TTL configuration in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does TTL affect cache expiration in NestJS?

Consider a NestJS cache configured with a TTL (time-to-live) of 5 seconds. What happens when you try to retrieve a cached value after 6 seconds?

NestJS

import { CacheModule, Injectable, Inject, CACHE_MANAGER } from '@nestjs/common';

@Injectable()
export class SampleService {
  constructor(@Inject(CACHE_MANAGER) private readonly cacheManager) {}

  async getCachedValue() {
    return await this.cacheManager.get('key');
  }
}

// CacheModule registered with TTL: 5 seconds

A The cached value is null or undefined because it expired after 5 seconds.

B The cached value is returned because TTL only affects cache insertion, not retrieval.

C An error is thrown because the cache key is invalid after TTL expires.

D The cache automatically refreshes the value after TTL expires, so the value is returned.

💡 Hint

Think about what TTL means for cached data lifetime.

TTL defines how long a cached item stays valid. After 5 seconds, the cached value expires and is removed, so retrieval returns null or undefined.

📝 Syntax

intermediate

Which option correctly sets a TTL of 10 seconds in NestJS CacheModule?

Choose the correct way to configure the CacheModule with a TTL of 10 seconds.

NestJS

import { CacheModule } from '@nestjs/common';

CacheModule.register({
  // TTL configuration here
});

A ttl: 10 * 1000

B ttl: 10

C ttl: 10000

D ttl: '10s'

💡 Hint

TTL is specified in seconds as a number.

The TTL option expects a number representing seconds. So ttl: 10 means 10 seconds.

🔧 Debug

advanced

Why does the cached value never expire despite TTL set?

Given this NestJS cache configuration, the cached values never expire. What is the likely cause?

NestJS

CacheModule.register({
  ttl: 5,
  max: 100
});

// Usage:
await cacheManager.set('key', 'value');
// After 10 seconds
const val = await cacheManager.get('key'); // val is still 'value'

A The max option disables TTL expiration when set.

B The TTL value must be set in milliseconds, not seconds.

C The cache store used does not support TTL expiration.

D The cacheManager.set method overrides TTL to infinite by default.

💡 Hint

Consider the cache store implementation and its TTL support.

Some cache stores or custom stores may not support TTL expiration. Setting TTL has no effect if the store ignores it.

❓ state_output

advanced

What is the output after setting different TTLs for cache keys?

Consider this code snippet using NestJS cache manager:

NestJS

await cacheManager.set('short', 'A', { ttl: 2 });
await cacheManager.set('long', 'B', { ttl: 5 });

// After 3 seconds
const shortVal = await cacheManager.get('short');
const longVal = await cacheManager.get('long');

console.log(shortVal, longVal);

A undefined 'B'

B 'A' undefined

C undefined undefined

D 'A' 'B'

💡 Hint

Think about which keys expire after 3 seconds.

The key 'short' expires after 2 seconds, so after 3 seconds it is gone (undefined). The key 'long' expires after 5 seconds, so it still returns 'B'.

🧠 Conceptual

expert

How does TTL configuration interact with distributed cache in NestJS?

In a NestJS app using Redis as a distributed cache, what is the effect of setting TTL in CacheModule configuration?

A TTL causes Redis to replicate keys indefinitely without expiration.

B TTL is ignored because Redis does not support key expiration.

C TTL only applies locally in NestJS memory cache, not in Redis.

D TTL is enforced by Redis, so cached keys expire automatically after the set time.

💡 Hint

Think about Redis capabilities and how TTL works in distributed caches.

Redis supports key expiration natively. When TTL is set in NestJS CacheModule using Redis store, Redis enforces expiration automatically.

Practice

1. What does TTL stand for in NestJS caching configuration?

easy

A. Timed Token Lock, a security feature for cache access

B. Time To Live, the duration cached data stays before removal

C. Temporary Transfer Lock, a pause on cache updates

D. Total Transaction Limit, the max number of cache writes

Solution

1. Step 1: Understand TTL meaning in caching

   TTL means how long cached data remains valid before it is removed automatically.

2. Step 2: Match TTL to the correct full form

   TTL stands for Time To Live, which fits the caching context perfectly.

