestjs/common to enable caching features.

2fill in blank

medium

Complete the code to create a custom cache key by overriding the trackBy method.

NestJS

export class CustomCacheInterceptor extends CacheInterceptor {
  protected trackBy(context: ExecutionContext): string | undefined {
    const request = context.switchToHttp().getRequest();
    return request.method + ':' + [1];
  }
}

Drag options to blanks, or click blank then click option'

Arequest.headers

Brequest.body

Crequest.params

Drequest.url

Attempts:

3 left

💡 Hint

Common Mistakes

Using request.body which may vary and cause cache misses

Using request.headers which is not a stable cache key

The cache key is often based on the HTTP method and the request URL to uniquely identify requests.

3fill in blank

hard

Fix the error in the custom cache key function to correctly handle query parameters.

NestJS

protected trackBy(context: ExecutionContext): string | undefined {
  const request = context.switchToHttp().getRequest();
  const key = request.method + ':' + request.url + '?' + [1];
  return key;
}

Drag options to blanks, or click blank then click option'

Arequest.params

Brequest.query

Crequest.body

Drequest.headers

Attempts:

3 left

💡 Hint

Common Mistakes

Using request.params which holds route parameters, not query parameters

Using request.body which is for POST data

Query parameters are accessed via request.query to include them in the cache key.

4fill in blank

hard

Fill both blanks to create a cache key that includes method, URL, and a user ID from headers.

NestJS

protected trackBy(context: ExecutionContext): string | undefined {
  const request = context.switchToHttp().getRequest();
  const userId = request.headers['[1]'];
  return request.method + ':' + request.url + ':' + [2];
}

Drag options to blanks, or click blank then click option'

Ax-user-id

Bauthorization

CuserId

Duser-agent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'authorization' header which contains tokens, not user ID

Using 'user-agent' which is unrelated to user identity

The user ID is often sent in a custom header like x-user-id. We retrieve it and include it in the cache key.

5fill in blank

hard

Fill all three blanks to define a custom cache interceptor class with a trackBy method that uses method, URL, and a query param 'lang'.

NestJS

import { CacheInterceptor, ExecutionContext } from '@nestjs/common';

export class CustomCacheInterceptor extends [1] {
  protected trackBy(context: [2]): string | undefined {
    const request = context.switchToHttp().getRequest();
    const lang = request.query['[3]'];
    return request.method + ':' + request.url + ':' + lang;
  }
}

Drag options to blanks, or click blank then click option'

ACacheInterceptor

BExecutionContext

Clang

DCacheService

Attempts:

3 left

💡 Hint

Common Mistakes

Using CacheService instead of CacheInterceptor as base class

Using wrong type for context parameter

Using incorrect query parameter key

The class extends CacheInterceptor, the method argument is of type ExecutionContext, and the query parameter key is 'lang'.

Practice

1. What is the main purpose of using @CacheKey() in a NestJS service?

easy

A. To set the time duration for cache expiration

B. To clear all cached data automatically

C. To define a custom name for the cached data key

D. To disable caching for a method

Solution

1. Step 1: Understand the role of @CacheKey()

   This decorator is used to specify a custom key name for the cached data instead of using a default or generated key.

2. Step 2: Differentiate from other cache decorators

   @CacheTTL() sets cache duration, clearing cache is manual or by other methods, and disabling cache is not done by @CacheKey().

3. Final Answer:

   To define a custom name for the cached data key -> Option C

4. Quick Check:

   Custom cache key = @CacheKey() [OK]

Hint: Remember: CacheKey sets the cache name, TTL sets time [OK]

Common Mistakes:

• Confusing @CacheKey() with @CacheTTL()
• Thinking @CacheKey() clears cache
• Assuming @CacheKey() disables caching

2. Which of the following is the correct way to apply a custom cache key to a method in NestJS?

easy

A. @CacheKey = 'myKey'\nasync getData() { return data; }

B. @CacheTTL(300)\nasync getData() { return data; }

C. cacheKey('myKey')\nasync getData() { return data; }

D. @CacheKey('myKey')\nasync getData() { return data; }

Solution

1. Step 1: Identify correct decorator syntax

   In NestJS, decorators are applied with @DecoratorName(parameters) syntax above the method.

