Cache stores (memory, Redis) in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a cache store in the context of NestJS?

A cache store in NestJS is a place where data is temporarily saved to speed up repeated requests. It helps avoid fetching or computing the same data multiple times.

beginner

How does the in-memory cache store work in NestJS?

The in-memory cache store keeps data inside the app's memory. It is fast but data is lost if the app restarts. It is good for small, temporary caching.

intermediate

Why use Redis as a cache store instead of memory in NestJS?

Redis stores cache data outside the app in a separate server. It keeps data even if the app restarts and can share cache between multiple app instances.

intermediate

How do you enable caching with Redis in a NestJS app?

You install Redis, then configure NestJS CacheModule with a Redis store by providing Redis connection details. This lets NestJS use Redis to save and get cached data.

beginner

What is a key benefit of using cache stores in web applications?

Cache stores reduce the time to get data by saving results of expensive operations. This makes apps faster and reduces load on databases or APIs.

Which cache store keeps data only while the app is running?

A In-memory cache

B Redis cache

C Database cache

D File system cache

In-memory cache stores data inside the app's memory and loses it when the app stops.

What is a main advantage of Redis over in-memory cache in NestJS?

A It is slower

B Data persists after app restarts

C It uses more CPU

D It only works locally

Redis stores data outside the app, so cached data stays even if the app restarts.

How do you add caching with Redis in a NestJS app?

A Configure CacheModule with Redis store and connection details

B Use in-memory cache only

C Install Redis but do not configure NestJS

D Write cache logic manually without modules

NestJS CacheModule can be configured to use Redis by providing connection info.

Which of these is NOT a benefit of caching in web apps?

A Faster data retrieval

B Reduced database load

C Permanent data storage

D Improved app performance

Cache is temporary storage, not meant for permanent data saving.

What happens to in-memory cache data when the NestJS app restarts?

A Data moves to Redis

B Data is saved automatically

C Data is backed up to disk

D Data is lost

In-memory cache is cleared when the app stops or restarts.

Explain how caching with Redis works in a NestJS application and why it might be preferred over in-memory caching.

Think about app restarts and multiple servers.

Describe the role of cache stores in improving web app performance and give examples of when to use memory cache versus Redis cache.

Consider speed, persistence, and app size.

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