Junior Caching & Redis Interview Questions (50 MCQs)

Question 1

What is the main purpose of using a cache in a backend system?

Accepted Answer

Reduce latency and load on the source by keeping frequently used data in faster storage

Answer

Guarantee permanent durability of data instead of the database

Answer

Increase data security by encrypting it automatically

Answer

Fully replace the database for every kind of query

Question 2

What does a cache miss mean?

Accepted Answer

The requested data was not in the cache and must be read from the source

Answer

The data was in the cache and was returned directly

Answer

The cache crashed because memory was full

Answer

The data in the cache was overwritten with a new value

Question 3

If 800 of 1000 requests are served from the cache, what is the hit ratio?

Accepted Answer

80%

Answer

20%

Answer

40%

Answer

125%

Question 4

In the cache-aside (lazy loading) pattern, what happens on a read?

Accepted Answer

The app checks the cache; on a miss it reads the DB and writes the result to the cache

Answer

The cache itself loads from the DB and the app only talks to the cache

Answer

All data is loaded into the cache at once when the app starts

Answer

The app always reads from the DB and the cache is only for writes

Question 5

What does TTL mean in caching?

Accepted Answer

How long an item stays valid in the cache before it expires

Answer

The maximum number of keys the cache can hold

Answer

The time needed for the app to connect to the cache server

Answer

How many times a key can be read

Question 6

Which of these is NOT a common caching layer in a web request path?

Accepted Answer

The server's log file

Answer

The browser cache

Answer

A CDN

Answer

A distributed cache like Redis

Question 7

What kind of tool is Redis fundamentally?

Accepted Answer

An in-memory key-value store with rich data structures

Answer

A disk-based relational database with SQL support

Answer

A message broker that only supports AMQP queues

Answer

A full-text search engine based on inverted indexes

Question 8

What is the main reason Redis is so fast?

Accepted Answer

Data is stored and processed in memory (RAM)

Answer

It spawns a separate thread for every request

Answer

It stores all data compressed on SSD

Answer

It uses optimized SQL to execute queries

Question 9

What does `SET session:42 abc EX 60` do in Redis?

Accepted Answer

Stores `abc` under the key and expires it after 60 seconds

Answer

Repeats the value 60 times only if the key exists

Answer

Stores the value and expires it after 60 milliseconds

Answer

Creates 60 new keys prefixed with `session:42`

Question 10

What does `TTL mykey` return for a key that exists but has no expiry?

Accepted Answer

`-1`

Answer

`-2`

Answer

`0`

Answer

`nil`

Question 11

Why is `INCR` well suited for counters (like page views) in Redis?

Accepted Answer

It is atomic, so concurrent clients increment without race conditions

Answer

It automatically flushes the value to disk so it's never lost

Answer

It only works on keys that have a TTL

Answer

It updates all replicas before replying

Question 12

Which of these is NOT a native Redis data type?

Accepted Answer

Table with typed columns

Answer

Sorted Set

Answer

Hash

Answer

Stream

Question 13

Which Redis type best fits storing a user's fields (name, email, age) under one key?

Accepted Answer

Hash with `HSET`

Answer

List with `LPUSH`

Answer

Set with `SADD`

Answer

HyperLogLog with `PFADD`

Question 14

Which Redis type fits storing a collection of unique tags for a post?

Accepted Answer

Set

Answer

List

Answer

String

Answer

Stream

Question 15

Which Redis type is used to build a simple FIFO queue with `LPUSH` and `RPOP`?

Accepted Answer

List

Answer

Sorted Set

Answer

Hash

Answer

Bitmap

Question 16

Which Redis type is best for a game leaderboard ranked by score?

Accepted Answer

Sorted Set (ZSET)

Answer

List

Answer

Set

Answer

String holding JSON

Question 17

What is the difference between `DEL key` and `EXPIRE key 10`?

Accepted Answer

`DEL` removes the key now; `EXPIRE` makes it disappear after 10 seconds

Answer

`DEL` only empties the value but keeps the key

Answer

`EXPIRE` deletes the key now and restores it after 10 seconds

Answer

They are identical, only the names differ

Question 18

What is the main risk of putting a cache in front of a database?

Accepted Answer

Serving stale data when the DB changed but the cache didn't

Answer

Every read becomes slower, even on a cache hit

Answer

Losing database data when the cache restarts

Answer

The database can no longer use indexes

Question 19

Which kind of data is the best candidate for caching?

