How to Configure HAProxy use-backend for Conditional Routing

Conditional backend routing with use-backend allows different services behind a single HAProxy instance. This guide walks through the specific troubleshooting steps to diagnose and resolve conditional routing 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

default_backend app (single backend for all traffic)

Wrong: Single backend for all request types

Right

acl is_api path_beg /api/\nuse_backend api if is_api\nuse_backend admin if is_admin\ndefault_backend app

Right: Conditional backend selection with use-backend

Output

Conditional routing:\n  /api/* → api backend\n  admin.example.com → admin backend\n  Everything else → app backend

Prevention

To avoid future issues, follow these best practices:

• Place use-backend rules before default_backend
• ACLs are evaluated in order -- first match wins
• Use multiple ACLs in one use-backend with or/and
• Use default_backend as the catch-all
• Test with haproxy -c -f haproxy.cfg

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 use backend

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world HAPROXY 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