2. Step 2: Check each option

   @CacheKey('myKey')\nasync getData() { return data; } uses @CacheKey('myKey') correctly. @CacheTTL(300)\nasync getData() { return data; } uses @CacheTTL which sets time, not key. @CacheKey = 'myKey'\nasync getData() { return data; } uses invalid assignment syntax. cacheKey('myKey')\nasync getData() { return data; } uses a non-existent function.

3. Final Answer:

   @CacheKey('myKey')\nasync getData() { return data; } -> Option D

4. Quick Check:

   Decorator syntax = @CacheKey('key') [OK]

Hint: Decorators start with @ and parentheses for parameters [OK]

Common Mistakes:

• Using assignment instead of decorator syntax
• Mixing @CacheKey with @CacheTTL
• Calling cacheKey as a function instead of decorator

3. Given this NestJS method with caching:

@CacheKey('userList')
@CacheTTL(60)
async getUsers() {
  return ['Alice', 'Bob'];
}

What will be the cache key and TTL for the cached data?

medium

A. Key: 'getUsers', TTL: default (no TTL set)

B. Key: 'userList', TTL: 60 seconds

C. Key: 'userList', TTL: default (no TTL set)

D. Key: 'getUsers', TTL: 60 seconds

Solution

1. Step 1: Identify the cache key from @CacheKey()

   The decorator @CacheKey('userList') sets the cache key explicitly to 'userList'.

2. Step 2: Identify the TTL from @CacheTTL()

   The decorator @CacheTTL(60) sets the cache time to 60 seconds.

3. Final Answer:

   Key: 'userList', TTL: 60 seconds -> Option B

4. Quick Check:

   Custom key + TTL = 'userList' & 60s [OK]

Hint: @CacheKey sets key, @CacheTTL sets time in seconds [OK]

Common Mistakes:

• Assuming method name is cache key when @CacheKey is used
• Ignoring @CacheTTL and thinking no TTL is set
• Confusing TTL units (seconds vs milliseconds)

4. You wrote this code:

@CacheKey('dataKey')
@CacheTTL('300')
async fetchData() {
  return 'info';
}

Why does the cache TTL not work as expected?

medium

A. Because TTL value should be a number, not a string

B. Because @CacheKey must come after @CacheTTL

C. Because async methods cannot use caching decorators

D. Because the cache key 'dataKey' is invalid

Solution

1. Step 1: Check the @CacheTTL parameter type

   The TTL decorator expects a number (seconds), but '300' is a string, which causes it to fail.

2. Step 2: Verify other decorator rules

   Decorator order does not affect TTL, async methods support caching, and 'dataKey' is a valid string key.

3. Final Answer:

   Because TTL value should be a number, not a string -> Option A

4. Quick Check:

   TTL must be number, not string [OK]

Hint: TTL needs number type, not quoted string [OK]

Common Mistakes:

• Passing TTL as string instead of number
• Thinking decorator order matters here
• Assuming async disables caching

5. You want to cache user data with a dynamic key based on user ID. Which approach correctly creates a custom cache key in NestJS for method getUserById(id: string)?

hard

A. Use @CacheKey(() => `user_${id}`) above the method

B. Manually generate the key inside the method and use @CacheKey() without parameters

C. Use @CacheKey('user_' + id) directly above the method

D. Use @CacheKey() and pass the key as a method argument

Solution

1. Step 1: Understand dynamic cache keys in NestJS

   To create a dynamic cache key, you can pass a function to @CacheKey() that returns the key string using method parameters.

2. Step 2: Evaluate each option

   Use @CacheKey(() => `user_${id}`) above the method correctly uses a function returning a template string with id. Manually generate the key inside the method and use @CacheKey() without parameters is invalid because @CacheKey() needs the key at decoration time. Use @CacheKey('user_' + id) directly above the method tries to concatenate directly but id is not available at decoration time. Use @CacheKey() and pass the key as a method argument is invalid as @CacheKey() does not accept method arguments.

3. Final Answer:

   Use @CacheKey(() => `user_${id}`) above the method -> Option A

4. Quick Check:

   Dynamic key = function in @CacheKey() [OK]

Hint: Use function in @CacheKey() for dynamic keys [OK]

Common Mistakes:

• Trying to use method parameters directly in decorator string
• Assuming @CacheKey() accepts method arguments
• Generating key inside method but not linking to cache