Counter pattern in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to increment a counter named 'page_views' by 1.

Redis

INCR [1]

Drag options to blanks, or click blank then click option'

Acounter

Bviews_count

Cpage_views

Dvisits

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different key name that does not exist.

Forgetting to use the INCR command.

The INCR command increases the value of the given key by 1. Here, the counter key is 'page_views'.

2fill in blank

medium

Complete the code to get the current value of the counter 'user_logins'.

Redis

GET [1]

Drag options to blanks, or click blank then click option'

Alogins

Buser_logins

Clogin_count

Dcount_users

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong key name that does not hold the counter.

Using a command other than GET to retrieve the value.

The GET command retrieves the current value stored at the key. Here, the key is 'user_logins'.

3fill in blank

hard

Fix the error in the code to decrement the counter 'active_sessions' by 1.

Redis

DECR [1]

Drag options to blanks, or click blank then click option'

Aactive_sessions

Bsessions_active

CactiveSession

Dsession_count

Attempts:

3 left

💡 Hint

Common Mistakes

Using a key name with different spelling or casing.

Using INCR instead of DECR for decrementing.

The DECR command decreases the value of the given key by 1. The key must be exactly 'active_sessions'.

4fill in blank

hard

Fill both blanks to set the counter 'error_count' to 0 and then increment it by 1.

Redis

SET [1] 0
INCR [2]

Drag options to blanks, or click blank then click option'

Aerror_count

BerrorCount

Cerrors

Dcount_errors

Attempts:

3 left

💡 Hint

Common Mistakes

Using different key names in SET and INCR commands.

Misspelling the key name.

Both commands must use the exact key name 'error_count' to reset and then increment the counter.

5fill in blank

hard

Fill all three blanks to atomically increment 'page_hits' by 1, set an expiration of 60 seconds, and then get its value.

Redis

MULTI
INCR [1]
EXPIRE [2] [3]
EXEC

Drag options to blanks, or click blank then click option'

Apage_hits

BpageHits

C60

D120

Attempts:

3 left

💡 Hint

Common Mistakes

Using different key names in the transaction.

Setting wrong expiration time.

Forgetting to use MULTI/EXEC for atomicity.

All commands must use the exact key 'page_hits'. The expiration time is set to 60 seconds.

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