INCR and DECR for counters in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command INCR do?

INCR increases the integer value of a key by 1. If the key does not exist, it is set to 0 before incrementing.

beginner

What happens if you use DECR on a key that does not exist in Redis?

The key is set to 0 first, then decreased by 1, resulting in a value of -1.

intermediate

Can INCR and DECR be used on keys holding non-integer values?

No. INCR and DECR only work on keys holding integer values. Using them on non-integer values causes an error.

intermediate

How would you atomically increase a counter by 5 in Redis?

Use the INCRBY command with the key and 5 as arguments, e.g., INCRBY key 5.

beginner

Why are INCR and DECR commands useful for counters in Redis?

They provide a simple, fast, and atomic way to increase or decrease counters without race conditions.

What is the result of INCR on a key that does not exist?

A Does nothing

B Returns an error

C Sets the key to 0

D Sets the key to 1

INCR sets the key to 0 first, then increments it by 1, so the final value is 1.

Which command decreases the value of a Redis key by 1?

A INCR

B DECR

C INCRBY

D DECRBY

DECR decreases the integer value of a key by 1.

What happens if you run INCR on a key holding the string 'hello'?

A An error is returned

B The value becomes 1

C The value becomes 'hello1'

D The key is deleted

INCR only works on integer values; using it on a non-integer string causes an error.

How can you increase a Redis counter by 10 in one command?

A INCR key 10

B DECRBY key 10

C INCRBY key 10

D INCR key; INCR key; ... (10 times)

INCRBY key 10 increases the key's value by 10 atomically.

Why is using INCR and DECR commands better than getting and setting values manually?

A They are atomic and avoid race conditions

B They are slower but safer

C They use less memory

D They allow floating point increments

INCR and DECR are atomic operations, so they prevent race conditions in concurrent environments.

Explain how INCR and DECR commands work in Redis and why they are useful for counters.

Think about how counters change and why atomicity matters.

Describe what happens if you use INCR or DECR on a key with a non-integer value in Redis.

Consider data types and command restrictions.

Practice

1. What does the Redis command INCR mycounter do if mycounter does not exist yet?

easy

A. It creates mycounter with value 1.

B. It returns an error because the key does not exist.

C. It creates mycounter with value 0.

D. It sets mycounter to -1.

Solution

1. Step 1: Understand INCR behavior on missing keys

   When INCR is called on a key that does not exist, Redis treats it as 0 before incrementing.

2. Step 2: Apply increment to initial value

   Incrementing 0 by 1 results in 1, so Redis sets mycounter to 1.

3. Final Answer:

   It creates mycounter with value 1. -> Option A

4. Quick Check:

   INCR on missing key = 1 [OK]

Hint: INCR on missing key starts at 1 automatically [OK]

Common Mistakes:

• Thinking INCR returns error if key missing
• Assuming key starts at 0 after INCR
• Confusing INCR with setting key manually

2. Which of the following is the correct syntax to decrease a Redis key named pageviews by 1?

easy

A. DECREMENT pageviews

B. DECR pageviews

C. INCR pageviews -1

D. DECR pageviews 1

Solution

1. Step 1: Recall Redis command for decrement by 1

   The correct Redis command to decrease a key by 1 is DECR key.

2. Step 2: Check options for correct syntax

   DECR pageviews uses DECR pageviews, which is valid. DECREMENT pageviews is invalid command, C is incorrect syntax, and D is invalid because DECR does not accept an amount argument.

3. Final Answer:

   DECR pageviews -> Option B

4. Quick Check:

   Use DECR key to subtract 1 [OK]

Hint: Use DECR key to subtract 1 exactly [OK]

Common Mistakes:

• Using DECREMENT instead of DECR
• Trying INCR with negative number
• Confusing DECR and DECRBY syntax

3. Given the following Redis commands executed in order:

INCR visits
INCR visits
DECR visits
INCR visits

What is the final value of the key visits?

medium

A. 4

B. 3

C. 1

D. 2

Solution

1. Step 1: Track each command's effect on visits

   Start: visits does not exist, treated as 0.
   INCR visits -> 1
   INCR visits -> 2
   DECR visits -> 1
   INCR visits -> 2

2. Step 2: Recalculate carefully

   Re-check: Actually, after DECR visits from 2, visits = 1.
   Then INCR visits from 1, visits = 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   INCR twice, DECR once, INCR once = 2 [OK]

Hint: Count increments and decrements stepwise [OK]

Common Mistakes:

• Adding all INCRs without subtracting DECR
• Assuming visits starts at 1
• Mixing order of commands

4. You run the command DECR mycount on a Redis key mycount that currently holds the string value "hello". What will happen?

medium

A. Redis will convert "hello" to 0 and then decrement to -1.

B. The value will become "hello-1".

C. Redis will return an error because the value is not an integer.

D. The value will remain "hello" unchanged.

Solution

1. Step 1: Understand DECR command requirements

   DECR expects the key's value to be an integer or missing (treated as 0). A non-integer string causes an error.

2. Step 2: Apply to given value

   Since mycount holds "hello", which is not an integer, Redis returns an error.

3. Final Answer:

   Redis will return an error because the value is not an integer. -> Option C

4. Quick Check:

   DECR on non-integer string = error [OK]

Hint: DECR only works on integer values [OK]

Common Mistakes:

• Assuming Redis converts strings to 0 automatically
• Expecting string concatenation
• Thinking DECR silently fails

5. You want to track the number of active users on a website using Redis. Which sequence of commands correctly increases the count when a user logs in and decreases it when a user logs out, ensuring the count never goes below zero?

hard

A. Use INCR active_users on login and before DECR active_users on logout, check if value > 0.

B. Use INCRBY active_users 1 on login and DECRBY active_users 1 on logout without checks.

C. Use INCR active_users on login and DECR active_users on logout without checks.

D. Use INCR active_users on login and DEL active_users on logout.

Solution

1. Step 1: Understand counting active users

   Increment on login is simple with INCR. Decrement on logout must avoid negative counts.

2. Step 2: Prevent negative counts on logout

   Before calling DECR, check if active_users > 0 to avoid going below zero.

3. Final Answer:

   Use INCR on login and check value > 0 before DECR on logout. -> Option A

4. Quick Check:

   Check before DECR to avoid negatives [OK]

Hint: Check count > 0 before DECR to prevent negatives [OK]

Common Mistakes:

• Not checking before DECR causes negative counts
• Deleting key on logout resets count incorrectly
• Assuming DECRBY and DECR behave differently here