Key design patterns in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Given the following Redis commands executed in order:

1. SET user:1000:name "Alice"
2. SET user:1000:age 30
3. HGETALL user:1000

What will be the output of the last command?

Redis

SET user:1000:name "Alice"
SET user:1000:age 30
HGETALL user:1000

A An empty list ([]) because user:1000 is not a hash key

B ["user:1000:name", "Alice", "user:1000:age", "30"]

C ["name", "Alice", "age", "30"]

D Error: WRONGTYPE Operation against a key holding the wrong kind of value

💡 Hint

Remember that SET creates string keys, not hashes.

The keys user:1000:name and user:1000:age are separate string keys. The key user:1000 does not exist as a hash, so HGETALL returns an empty list.

📝 Syntax

intermediate

Which Redis command correctly implements a simple rate limiter using a sorted set?

You want to limit a user to 5 actions per minute using Redis sorted sets. Which command correctly adds a timestamp to the sorted set for user:123 and removes old entries older than 60 seconds?

A

ZADD user:123 action1 1620000000
ZREMRANGEBYSCORE user:123 -inf 1619999940

B

ZADD user:123 action1 1620000000
ZREMRANGEBYSCORE user:123 1619999940 +inf

C

ZADD user:123 1620000000 action1
ZREMRANGEBYSCORE user:123 1619999940 +inf

D

ZADD user:123 1620000000 action1
ZREMRANGEBYSCORE user:123 -inf 1619999940

💡 Hint

ZADD syntax is ZADD key score member. ZREMRANGEBYSCORE removes by score range.

Option D correctly adds the action with the timestamp as score and removes entries with scores less than or equal to 1619999940 (60 seconds ago).

🧠 Conceptual

advanced

Why is the Redis key naming pattern with colons (:) recommended?

Redis keys often use colons to separate parts, like user:1000:profile. What is the main advantage of this pattern?

A It helps organize keys logically and supports efficient use of commands like SCAN and KEYS with patterns.

B It allows Redis to automatically create nested data structures.

C It compresses keys to save memory in Redis.

D It enables Redis to enforce access control on key namespaces.

💡 Hint

Think about how you find keys by pattern.

Using colons creates a logical namespace that helps group related keys and makes pattern matching easier with commands like SCAN user:1000:*.

🔧 Debug

advanced

What error occurs when running this Redis Lua script?

Consider this Lua script run in Redis:

local val = redis.call('GET', 'counter')
return val + 1

What error will this script produce if 'counter' key does not exist?

Redis

local val = redis.call('GET', 'counter')
return val + 1

A Redis command error because GET is invalid in Lua

B nil value error because val is nil and cannot be added to 1

C Returns 1 because nil is treated as 0

D Syntax error in Lua script

💡 Hint

What does GET return if key does not exist?

GET returns nil if the key does not exist. Adding nil + 1 causes a Lua runtime error.

❓ optimization

expert

Which Redis data structure and pattern is best for implementing a leaderboard with frequent score updates and top 10 queries?

You want to build a leaderboard that updates user scores frequently and retrieves the top 10 users efficiently. Which Redis data structure and pattern is best suited?

A Use a Redis hash with user IDs as fields and scores as values

B Use a Redis list to push scores and sort on retrieval

C Use a Redis sorted set with user IDs as members and scores as scores

D Use multiple string keys per user to store scores and sort externally

💡 Hint

Think about data structures that keep elements sorted by score.

Redis sorted sets keep members sorted by score and support efficient range queries like top 10 with ZREVRANGE.