A/B testing with split_clients in Nginx - Mini Project: Build & Apply

A/B Testing with split_clients in Nginx

📖 Scenario: You are working as a web server administrator. Your website wants to test two different versions of a landing page to see which one users like better. This is called A/B testing.You will use Nginx's split_clients module to send 50% of visitors to version A and 50% to version B.

🎯 Goal: Build an Nginx configuration that uses split_clients to assign visitors to either variant_a or variant_b evenly based on their IP address.This will help the website show different content to different users for testing.

📋 What You'll Learn

Create a variable called $variant using split_clients

Split traffic 50% to variant_a and 50% to variant_b

Use $remote_addr as the key for splitting

Add a server block listening on port 8080

Return the assigned variant in the response body

💡 Why This Matters

🌍 Real World

Websites often want to test different versions of pages to improve user experience. Using Nginx's split_clients lets you do this easily at the server level.

💼 Career

DevOps engineers and site reliability engineers use Nginx configurations like this to manage traffic and run experiments without changing application code.

1

Create basic Nginx server block

Create an Nginx server block that listens on port 8080 and returns the text "Hello from Nginx" for any request.

Nginx

server {
    listen 8080;
    location / {
        # Your code here
    }
}

Hint

Use return 200 "Hello from Nginx"; inside the location / block.

2

Add split_clients block to define $variant

Add a split_clients block inside the server block that creates a variable called $variant. Use $remote_addr as the key. Split traffic 50% to variant_a and 50% to variant_b.

Nginx

server {
    listen 8080;

    # Your code here

    location / {
        return 200 "Hello from Nginx";
    }
}

Hint

Use split_clients "$remote_addr" $variant { 50% variant_a; * variant_b; } inside the server block.

3

Modify location block to return the variant

Change the location / block to return the text "User assigned to: $variant" using the return directive.

Nginx

server {
    listen 8080;

    split_clients "$remote_addr" $variant {
        50% variant_a;
        *    variant_b;
    }

    location / {
        # Your code here
    }
}

Hint

Use return 200 "User assigned to: $variant"; inside the location / block.

4

Test the configuration output

Run the Nginx server and send a request to http://localhost:8080/. The response should be either User assigned to: variant_a or User assigned to: variant_b depending on your IP.

Nginx

# Run the server and test the output
# Your code here

Hint

Use curl http://localhost:8080/ to test the response.

Practice

1. What is the main purpose of the split_clients directive in nginx for A/B testing?

easy

A. To redirect all traffic to a single URL

B. To cache static files for faster loading

C. To compress responses before sending to clients

D. To divide visitors into groups based on a variable for testing different versions

Solution

1. Step 1: Understand the role of split_clients

   The split_clients directive is designed to split traffic into groups based on a variable like IP or cookie.

2. Step 2: Connect to A/B testing purpose

   This splitting allows serving different site versions to different groups, which is the core of A/B testing.

3. Final Answer:

   To divide visitors into groups based on a variable for testing different versions -> Option D

4. Quick Check:

   split_clients divides visitors for testing [OK]

Hint: Remember: split_clients splits visitors into groups [OK]

Common Mistakes:

• Confusing split_clients with caching directives
• Thinking it redirects all traffic to one URL
• Assuming it compresses responses

2. Which of the following is the correct syntax to use split_clients in nginx to split traffic by cookie $cookie_user into two groups: 50% 'A' and 50% 'B'?

easy

A. split_clients $cookie_user $variant { 50% A; 50% B; }

B. split_clients $cookie_user { 50% A; 50% B; }

C. split_clients $cookie_user $variant { 50 A; 50 B; }

D. split_clients $cookie_user $variant { 50% "A"; 50% "B"; }

Solution

1. Step 1: Review split_clients syntax

   The correct syntax requires specifying the source variable, the destination variable, and the percentage groups with semicolons.

2. Step 2: Check each option

   split_clients $cookie_user $variant { 50% A; 50% B; } correctly uses split_clients $cookie_user $variant { 50% A; 50% B; }. split_clients $cookie_user { 50% A; 50% B; } misses the destination variable. split_clients $cookie_user $variant { 50 A; 50 B; } lacks the percent sign. split_clients $cookie_user $variant { 50% "A"; 50% "B"; } incorrectly quotes the group names.

