Custom cache keys in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom cache key in NestJS caching?

A custom cache key is a user-defined string or value used to uniquely identify cached data instead of the default key. It helps control how data is stored and retrieved from the cache.

beginner

Why would you use a custom cache key in NestJS?

You use a custom cache key to avoid collisions, organize cached data better, or include dynamic parts like user IDs or query parameters to cache different results separately.

intermediate

How do you define a custom cache key in a NestJS @Cacheable decorator?

You provide a function to the 'key' property in the @Cacheable decorator options. This function returns a string that will be used as the cache key based on method arguments or other logic.

intermediate

Example: What does this custom cache key function do? key: (args) => `user_${args[0]}_data`

It creates a cache key by taking the first argument (usually a user ID) and making a string like 'user_123_data'. This caches data specific to each user separately.

beginner

What happens if you don't use a custom cache key in NestJS caching?

NestJS uses a default key based on the method name and arguments. This can cause cache collisions or less control over cache entries, especially when arguments are complex or dynamic.

What is the main purpose of a custom cache key in NestJS?

A To disable caching

B To uniquely identify cached data

C To increase server memory

D To log cache hits

Custom cache keys uniquely identify cached data to avoid collisions and organize cache entries.

How do you specify a custom cache key in NestJS caching decorators?

A By renaming the cache folder

B By setting a global environment variable

C By passing a function to the 'key' option

D By using a special cache service method

You specify a custom cache key by providing a function to the 'key' option in the caching decorator.

Which of these is a good example of a custom cache key for user data?

A profile_cache

B cache_default

C data123

D user_${userId}_profile

A key like `user_${userId}_profile` uniquely identifies cache entries per user.

What could happen if you don't use custom cache keys when caching dynamic data?

A Cache collisions and incorrect data retrieval

B Faster cache performance

C Automatic cache clearing

D No caching at all

Without custom keys, different data might overwrite each other causing wrong data to be served.

In NestJS, the custom cache key function receives what as input?

A The method arguments

B The server configuration

C The cache store instance

D The HTTP request headers

The custom cache key function receives the method arguments to build a unique key.

Explain how to create and use a custom cache key in NestJS caching.

Think about how to make cache keys unique per input.

Why is it important to use custom cache keys when caching data that depends on user input?

Consider what happens if all users share the same cache key.

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