Why Cache stores (memory, Redis) in NestJS? - Purpose & Use Cases

The Big Idea

What if your app could remember answers and skip slow steps every time?

The Scenario

Imagine your NestJS app fetching user data from a database every time someone visits a profile page.

Each request repeats the same slow database query, making users wait longer and servers work harder.

The Problem

Manually repeating database calls wastes time and resources.

It causes slow responses, higher server load, and poor user experience.

Trying to store data yourself without a proper cache system leads to messy code and bugs.

The Solution

Cache stores like memory or Redis save frequently used data temporarily.

They let your app quickly return data without hitting the database every time.

This makes your app faster, lighter, and more reliable.

Before vs After

✗ Before

const user = await database.findUser(id); // every request hits DB

✓ After

const user = await cache.get(id) || await database.findUser(id); await cache.set(id, user);

What It Enables

Cache stores enable lightning-fast responses and scalable apps by reducing repeated work.

Real Life Example

When you refresh a social media feed, cached posts load instantly instead of waiting for the server to fetch them again.

Key Takeaways

Manual repeated data fetching slows apps and wastes resources.

Cache stores save data temporarily for quick reuse.

Using memory or Redis caches makes NestJS apps faster and more efficient.

Practice

1. What is the main advantage of using Redis as a cache store in a NestJS application compared to in-memory cache?

easy

A. Redis does not support expiration of cached data.

B. Redis cache is stored only in the local app memory.

C. Redis allows sharing cached data across multiple app instances.

D. Redis cache clears automatically every second.

Solution

1. Step 1: Understand in-memory cache scope

   In-memory cache stores data only inside the app instance's memory, so it is not shared.

2. Step 2: Understand Redis cache scope

   Redis is a separate server that multiple app instances can connect to, sharing cached data.

3. Final Answer:

   Redis allows sharing cached data across multiple app instances. -> Option C

4. Quick Check:

   Redis cache is shared [OK]

Hint: Redis shares cache across servers; memory cache is local only [OK]

Common Mistakes:

• Thinking in-memory cache shares data between servers
• Believing Redis cache is local to one app instance
• Assuming Redis cache clears every second automatically

2. Which of the following is the correct way to import and register the Redis cache store in a NestJS module?

easy

A. CacheModule.forRoot({ type: 'redis', url: 'redis://localhost:6379' })

B. CacheModule.register({ store: 'redis', host: 'localhost', port: 6379 })

C. CacheModule.useRedis({ host: 'localhost', port: 6379 })

D. CacheModule.registerRedis({ url: 'redis://localhost:6379' })

Solution

1. Step 1: Recall NestJS CacheModule syntax

   CacheModule.register() is used to configure cache with options including store type and connection details.

2. Step 2: Identify correct Redis store option

   Using store: 'redis' with host and port is the correct way to set Redis cache in NestJS.

3. Final Answer:

   CacheModule.register({ store: 'redis', host: 'localhost', port: 6379 }) -> Option B

4. Quick Check:

   Use CacheModule.register with store: 'redis' [OK]

Hint: Use CacheModule.register with store: 'redis' for Redis cache [OK]

Common Mistakes:

• Using non-existent methods like useRedis or registerRedis
• Confusing forRoot with register for cache setup
• Incorrect option keys like type instead of store

3. Given this NestJS code snippet using in-memory cache:

await cacheManager.set('key1', 'value1', { ttl: 5 });
const val1 = await cacheManager.get('key1');
await new Promise(r => setTimeout(r, 6000));
const val2 = await cacheManager.get('key1');

What will be the values of val1 and val2 respectively?

medium

A. 'value1' and undefined

B. 'value1' and 'value1'

C. undefined and 'value1'

D. undefined and undefined

Solution

1. Step 1: Understand TTL effect on cache

   The key 'key1' is set with a TTL (time to live) of 5 seconds, so it expires after 5 seconds.

2. Step 2: Analyze timing of gets

   val1 is read immediately after setting, so it returns 'value1'. val2 is read after 6 seconds, after TTL expires, so it returns undefined.

3. Final Answer:

   'value1' and undefined -> Option A

4. Quick Check:

   TTL expires after 5s, val2 is undefined [OK]

Hint: TTL expires cache after set time; later gets return undefined [OK]

Common Mistakes:

• Expecting cached value to persist beyond TTL
• Ignoring async delay before second get
• Confusing immediate and delayed cache reads

4. You wrote this NestJS Redis cache setup:

CacheModule.register({
  store: 'redis',
  host: 'localhost',
  port: '6379'
})

But your app throws a connection error. What is the likely cause?

medium

A. The port should be a number, not a string.

B. Redis store does not support host option.

C. CacheModule.register cannot be used for Redis.

D. Missing password option causes connection failure.

Solution

1. Step 1: Check data types in config

   Port should be a number (6379), not a string ('6379'), or connection fails.

2. Step 2: Validate other options

   Host is valid, CacheModule.register supports Redis, password is optional if Redis has no auth.

3. Final Answer:

   The port should be a number, not a string. -> Option A

4. Quick Check:

   Port must be number type [OK]

Hint: Use number type for port, not string, in Redis config [OK]

Common Mistakes:

• Passing port as string instead of number
• Assuming host option is invalid
• Thinking password is always required

5. You want to cache user profile data in Redis with a TTL of 10 minutes in NestJS. Which code snippet correctly sets this cache and retrieves it safely?

hard

A. await cacheManager.set('user_123', profile, 600); const data = cacheManager.get('user_123');

B. await cacheManager.set('user_123', profile, { expire: 600 }); const data = await cacheManager.get('user_123');

C. cacheManager.set('user_123', profile, { ttl: 600 }); const data = await cacheManager.get('user_123');

D. await cacheManager.set('user_123', profile, { ttl: 600 }); const data = await cacheManager.get('user_123');

Solution

1. Step 1: Use correct TTL option in set()

   TTL must be passed as an object property { ttl: 600 } for 10 minutes (600 seconds).

2. Step 2: Await both set and get calls

   Both set and get are async, so await is needed to ensure completion and data retrieval.

3. Final Answer:

   await cacheManager.set('user_123', profile, { ttl: 600 }); const data = await cacheManager.get('user_123'); -> Option D

4. Quick Check:

   TTL as { ttl: 600 } and await both calls [OK]

Hint: Pass TTL as { ttl: seconds } and await async cache calls [OK]

Common Mistakes:

• Passing TTL as a number directly instead of in object
• Not awaiting async cache get call
• Using wrong option name like expire instead of ttl