Accepted Answer

Data that's read often and changes rarely, like a product category list

Answer

A live bank balance that changes on every transaction

Answer

Data that's written once and never read

Answer

A one-time token that is invalidated immediately

Question 20

What does the response header `Cache-Control: max-age=3600` mean?

Accepted Answer

The response is fresh for 3600 seconds and can be reused without contacting the server

Answer

The server must respond within 3600 milliseconds

Answer

The response can be read from cache only 3600 times

Answer

The response must not be stored until 3600 seconds pass

Question 21

Which header is correct for a sensitive response that must not be stored in any cache?

Accepted Answer

`Cache-Control: no-store`

Answer

`Cache-Control: no-cache`

Answer

`Cache-Control: max-age=0`

Answer

`Cache-Control: public`

Question 22

What exactly does `Cache-Control: no-cache` specify?

Accepted Answer

The response may be stored but must be revalidated with the server before each use

Answer

The response must never be stored in any cache

Answer

The response is stored only in the CDN, not the browser

Answer

The response is fresh forever and never needs checking

Question 23

What is the role of the `ETag` header in HTTP caching?

Accepted Answer

It's a version identifier so the client can check for changes via `If-None-Match`

Answer

It specifies how many seconds the response stays fresh

Answer

It decides whether the CDN may store the response

Answer

It encrypts the response body for security

Question 24

When does a server return `304 Not Modified`?

Accepted Answer

When a conditional request finds the resource unchanged since the client's copy

Answer

When the resource has moved to another URL

Answer

When the client isn't allowed to access the resource

Answer

When the server cache is full and can't store the response

Question 25

What kind of response is `Cache-Control: private` meant for?

Accepted Answer

User-specific responses that only the user's browser may store, not shared CDNs/proxies

Answer

Responses that should be stored only in the CDN, not the browser

Answer

Responses that must be encrypted with HTTPS

Answer

Responses that only admins are allowed to see

Question 26

How does a CDN speed up delivery of static files?

Accepted Answer

By caching content on edge servers close to users

Answer

By compressing the origin server's database

Answer

By running backend code inside the user's browser

Answer

By removing the need for DNS to resolve the domain

Question 27

What does eviction mean in a cache?

Accepted Answer

Removing items to free space when the cache hits its memory limit

Answer

Moving data from the cache into the database on write

Answer

Encrypting old keys for extra security

Answer

Replicating data to every node in the cluster

Question 28

Which item does the LRU policy choose to evict?

Accepted Answer

The item that hasn't been accessed for the longest time

Answer

The item accessed the fewest number of times

Answer

The item inserted first, regardless of access

Answer

The item with the largest size

Question 29

On what basis does LFU evict items?

Accepted Answer

Lowest access frequency

Answer

Oldest last-access time

Answer

Shortest remaining TTL

Answer

Purely random selection

Question 30

What is the main weakness of FIFO eviction?

Accepted Answer

It may evict a hot item just because it was inserted early

Answer

It requires exact access counting, which is expensive

Answer

It never evicts anything

Answer

It only works on keys with a TTL

Question 31

What is the main problem with a local in-memory cache (e.g. a Map per instance) when running several instances?

Accepted Answer

Each instance has its own copy, so data becomes inconsistent across them

Answer

Accessing it is slower than Redis

Answer

It's impossible to set TTLs on it

Answer

It can only be implemented in compiled languages

Question 32

If Redis restarts with persistence fully disabled, what happens to the data?

Accepted Answer

All data is lost because it lived only in RAM

Answer

Data is automatically restored from the primary database

Answer

Only keys without a TTL are preserved

Answer

Redis refuses to start without persistence

Question 33

Which Redis key naming convention is common and recommended?

Accepted Answer

Colon-separated namespaces, like `user:42:profile`

Answer

Random numbers without prefixes for each key

Answer

Putting the whole JSON value inside the key name

Answer

Very long names that include a full timestamp

Question 34

Why is running `KEYS *` on a production Redis dangerous?

Accepted Answer

It walks every key in one O(N) call and blocks the single-threaded server

Answer

It deletes all keys after listing them

Answer

It resets the TTL of every key

Answer

It works only on replicas and errors on the master

Question 35

What does `EXISTS user:1` return if the key exists?

