Counter pattern in Redis - Step-by-Step Execution

Concept Flow - Counter pattern

Start

↓

Check if counter key exists

Yes↓

Increment counter by 1

↓

Return new counter value

↓

End

No↓

Create counter key with value 1

↓

Return 1

↓

End

The counter pattern checks if a key exists in Redis, increments it if yes, or creates it with 1 if no, then returns the current count.

Execution Sample

Redis

INCR page_views
INCR page_views
GET page_views

This code increments the 'page_views' counter twice and then retrieves its current value.

Execution Table

Step	Command	Key Exists?	Action	Counter Value After
1	INCR page_views	No	Create 'page_views' with 1	1
2	INCR page_views	Yes	Increment 'page_views' by 1	2
3	GET page_views	Yes	Retrieve 'page_views' value	2

💡 After step 3, the counter value is 2, showing two increments.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
page_views	null	1	2	2

Key Moments - 2 Insights

Why does the first INCR command create the key instead of failing?

In Redis, the INCR command automatically creates the key with value 1 if it does not exist, as shown in execution_table step 1.

What happens if we GET the counter before any INCR commands?

If the key does not exist, GET returns null or nil, meaning no value is set yet, unlike INCR which initializes it.

Visual Quiz - 3 Questions

Look at the execution table, what is the counter value after the first INCR command?

A null

B 0

C 1

D 2

💡 Hint

Check the 'Counter Value After' column in row 1 of the execution_table.

At which step does the counter key first exist in Redis?

A Step 3

B Step 1

C Step 2

D It never exists

💡 Hint

Look at the 'Key Exists?' column; it changes from No to Yes after step 1.

If we run INCR three times instead of two, what would be the counter value after step 3?

A 3

B 2

C 1

D 4

💡 Hint

Each INCR adds 1; see variable_tracker for increments after each step.

Concept Snapshot

Counter pattern in Redis:
- Use INCR key to increment a counter.
- If key doesn't exist, INCR creates it with 1.
- GET key retrieves current count.
- Useful for tracking counts like page views.
- Simple, atomic, and efficient increment operation.

Full Transcript

The Redis counter pattern uses the INCR command to increase a numeric value stored at a key. If the key does not exist, INCR creates it with the value 1 automatically. This pattern is useful for counting events like page views or clicks. The example increments the 'page_views' counter twice and then retrieves its value, which ends up as 2. The execution table shows each step's command, whether the key existed, the action taken, and the counter's value after the step. The variable tracker follows the 'page_views' value from null to 1, then 2. Key moments clarify why INCR creates the key and what happens if GET is called before any increments. The visual quiz tests understanding of the counter value after increments and when the key exists. This pattern is simple and efficient for counting in Redis.