A/B testing with split_clients in Nginx - Commands & Configuration

Introduction

A/B testing helps you show different versions of a website to different users to see which works better. The split_clients directive in Nginx lets you divide users into groups automatically based on their IP or other data, so you can test different content easily.

When you want to test two different webpage designs to see which users like more.

When you want to show a new feature to only some users without affecting everyone.

When you want to compare two different backend servers by sending users to each one randomly.

When you want to gradually roll out a change by increasing the percentage of users seeing it.

When you want to collect data on user behavior for different versions of your site.

Config File - nginx.conf

nginx.conf

http {
    split_clients "$remote_addr" $variant {
        50%     "A";
        *       "B";
    }

    server {
        listen 80;

        location / {
            if ($variant = "A") {
                return 200 "You are in group A";
            }
            if ($variant = "B") {
                return 200 "You are in group B";
            }
        }
    }
}

The split_clients directive divides users into groups based on their IP address ($remote_addr).

Here, 50% of users get the variant "A" and the rest get "B".

Inside the server block, requests are checked for their group and respond with a message accordingly.

Commands

This command tests the Nginx configuration file for syntax errors before applying it.

Terminal

nginx -t

Expected Output

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok

nginx: configuration file /etc/nginx/nginx.conf test is successful

This command reloads Nginx to apply the new configuration without stopping the server.

Terminal

systemctl reload nginx

Expected Output

No output (command runs silently)

This command sends a request to the local Nginx server to see which variant the user is assigned to.

Terminal

curl -s http://localhost/

Expected Output

You are in group A

→

-s - Runs curl silently without progress output

Run the request multiple times to observe that about half the requests return group A and half return group B.

Terminal

curl -s http://localhost/

Expected Output

You are in group B

→

-s - Runs curl silently without progress output

Key Concept

If you remember nothing else from this pattern, remember: split_clients divides users into groups based on a key like IP, letting you serve different content for A/B testing.

Common Mistakes

Using split_clients without a proper key like $remote_addr

Without a stable key, users may be assigned randomly on each request, breaking consistent A/B testing.

Always use a consistent key such as $remote_addr or a cookie to keep users in the same group.

Not reloading Nginx after changing the configuration

Changes won't take effect until Nginx reloads, so the test won't work as expected.

Run 'nginx -t' to check config, then 'systemctl reload nginx' to apply changes.

Summary

Use split_clients in Nginx to split users into groups based on their IP address.

Test the configuration syntax with 'nginx -t' before reloading Nginx.

Reload Nginx to apply changes and verify by sending requests to see which group users belong to.

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