Redis as cache vs data store vs message broker - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Primary use of Redis as a cache

What is the main reason Redis is used as a cache in applications?

A To store data permanently with complex queries

B To perform heavy data analytics and reporting

C To send messages between different parts of an application

D To temporarily store frequently accessed data for faster retrieval

💡 Hint

Think about why caching helps speed up applications.

Redis as a cache stores data temporarily to speed up access to frequently used information, reducing the need to fetch from slower databases.

🧠 Conceptual

intermediate

Redis as a message broker role

Which Redis feature allows it to act as a message broker?

A Key expiration for automatic data deletion

B Pub/Sub system for sending messages between clients

C Sorted sets for ranking data

D Hashes for storing user profiles

💡 Hint

Message brokers help parts of an app talk to each other.

Redis uses its Pub/Sub feature to send messages between clients, enabling it to work as a message broker.

❓ query_result

advanced

Result of Redis command for cache expiration

What is the output of the following Redis commands?

SET user:1 "Alice" EX 10
TTL user:1

Redis

SET user:1 "Alice" EX 10
TTL user:1

A 0

B -1

C 10

D nil

💡 Hint

EX sets expiration in seconds; TTL returns remaining time.

The SET command with EX 10 sets the key to expire in 10 seconds. TTL returns the remaining time to live, which is 10 seconds immediately after setting.

📝 Syntax

advanced

Correct Redis command to publish a message

Which Redis command correctly publishes the message "Hello" to the channel "news"?

A PUBLISH news Hello

B PUBLISH news "Hello"

C PUBLISH "news" "Hello"

D PUBLISH "news" Hello

💡 Hint

Redis commands do not require quotes for simple strings.

The PUBLISH command syntax is: PUBLISH channel message. Quotes are not needed unless the client requires them, but Redis CLI accepts unquoted simple strings.

🧠 Conceptual

expert

Choosing Redis as data store vs cache

Which scenario best justifies using Redis as a primary data store instead of just a cache?

A Maintaining real-time leaderboard data that must be highly available

B Caching product details that are also stored in a relational database

C Holding temporary API rate limit counters that reset often

D Storing session data that can be lost without issues

💡 Hint

Think about when data loss is unacceptable and fast access is critical.

Using Redis as a primary data store fits scenarios needing fast, reliable access to data that cannot be lost, like real-time leaderboards.

Practice

1. Which of the following best describes Redis when used as a cache?

easy

A. It sends messages between different parts of an app.

B. It permanently stores large amounts of data for apps.

C. It temporarily stores data to speed up app responses.

D. It analyzes data to find patterns automatically.

Solution

1. Step 1: Understand Redis cache role

   Redis cache stores data temporarily to help apps respond faster by avoiding repeated data fetching.

2. Step 2: Compare other roles

   Permanent storage is data store, messaging is message broker, and analysis is not Redis's main role.

3. Final Answer:

   It temporarily stores data to speed up app responses. -> Option C

4. Quick Check:

   Cache = Temporary fast storage [OK]

Hint: Cache means temporary fast storage to speed apps [OK]

Common Mistakes:

• Confusing cache with permanent data store
• Thinking Redis analyzes data automatically
• Mixing message broker role with cache

2. Which Redis command is used to publish a message to a channel when Redis acts as a message broker?

easy

A. SET

B. PUBLISH

C. GET

D. SUBSCRIBE

Solution

1. Step 1: Identify publish command in Redis

   The PUBLISH command sends a message to subscribers on a channel in Redis's message broker role.

2. Step 2: Differentiate from other commands

   SET stores data, GET retrieves data, SUBSCRIBE listens to messages but does not send them.

3. Final Answer:

   PUBLISH -> Option B

4. Quick Check:

   Send message = PUBLISH [OK]

Hint: Publish sends messages; subscribe listens [OK]

Common Mistakes:

• Using SET or GET to send messages
• Confusing SUBSCRIBE with PUBLISH
• Thinking GET publishes messages

3. Consider this Redis usage: You store user session data with expiration times to speed up login checks. What role is Redis playing here?

medium

A. Cache

B. Message broker

C. Data store

D. Analytics engine

Solution

1. Step 1: Analyze session data with expiration

   Storing data with expiration to speed up access fits caching, as data is temporary and speeds up app.

2. Step 2: Exclude other roles

   Message broker sends messages, data store keeps permanent data, analytics engine is unrelated.

3. Final Answer:

   Cache -> Option A

4. Quick Check:

   Temporary session data = Cache [OK]

Hint: Temporary data with expiry means cache [OK]

Common Mistakes:

• Thinking session data is permanent storage
• Confusing message broker with cache
• Assuming analytics role for session data

4. You wrote this Redis command to send a message: SUBSCRIBE news. What is wrong if your goal was to send a message?

medium

A. SUBSCRIBE deletes the channel.

B. SUBSCRIBE command syntax is incorrect.

C. SUBSCRIBE stores messages permanently.

D. SUBSCRIBE is for receiving messages, not sending.

Solution

1. Step 1: Understand SUBSCRIBE command purpose

   SUBSCRIBE listens to messages on a channel; it does not send messages.

2. Step 2: Identify correct sending command

   To send messages, use PUBLISH, not SUBSCRIBE.

3. Final Answer:

   SUBSCRIBE is for receiving messages, not sending. -> Option D

4. Quick Check:

   SUBSCRIBE = receive, PUBLISH = send [OK]

Hint: Subscribe listens; publish sends messages [OK]

Common Mistakes:

• Using SUBSCRIBE to send messages
• Thinking SUBSCRIBE deletes channels
• Believing SUBSCRIBE stores messages

5. You want to use Redis to both speed up data access and send notifications between app parts. Which setup correctly uses Redis roles?

hard

A. Use Redis as a cache for data and use PUBLISH/SUBSCRIBE for messaging.

B. Use Redis only as a data store and ignore messaging features.

C. Use Redis as a message broker only, not for caching.

D. Use Redis to permanently store messages and cache data simultaneously.

Solution

1. Step 1: Identify Redis roles for caching and messaging

   Redis can cache data to speed access and use PUBLISH/SUBSCRIBE commands to send messages between app parts.

2. Step 2: Exclude incorrect setups

   Ignoring messaging misses notification need; message broker only misses caching; permanent message storage is not typical.

3. Final Answer:

   Use Redis as a cache for data and use PUBLISH/SUBSCRIBE for messaging. -> Option A

4. Quick Check:

   Cache + Pub/Sub = correct multi-role use [OK]

Hint: Combine cache and pub/sub for speed and messaging [OK]

Common Mistakes:

• Using Redis only for one role when multiple needed
• Trying to store messages permanently in Redis
• Ignoring pub/sub for notifications