Memory-efficient data structures in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Consider the following Redis commands executed in order:

HMSET user:1 name "Alice" age 30 city "NY"
HGETALL user:1

What is the output of HGETALL user:1?

Redis

HMSET user:1 name "Alice" age 30 city "NY"
HGETALL user:1

A ["user:1", "name", "Alice", "age", "30", "city", "NY"]

B {"name": "Alice", "age": 30, "city": "NY"}

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

D ["name", "Alice", "age", 30, "city", "NY"]

💡 Hint

Remember that HGETALL returns a flat list of fields and values as strings.

The HGETALL command returns a flat list of field-value pairs as strings, not a JSON object or dictionary.

🧠 Conceptual

intermediate

Which Redis data structure is most memory-efficient for storing a large set of unique integers?

You need to store millions of unique integers in Redis with minimal memory usage. Which data structure should you choose?

A Redis Bitmap

B Redis Sorted Set

C Redis Set

D Redis List

💡 Hint

Think about bit-level storage for integers.

Redis Bitmaps use bits to represent presence or absence of integers, making them very memory-efficient for large sets of integers.

📝 Syntax

advanced

Which Redis command correctly creates a memory-efficient small hash?

You want to create a small hash with fields 'a' and 'b' having values '1' and '2' respectively, optimized for memory. Which command is correct?

A HSET myhash {a 1} {b 2}

B HSET myhash a 1 b 2

C HMSET myhash a 1 b 2

D HSET myhash a=1 b=2

💡 Hint

Check the correct syntax for HSET with multiple fields.

HSET supports multiple field-value pairs as separate arguments. HMSET is deprecated. Options A and D have invalid syntax.

❓ optimization

advanced

How to optimize memory usage for a Redis list with many small elements?

You have a Redis list with many small string elements. Which approach reduces memory usage the most?

A Use a Redis List as is

B Use Redis Sorted Set instead

C Store elements as separate keys

D Convert the list to a Redis Ziplist-encoded list by keeping it small

💡 Hint

Redis uses special encodings for small lists to save memory.

Redis automatically uses Ziplist encoding for small lists, which is memory efficient. Keeping the list small enables this optimization.

🔧 Debug

expert

Why does this Redis command cause a memory spike?

You run:
LPUSH mylist "a" "b" "c" ... "z"
and notice a sudden memory spike. What is the most likely cause?

Redis

LPUSH mylist "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"

A Redis converted the list from Ziplist to Linkedlist due to size

B LPUSH command syntax is incorrect causing memory leak

C Redis is duplicating the list elements internally

D Redis is compressing the list causing temporary memory spike

💡 Hint

Redis changes internal encoding based on list size.

When a list grows beyond a threshold, Redis switches from Ziplist (compact) to Linkedlist encoding, increasing memory usage.