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.