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.

Practice

1. What does the Redis command INCR do in the counter pattern?

easy

A. It increases the value of a key by 1.

B. It deletes the key from the database.

C. It sets the key to a specific string value.

D. It retrieves the current value of the key.

Solution

1. Step 1: Understand the purpose of INCR

   The INCR command is designed to increase the integer value stored at a key by 1.

2. Step 2: Compare with other commands

   Unlike GET which retrieves a value, or DEL which deletes a key, INCR modifies the value by incrementing it.

3. Final Answer:

   It increases the value of a key by 1. -> Option A

4. Quick Check:

   INCR means increment by 1 [OK]

Hint: INCR always adds one to the number stored [OK]

Common Mistakes:

• Thinking INCR deletes the key
• Confusing INCR with GET
• Assuming INCR sets a string value

2. Which of the following is the correct syntax to increment a counter named page_views in Redis?

easy

A. INCREMENT page_views

B. INCR('page_views')

C. increment page_views

D. INCR page_views

Solution

1. Step 1: Recall Redis command syntax

   Redis commands are issued as uppercase words followed by the key name without parentheses.

2. Step 2: Identify the correct command

   INCR page_views is the correct syntax; other options use invalid syntax or wrong command names.

3. Final Answer:

   INCR page_views -> Option D

4. Quick Check:

   Redis commands use space-separated syntax [OK]

Hint: Redis commands have no parentheses, just command and key [OK]

Common Mistakes:

• Using parentheses like a function call
• Typing 'increment' instead of 'INCR'
• Using lowercase command names

3. Given the following Redis commands executed in order:
INCR visits
INCR visits
INCR visits
GET visits
What will be the output of the last command?

medium

A. 3

B. 1

C. 0

D. Error

Solution

1. Step 1: Count increments

   Each INCR visits increases the value by 1. Three increments mean the value is 3.

2. Step 2: Retrieve the value

   GET visits returns the current value, which is 3 after three increments.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 increments = 3 [OK]

Hint: Count how many times INCR runs before GET [OK]

Common Mistakes:

• Assuming value starts at 0 without increments
• Confusing GET output with command name
• Expecting an error if key does not exist initially

4. You run the command INCR user_likes but get an error: "ERR value is not an integer or out of range". What is the most likely cause?

medium

A. The Redis server is down.

B. The key user_likes contains a non-integer value.

C. The command INCR is misspelled.

D. The key user_likes does not exist.

Solution

1. Step 1: Understand the error message

   The error says the value is not an integer or out of range, meaning the key exists but holds a non-integer.

2. Step 2: Check other options

   If the key did not exist, Redis would create it with value 1. Misspelling causes a different error. Server down means no response.

3. Final Answer:

   The key user_likes contains a non-integer value. -> Option B

4. Quick Check:

   INCR needs integer value to work [OK]

Hint: Check key's value type before INCR [OK]

Common Mistakes:

• Assuming key absence causes this error
• Thinking server down causes this error
• Ignoring value type requirements

5. You want to count unique visitors per day using Redis counters. Which approach correctly uses the counter pattern to track daily counts without mixing days?

hard

A. Use a list to store all visitor IDs and count its length.

B. Use a single key visits and reset it to zero every day at midnight.

C. Use keys like visits:2024-06-01, visits:2024-06-02 and increment each day separately.

D. Use INCR on a key named visits without any date information.

Solution

1. Step 1: Understand the need for daily separation

   To track counts per day, keys must be unique per day to avoid mixing counts.

2. Step 2: Evaluate options

   Using keys with date suffixes like visits:2024-06-01 keeps counts separate. Resetting a single key risks losing data if reset fails. Lists are not efficient counters. Using a single key mixes all days.

3. Final Answer:

   Use keys like visits:2024-06-01, visits:2024-06-02 and increment each day separately. -> Option C

4. Quick Check:

   Unique keys per day keep counts separate [OK]

Hint: Add date to key name for daily counters [OK]

Common Mistakes:

• Resetting counters can cause data loss
• Using single key mixes all days
• Using lists instead of counters