Counter pattern in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Incrementing a counter in Redis

What will be the value of the key page_views after running these commands in order?

1. SET page_views 10
2. INCR page_views
3. INCRBY page_views 5

Redis

SET page_views 10
INCR page_views
INCRBY page_views 5

A 11

B 10

C 15

D 16

💡 Hint

Remember that INCR increases by 1 and INCRBY increases by the specified number.

The initial value is 10. INCR adds 1 making it 11. INCRBY adds 5 making it 16.

📝 Syntax

intermediate

Correct syntax to initialize a counter

Which Redis command correctly initializes a counter named likes to zero?

A SET likes 0

B INCR likes 0

C INCRBY likes 0

D SET likes

💡 Hint

To set a key to a value, use SET with key and value.

SET likes 0 sets the key 'likes' to zero. INCR and INCRBY require the key to exist or treat missing as zero but cannot set directly to zero with arguments.

❓ optimization

advanced

Efficiently increment multiple counters

You want to increment counters counter1, counter2, and counter3 by 1 each in Redis. Which approach is the most efficient?

A Use a Lua script that increments all three counters in one call

B Run three separate INCR commands: INCR counter1, INCR counter2, INCR counter3

C Use a Redis transaction (MULTI/EXEC) with three INCR commands inside

D Use INCRBY with a list of keys and values

💡 Hint

Reducing round trips to Redis improves performance.

A Lua script runs atomically and reduces network round trips by incrementing all counters in one call. MULTI/EXEC groups commands but still sends multiple commands. Separate INCR commands cause multiple round trips. INCRBY does not accept multiple keys.

🔧 Debug

advanced

Why does INCR fail on this key?

You run SET user_count "ten" and then INCR user_count. What error will Redis return?

A 0

B OK

C ERR value is not an integer or out of range

D ERR unknown command 'INCR'

💡 Hint

INCR expects the key to hold an integer value or not exist.

Since the value is the string "ten", which is not an integer, INCR cannot increment it and returns an error.

🧠 Conceptual

expert

Atomicity of counter increments in Redis

Which statement best describes the atomicity of the INCR command in Redis when multiple clients increment the same counter simultaneously?

A INCR requires explicit locking to be atomic

B INCR is atomic; Redis ensures increments are processed one at a time without race conditions

C INCR is not atomic; increments can be lost if clients run concurrently

D INCR batches increments and applies them periodically

💡 Hint

Redis commands are atomic by design.

Redis processes commands sequentially in a single thread, so INCR commands are atomic and safe from race conditions even with concurrent clients.