3. Final Answer:

   Time To Live, the duration cached data stays before removal -> Option B

4. Quick Check:

   TTL = Time To Live [OK]

Hint: TTL means how long cache data lives before removal [OK]

Common Mistakes:

• Confusing TTL with security or transaction terms
• Thinking TTL is a cache size limit
• Assuming TTL is a manual cache clear command

2. Which is the correct way to set a global TTL of 10 seconds in NestJS cache module?

easy

A. CacheModule.register({ ttl: 10 })

B. CacheModule.register({ timeToLive: 10 })

C. CacheModule.register({ ttl: 10000 })

D. CacheModule.register({ expire: 10 })

Solution

1. Step 1: Identify TTL unit in NestJS cache config

   TTL is set in seconds, so 10 means 10 seconds.

2. Step 2: Check correct property name and value type

   The property is ttl, and the value is a number in seconds, so ttl: 10 is correct.

3. Final Answer:

   CacheModule.register({ ttl: 10 }) -> Option A

4. Quick Check:

   TTL in seconds uses 'ttl' property [OK]

Hint: TTL value is seconds, use 'ttl' property exactly [OK]

Common Mistakes:

• Using milliseconds instead of seconds
• Using wrong property names like timeToLive or expire
• Passing ttl as string instead of number

3. Given this NestJS cache config snippet:

CacheModule.register({ ttl: 5 })

What happens to cached data after 6 seconds?

medium

A. Cached data throws an error after 6 seconds

B. Cached data remains indefinitely until manually cleared

C. Cached data is refreshed automatically every 6 seconds

D. Cached data is automatically removed after 5 seconds

Solution

1. Step 1: Understand TTL effect on cached data

   TTL of 5 means cached data expires and is removed after 5 seconds.

2. Step 2: Check what happens after 6 seconds

   Since 6 seconds is more than 5, cached data is no longer valid and is removed.

3. Final Answer:

   Cached data is automatically removed after 5 seconds -> Option D

4. Quick Check:

   TTL 5 seconds means removal after 5 seconds [OK]

Hint: Data expires after TTL seconds, so 6 > 5 means removal [OK]

Common Mistakes:

• Thinking data stays forever without manual clear
• Assuming TTL refreshes cache automatically
• Believing cache throws errors on expiry

4. Identify the error in this NestJS cache TTL setup:

CacheModule.register({ ttl: '30' })

medium

A. TTL property name is incorrect, should be timeToLive

B. TTL cannot be set globally, only per item

C. TTL value should be a number, not a string

D. TTL value must be in milliseconds, not seconds

Solution

1. Step 1: Check TTL value type

   TTL expects a number representing seconds, but '30' is a string.

2. Step 2: Confirm correct property and units

   The property ttl is correct and uses seconds, so only the value type is wrong.

3. Final Answer:

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

4. Quick Check:

   TTL must be numeric seconds, not string [OK]

Hint: TTL must be a number, not quoted string [OK]

Common Mistakes:

• Using string instead of number for TTL
• Changing property name incorrectly
• Confusing seconds with milliseconds

5. You want to cache user data with a TTL of 60 seconds, but override it to 120 seconds for admin users only. How do you configure TTL in NestJS?

hard

A. Set global TTL to 60, then use @CacheTTL(120) decorator on admin methods

B. Set global TTL to 120, then manually clear cache for non-admin users

C. Set TTL only in controller constructor based on user role

D. Use two separate CacheModules with different TTLs and switch dynamically

Solution

1. Step 1: Set global TTL for general users

   Configure CacheModule with ttl: 60 to cover normal users.

2. Step 2: Override TTL for admin users

   Use the @CacheTTL(120) decorator on admin-specific methods to extend TTL.

3. Final Answer:

   Set global TTL to 60, then use @CacheTTL(120) decorator on admin methods -> Option A

4. Quick Check:

   Global TTL + @CacheTTL override [OK]

Hint: Use global TTL plus @CacheTTL decorator for overrides [OK]

Common Mistakes:

• Trying to set TTL only in constructor without decorators
• Using multiple CacheModules unnecessarily
• Setting global TTL too high and clearing cache manually