Accepted Answer

`1`

Answer

`OK`

Answer

The key's value

Answer

`true`

Question 36

Why is Redis a common choice for storing user sessions in multi-instance services?

Accepted Answer

All instances share one fast store, and TTL support is built in

Answer

Redis automatically stores the session in the browser cookie

Answer

Redis is the only database that supports encryption

Answer

With Redis, user authentication isn't needed

Question 37

In the write-through pattern, what happens on a write?

Accepted Answer

Data is written to both the cache and the DB synchronously before acknowledging

Answer

Data is written only to the cache and flushed to the DB asynchronously later

Answer

Data is written only to the DB and the cache is untouched

Answer

The entire cache is flushed after every write

Question 38

In cache-aside, after updating a record in the DB, what's the most common action on its cache key?

Accepted Answer

Delete (invalidate) the key so the next read loads the fresh value

Answer

Extend the current key's TTL without changing its value

Answer

Run `FLUSHALL` to wipe the whole cache

Answer

Nothing, since the cache detects the change by itself

Question 39

What is a cold cache?

Accepted Answer

An empty cache (e.g. after a restart) where most requests miss

Answer

A cache running on a low-temperature server

Answer

A cache that stores only archived data

Answer

A read-only cache that rejects writes

Question 40

How do you store a JavaScript object in Redis as a string?

Accepted Answer

Serialize with `JSON.stringify` and rebuild with `JSON.parse` on read

Answer

Pass the object directly; Redis converts it to JSON itself

Answer

Use `toString()` so all fields are kept

Answer

Redis doesn't allow storing non-numeric data

Question 41

What is the main difference between Memcached and Redis?

Accepted Answer

Memcached offers only plain string key-value; Redis adds data structures and persistence

Answer

Memcached stores on disk while Redis stores in memory

Answer

Memcached doesn't support TTLs

Answer

Redis only runs on Windows

Question 42

Why is reading 100 keys with one `MGET` better than 100 separate `GET`s?

Accepted Answer

Only one network round-trip is made, reducing total latency

Answer

`MGET` compresses data and uses less memory

Answer

`MGET` also reads from the browser cache

Answer

`GET` only works on replicas

Question 43

What role does the buffer pool play in databases like PostgreSQL or MySQL?

Accepted Answer

Keeps hot data and index pages in RAM to reduce disk reads

Answer

Caches complete query results for clients

Answer

Manages client network connections

Answer

Holds daily backups in memory

Question 44

In Redis Pub/Sub, what happens to a message if no subscriber is connected at `PUBLISH` time?

Accepted Answer

The message is dropped and not stored

Answer

It is queued until the first subscriber connects

Answer

It is automatically stored in a Stream

Answer

Redis returns an error and rejects it

Question 45

What does the `NX` option do in `SET key value NX`?

Accepted Answer

Sets the value only if the key does not already exist

Answer

Sets the value only if the key already exists

Answer

Stores the key with no TTL, forever

Answer

Returns the old value and deletes the key

Question 46

When caching `GET /products?page=2&sort=price`, what should the cache key include?

Accepted Answer

All parameters that affect the result, e.g. `products:page=2:sort=price`

Answer

Just `products`, since the path is the same

Answer

A random UUID for each request

Answer

Only the request timestamp

Question 47

What is the consequence of a very long TTL for data that changes occasionally?

Accepted Answer

Hit ratio rises, but users see stale data for longer

Answer

Hit ratio drops and the DB gets more load

Answer

Redis rejects the key since the max TTL is one hour

Answer

No effect, as TTL is only for security

Question 48

Which tool is an example of a reverse proxy cache in front of an app server?

Accepted Answer

Varnish or Nginx with `proxy_cache`

Answer

PostgreSQL

Answer

Kafka

Answer

Elasticsearch

Question 49

Why shouldn't Redis usually be the only store for critical data like financial orders?

Accepted Answer

Data lives in RAM and, depending on persistence settings, may be lost on a crash

Answer

Because Redis can't store numbers

Answer

Because Redis supports only 100 keys

Answer

Because reading Redis is slower than a relational DB

Question 50

Which header suits fingerprinted static files like `app.3f9a1c.js`?

Accepted Answer

`Cache-Control: public, max-age=31536000, immutable`

Answer

`Cache-Control: no-store`

Answer

`Cache-Control: no-cache`

Answer

`Cache-Control: private, max-age=0`