3. Final Answer:

   split_clients $cookie_user $variant { 50% A; 50% B; } -> Option A

4. Quick Check:

   Syntax requires source, destination, and percent groups [OK]

Hint: Syntax: split_clients source_var dest_var { percent group; } [OK]

Common Mistakes:

• Omitting the destination variable
• Forgetting percent signs (%)
• Using quotes around group names

3. Given this nginx config snippet:

split_clients $remote_addr $group {
  30% A;
  70% B;
}

server {
  location / {
    if ($group = A) {
      return 200 'Group A content';
    }
    return 200 'Group B content';
  }
}

What response do 70% of visitors receive?

medium

A. Group A content

B. Error: invalid syntax

C. Group B content

D. No response, connection closed

Solution

1. Step 1: Understand split_clients distribution

   The split_clients directive splits visitors by IP into 30% group A and 70% group B.

2. Step 2: Identify the majority group

   The configuration assigns 70% of visitors to group B based on the hash distribution, so they receive 'Group B content'.

3. Final Answer:

   Group B content -> Option C

4. Quick Check:

   70% of visitors get Group B content [OK]

Hint: Larger split_clients percentage gets more visitors [OK]

Common Mistakes:

• Assuming visitors always get Group A content
• Thinking syntax error occurs
• Misunderstanding the traffic split percentages

4. You wrote this nginx config:

split_clients $cookie_user $group {
  50% A
  50% B
}

But nginx fails to start. What is the error?

medium

A. Percentages must add up to 1000%

B. Missing semicolons after percentage groups

C. Destination variable $group is not declared

D. Using $cookie_user instead of $remote_addr

Solution

1. Step 1: Check syntax for split_clients block

   Each percentage group line must end with a semicolon (;). The config misses semicolons after '50% A' and '50% B'.

2. Step 2: Validate other options

   Using $cookie_user is valid. $group is declared as destination variable. Percentages add to 100%, not 1000%.

3. Final Answer:

   Missing semicolons after percentage groups -> Option B

4. Quick Check:

   Each group line ends with ; [OK]

Hint: Always end split_clients group lines with semicolons [OK]

Common Mistakes:

• Forgetting semicolons after group percentages
• Thinking variable names must be IP only
• Misreading percentage sums

5. You want to run an A/B test with three groups: 20% 'A', 30% 'B', and 50% 'C'. You also want to ensure users stay in the same group on repeat visits using a cookie $cookie_user. Which nginx config snippet correctly implements this?

hard

A. split_clients $cookie_user $group {
20% A;
30% B;
50% C;
}

server {
location / {
if ($group = A) { return 200 'A version'; }
if ($group = B) { return 200 'B version'; }
return 200 'C version';
}
}

B. split_clients $remote_addr $group {
20% A;
30% B;
50% C;
}

server {
location / {
if ($group = A) { return 200 'A version'; }
if ($group = B) { return 200 'B version'; }
return 200 'C version';
}
}

C. split_clients $cookie_user $group {
20 A;
30 B;
50 C;
}

server {
location / {
if ($group = A) { return 200 'A version'; }
if ($group = B) { return 200 'B version'; }
return 200 'C version';
}
}

D. split_clients $cookie_user $group {
20% A
30% B
50% C
}

server {
location / {
if ($group = A) { return 200 'A version'; }
if ($group = B) { return 200 'B version'; }
return 200 'C version';
}
}

Solution

1. Step 1: Confirm correct split_clients syntax and variable

   split_clients $cookie_user $group { 20% A; 30% B; 50% C; } server { location / { if ($group = A) { return 200 'A version'; } if ($group = B) { return 200 'B version'; } return 200 'C version'; } } uses $cookie_user to keep users in the same group on repeat visits and correctly uses percentages with semicolons.

2. Step 2: Check other options for errors

   split_clients $remote_addr $group { 20% A; 30% B; 50% C; } server { location / { if ($group = A) { return 200 'A version'; } if ($group = B) { return 200 'B version'; } return 200 'C version'; } } uses $remote_addr which may change and not guarantee consistent grouping. split_clients $cookie_user $group { 20 A; 30 B; 50 C; } server { location / { if ($group = A) { return 200 'A version'; } if ($group = B) { return 200 'B version'; } return 200 'C version'; } } misses percent signs. split_clients $cookie_user $group { 20% A 30% B 50% C } server { location / { if ($group = A) { return 200 'A version'; } if ($group = B) { return 200 'B version'; } return 200 'C version'; } } misses semicolons after group lines.

3. Final Answer:

   split_clients $cookie_user $group { 20% A; 30% B; 50% C; } server { location / { if ($group = A) { return 200 'A version'; } if ($group = B) { return 200 'B version'; } return 200 'C version'; } } -> Option A

4. Quick Check:

   Use cookie variable and proper syntax for consistent groups [OK]

Hint: Use cookie variable and semicolons for stable multi-group split [OK]

Common Mistakes:

• Using IP instead of cookie for consistent user grouping
• Omitting percent signs or semicolons
• Misordering or missing group percentages