Custom cache keys in NestJS - Mini Project: Build & Apply

Custom Cache Keys in NestJS

📖 Scenario: You are building a NestJS service that caches user data. To improve cache management, you want to create custom cache keys based on user IDs.

🎯 Goal: Build a NestJS service that uses a custom cache key function to store and retrieve cached user data.

📋 What You'll Learn

Create a NestJS service with a method to get user data.

Add a custom cache key function that uses the user ID.

Use the cache interceptor with the custom cache key function.

Ensure the cache key is unique per user ID.

💡 Why This Matters

🌍 Real World

Custom cache keys help store and retrieve cached data uniquely, improving performance and reducing redundant data fetching in web applications.

💼 Career

Understanding custom cache keys in NestJS is useful for backend developers working on scalable and efficient server-side applications.

1

Create a NestJS service with a method to get user data

Create a NestJS service class called UserService with a method getUserData that takes a parameter userId and returns a string `User data for ${userId}`.

NestJS

export class UserService {
  // Create getUserData method here
  // Your code here
}

Hint

Define a class with a method that returns a template string using the userId parameter.

2

Add a custom cache key function using the user ID

Create a function called customCacheKey that takes context as a parameter and returns a string cache key in the format `user-cache-${userId}`. Extract userId from context.args[0].

NestJS

export class UserService {
  getUserData(userId: string): string {
    return `User data for ${userId}`;
  }
}

// Create customCacheKey function here
// Your code here

Hint

Use context.args[0] to get the userId and return a string with that ID.

3

Use the cache interceptor with the custom cache key function

In a NestJS controller class called UserController, create a method getUser that takes userId as a parameter and calls userService.getUserData(userId). Use the @UseInterceptors(CacheInterceptor) decorator with options to set cacheKeyFactory to customCacheKey. Inject UserService in the constructor.

NestJS

export class UserService {
  getUserData(userId: string): string {
    return `User data for ${userId}`;
  }
}

function customCacheKey(context: any): string {
  const userId = context.args[0];
  return `user-cache-${userId}`;
}

// Create UserController with getUser method and cache interceptor
// Your code here

Hint

Use @Controller, inject UserService, and decorate getUser with @UseInterceptors(CacheInterceptor).

4

Configure the cache interceptor to use the custom cache key factory

Modify the @UseInterceptors(CacheInterceptor) decorator on UserController to pass options with cacheKeyFactory set to customCacheKey.

NestJS

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

export class UserService {
  getUserData(userId: string): string {
    return `User data for ${userId}`;
  }
}

function customCacheKey(context: any): string {
  const userId = context.args[0];
  return `user-cache-${userId}`;
}

// Modify the UseInterceptors decorator to use cacheKeyFactory
@UseInterceptors(CacheInterceptor)
@Controller('users')
export class UserController {
  constructor(private readonly userService: UserService) {}

  @Get(':userId')
  @UseInterceptors(CacheInterceptor)
  getUser(@Param('userId') userId: string): string {
    return this.userService.getUserData(userId);
  }
}

Hint

Pass an options object with cacheKeyFactory: customCacheKey to the @UseInterceptors decorator.

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