Why INCRBY and DECRBY in Redis? - Purpose & Use Cases

The Big Idea

What if you could update scores instantly without any chance of mistakes?

The Scenario

Imagine you have a list of scores for a game stored in a notebook. Every time a player scores, you have to find their score and add the points manually. If the player loses points, you have to subtract them by hand. This gets confusing and slow as the game goes on and the list grows.

The Problem

Manually updating scores is slow and easy to mess up. You might write the wrong number, forget to update a score, or lose track of who has what points. It's hard to keep everything accurate and up-to-date, especially when many players are involved.

The Solution

INCRBY and DECRBY commands let you add or subtract numbers directly in the database without reading or rewriting the whole value. This means the database handles the math safely and quickly, so you never lose track or make mistakes.

Before vs After

✗ Before

score = get_score(player)
new_score = score + points
set_score(player, new_score)

✓ After

INCRBY player points
DECRBY player points

What It Enables

It makes updating numbers fast, safe, and easy, even when many users change data at the same time.

Real Life Example

In a live online game, players earn or lose points constantly. Using INCRBY and DECRBY keeps scores accurate instantly without slowing down the game.

Key Takeaways

Manual updates are slow and error-prone.

INCRBY and DECRBY update numbers directly in the database.

This keeps data accurate and operations fast.

Practice

1. What does the Redis command INCRBY key 5 do?

easy

A. Sets the value of key to 5.

B. Subtracts 5 from the current integer value stored at key.

C. Deletes the key from Redis.

D. Adds 5 to the current integer value stored at key.

Solution

1. Step 1: Understand the INCRBY command

   The INCRBY command increases the integer value stored at the specified key by the given amount.

2. Step 2: Apply the command to the example

   Using INCRBY key 5 adds 5 to the current value of key.

3. Final Answer:

   Adds 5 to the current integer value stored at key. -> Option D

4. Quick Check:

   INCRBY adds number = Adds 5 [OK]

Hint: INCRBY always adds the number to the key's value [OK]

Common Mistakes:

• Confusing INCRBY with DECRBY
• Thinking INCRBY sets the value instead of adding
• Assuming it deletes the key

2. Which of the following is the correct syntax to subtract 3 from a Redis key named counter?

easy

A. DECR counter 3

B. INCR counter -3

C. DECRBY counter 3

D. SUBTRACT counter 3

Solution

1. Step 1: Identify the correct command for subtraction

   The Redis command to subtract a number from a key is DECRBY key number.

2. Step 2: Check the syntax options

   DECRBY counter 3 correctly subtracts 3 from counter. INCR counter -3 is invalid (INCR takes only one argument). DECR counter 3 and SUBTRACT counter 3 are invalid.

3. Final Answer:

   DECRBY counter 3 -> Option C

4. Quick Check:

   DECRBY subtracts number = DECRBY counter 3 [OK]

Hint: Use DECRBY key number to subtract correctly [OK]

Common Mistakes:

• Using DECR with two arguments (only one allowed)
• Using INCR with an increment argument (invalid syntax)
• Using non-existent commands like SUBTRACT

3. Given the following Redis commands executed in order:
SET score 10
INCRBY score 7
DECRBY score 4
What is the final value of score?

medium

A. 10

B. 13

C. 3

D. 17

Solution

1. Step 1: Set initial value of score

   SET score 10 sets the value of score to 10.

2. Step 2: Apply INCRBY and DECRBY commands

   INCRBY score 7 adds 7, making it 17. Then DECRBY score 4 subtracts 4, resulting in 13.

3. Final Answer:

   13 -> Option B

4. Quick Check:

   10 + 7 - 4 = 13 [OK]

Hint: Add then subtract values stepwise to find final number [OK]

Common Mistakes:

• Ignoring the order of commands
• Subtracting before adding
• Confusing initial value with final

4. You run the command DECRBY visits 2 but get an error. Which is the most likely cause?

medium

A. The key visits is not an integer.

B. DECRBY requires three arguments, but only two were given.

C. DECRBY cannot subtract numbers, only add.

D. The Redis server is down.

Solution

1. Step 1: Understand DECRBY requirements

   DECRBY subtracts a number from an integer stored at the key. If the key does not exist, Redis treats it as 0. But if the key holds a non-integer value, an error occurs.

2. Step 2: Analyze the error cause

   The error likely means visits exists but is not an integer, causing DECRBY to fail.

3. Final Answer:

   The key visits is not an integer. -> Option A

4. Quick Check:

   Non-integer key causes DECRBY error [OK]

Hint: DECRBY needs integer key value; else error occurs [OK]

Common Mistakes:

• Thinking DECRBY needs 3 arguments
• Believing DECRBY only adds
• Assuming server is down without checking

5. You want to track page views in Redis using a key page_views. Initially, it does not exist. You run:
INCRBY page_views 10
DECRBY page_views 3
INCRBY page_views 5
What is the final value of page_views and why?

hard

A. 12, because Redis treats missing keys as 0 and updates accordingly.

B. 15, because DECRBY does not affect the value.

C. Error, because the key did not exist initially.

D. 10, because only the first INCRBY counts.

Solution

1. Step 1: Understand Redis behavior with missing keys

   If a key does not exist, Redis treats its value as 0 for INCRBY and DECRBY commands.

2. Step 2: Calculate the final value step-by-step

   Starting at 0, INCRBY page_views 10 sets it to 10. Then DECRBY page_views 3 subtracts 3, making 7. Finally, INCRBY page_views 5 adds 5, resulting in 12.

3. Final Answer:

   12, because Redis treats missing keys as 0 and updates accordingly. -> Option A

4. Quick Check:

   0 + 10 - 3 + 5 = 12 [OK]

Hint: Missing keys start at 0 for INCRBY/DECRBY [OK]

Common Mistakes:

• Expecting error on missing key
• Ignoring DECRBY effect
• Assuming only first command counts