INCR and DECR for counters in Redis - Step-by-Step Execution

Concept Flow - INCR and DECR for counters

Start with a key holding a number

↓

Receive INCR or DECR command

↓

Fetch current value of key

↓

Add 1 if INCR, subtract 1 if DECR

↓

Store new value back to key

↓

Return new value as result

↓

End

The flow shows how Redis increments or decrements a numeric value stored at a key, updating and returning the new value.

Execution Sample

Redis

SET counter 5
INCR counter
DECR counter
INCR counter

This sequence sets a counter to 5, increments it, decrements it, then increments it again.

Execution Table

Step	Command	Current Value	Operation	New Value	Output
1	SET counter 5	N/A	Set to 5	5	OK
2	INCR counter	5	+1	6	6
3	DECR counter	6	-1	5	5
4	INCR counter	5	+1	6	6
5	END	6	No more commands	6	Execution stops

💡 No more commands to execute, final counter value is 6

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
counter	N/A	5	6	5	6

Key Moments - 2 Insights

Why does the counter value change after each INCR or DECR command?

Each INCR adds 1 and each DECR subtracts 1 from the current value, as shown in steps 2, 3, and 4 of the execution_table.

What happens if the key does not exist before INCR or DECR?

Redis treats a missing key as 0, so INCR sets it to 1 and DECR sets it to -1, but this example starts with SET so the key exists.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after the first INCR command?

A 6

B 5

C 7

D Error

💡 Hint

Check row 2 in the execution_table under Output column

At which step does the counter value return to 5?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the New Value column in execution_table row 3

If we remove the initial SET command, what would INCR counter return on the first call?

A 0

B Error

C 1

D N/A

💡 Hint

Redis treats missing keys as 0 before incrementing, so INCR returns 1

Concept Snapshot

INCR and DECR commands in Redis:
- INCR key: increases the integer value by 1
- DECR key: decreases the integer value by 1
- If key does not exist, treated as 0 before operation
- Returns the new value after operation
- Useful for counters and simple numeric tracking

Full Transcript

This visual execution trace shows how Redis commands INCR and DECR work on a counter key. Starting with setting the counter to 5, each INCR adds 1 and each DECR subtracts 1. The execution table tracks each step's command, current value, operation, new value, and output. The variable tracker shows how the counter changes over time. Key moments clarify why values change and what happens if the key is missing. The quiz tests understanding of outputs and behavior. This helps beginners see exactly how Redis updates counters step-by-step.