Unique visitor tracking with sets in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Counting unique visitors using Redis sets

Given the following Redis commands executed in order:

1. SADD visitors 101 102 103
2. SADD visitors 102 104
3. SADD visitors 105
4. SCARD visitors

What is the output of the last command?

Redis

SADD visitors 101 102 103
SADD visitors 102 104
SADD visitors 105
SCARD visitors

A 3

B 4

C 6

D 5

💡 Hint

Remember that sets only store unique elements.

The set 'visitors' after all SADD commands contains unique IDs: 101, 102, 103, 104, 105. So the count is 5.

🧠 Conceptual

intermediate

Why use sets for unique visitor tracking?

Why are Redis sets a good choice for tracking unique visitors on a website?

A Because sets store data in sorted order for fast retrieval.

B Because sets automatically prevent duplicate entries, ensuring each visitor is counted once.

C Because sets allow storing key-value pairs for visitor details.

D Because sets compress data to save memory.

💡 Hint

Think about how duplicates are handled.

Redis sets store unique elements only, so adding the same visitor ID multiple times does not increase the count.

📝 Syntax

advanced

Identify the correct Redis command to add multiple visitors

Which of the following Redis commands correctly adds visitor IDs 201, 202, and 203 to the set named 'daily_visitors'?

A SADD daily_visitors [201,202,203]

B ADD daily_visitors 201 202 203

C SADD daily_visitors 201 202 203

D SADD daily_visitors {201,202,203}

💡 Hint

Check the exact Redis command name and argument format.

The correct command is SADD followed by the set name and the list of members separated by spaces.

❓ optimization

advanced

Efficiently tracking unique visitors across multiple days

You have daily sets of visitors: 'visitors_day1', 'visitors_day2', and 'visitors_day3'. You want to find the total unique visitors over these three days. Which Redis command is the most efficient to get this count without creating a new set?

A SUNION visitors_day1 visitors_day2 visitors_day3

B SUNIONSTORE total_visitors visitors_day1 visitors_day2 visitors_day3; SCARD total_visitors

C SCARD visitors_day1 + SCARD visitors_day2 + SCARD visitors_day3

D SINTER visitors_day1 visitors_day2 visitors_day3

💡 Hint

Look for a command that returns the union without storing it.

SUNION returns the union of sets without storing it. Then count the number of elements in the returned set client-side. Option A stores a new set, which is less efficient.

🔧 Debug

expert

Debugging incorrect unique visitor count

A developer runs these commands:

1. SADD visitors 301 302 303
2. SADD visitors 302 304
3. DEL visitors
4. SCARD visitors

The SCARD command returns 0, but the developer expected 4. What is the reason?

A The DEL command deleted the 'visitors' set, so SCARD returns 0 because the set no longer exists.

B SCARD only counts members added in the last SADD command.

C SCARD returns the count of unique members added in the current session only.

D SADD commands did not add members because of a syntax error.

💡 Hint

Think about what DEL does to a key.

DEL removes the entire key and its data. After deletion, the set does not exist, so SCARD returns 0.