How to Use Nginx split_clients for A/B Testing

split_clients provides deterministic traffic splitting for A/B testing and canary deployments. Same clients consistently see the same variant. This guide walks through the specific troubleshooting steps to diagnose and resolve split_clients issues.

Before You Begin

Before you begin, be sure to have the following in place:

• A Linux server with the relevant software installed
• Access to the command line interface
• Appropriate permissions (root or sudo)

Quick Fix

Wrong

if ($remote_addr ~ ...) { ... } for traffic splitting

Wrong: Using if blocks for A/B testing

Right

split_clients "${remote_addr}${http_user_agent}" $variant {\n    50%     variant-a;\n    50%     variant-b;\n}

Right: Using split_clients for deterministic traffic splitting

Output

Split clients configured:\n  50% → variant-a\n  50% → variant-b\n  Deterministic: same client always gets same variant

Prevention

To avoid future issues, follow these best practices:

• Use split_clients for consistent A/B testing (same client sees same variant)
• The hash string must include unique client identifiers
• Percentages must add up to 100%
• Use the result variable in other directives for variant handling
• Split is deterministic -- based on hash of the input string

DodaTech Tools

For further assistance with any of the above issues, consider using DodaTech consulting services or DodaTech tutorials for more in-depth guidance.

Common Mistakes with split clients

1. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
2. Using head and tail instead of pattern matching, causing runtime errors on empty lists
3. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks

These mistakes appear frequently in real-world NGINX code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