Unique visitor tracking with sets in Redis - Step-by-Step Execution

Concept Flow - Unique visitor tracking with sets

User visits website

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Add user ID to Redis set

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Redis set stores unique IDs

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Query set size for unique visitor count

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Return count of unique visitors

Each visitor's ID is added to a Redis set, which automatically keeps only unique IDs. The set size gives the count of unique visitors.

Execution Sample

Redis

SADD visitors user1
SADD visitors user2
SADD visitors user1
SCARD visitors

Add user1 and user2 to the 'visitors' set, try adding user1 again, then get the count of unique visitors.

Execution Table

Step	Command	Action	Set Content	Output
1	SADD visitors user1	Add 'user1' to set	{user1}	1 (added)
2	SADD visitors user2	Add 'user2' to set	{user1, user2}	1 (added)
3	SADD visitors user1	Try add 'user1' again	{user1, user2}	0 (already exists)
4	SCARD visitors	Count unique visitors	{user1, user2}	2

💡 All commands executed; final unique visitor count is 2

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
visitors set	{}	{user1}	{user1, user2}	{user1, user2}	{user1, user2}

Key Moments - 2 Insights

Why does adding 'user1' the second time not increase the set size?

Because Redis sets only store unique elements, the second SADD for 'user1' does not add a duplicate, as shown in execution_table step 3 where output is 0.

How do we get the number of unique visitors?

By using the SCARD command on the set, which returns the count of unique elements, as shown in execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the output of the second SADD command?

A 1 (added)

B 0 (already exists)

C 2 (added)

D Error

💡 Hint

Check execution_table row 2 under Output column

At which step does the set stop changing?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at Set Content column in execution_table rows 3 and 4

If we add a new user 'user3' after step 3, what will SCARD return?

A 2

B 3

C 1

D 0

💡 Hint

Adding a new unique user increases set size by 1; see variable_tracker final state

Concept Snapshot

Use Redis sets to track unique visitors.
SADD adds user IDs; duplicates ignored.
SCARD returns count of unique IDs.
Set automatically keeps unique values.
Simple commands give real-time unique visitor count.

Full Transcript

This example shows how Redis sets track unique visitors by adding user IDs with SADD. Each SADD adds a user if not already present. Duplicate adds return 0 and do not change the set. The SCARD command returns the number of unique visitors by counting set members. The execution table traces each command's effect on the set and output. The variable tracker shows the set content after each step. This method efficiently counts unique visitors without duplicates.