Kong Request Transformer Not Applied

You encounter a kong configuration issue that prevents your workflow from completing. This guide walks through the fix step by step.

Wrong ❌

curl -X POST http://localhost:8001/routes/my-route/plugins --data "name=request-transformer" --data "config.add.headers[]=x-custom:$(consumer.username)" # Wrong template syntax

Wrong Output

Transformer runs but header value is literal "$(consumer.username)" instead of the consumer's username.

Right ✅

curl -X POST http://localhost:8001/routes/my-route/plugins --data "name=request-transformer" --data "config.add.headers[]=x-consumer-id:{$consumer.id}" --data "config.add.headers[]=x-consumer-username:{$consumer.username}" --data "config.remove.headers[]=authorization"

Right Output

Headers transformed: x-consumer-id and x-consumer-username added. Authorization header removed.

Prevention

• Use {$var} template syntax, not $(var).
• Valid templates: {$consumer.username}, {$route.name}, {$service.name}.
• Test transformations with Kong's debug header: Kong-Debug=1.
• Use remove.headers for security-sensitive upstream headers.
• Log transformed requests during debugging.

DodaTech applies similar defensive patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro infrastructure for production reliability.

Common Mistakes with request transformer

1. Using return to exit a function early instead of wrapping a pure value in the monad
2. Mixing let bindings with <- bindings in do notation, producing type errors
3. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors

These mistakes appear frequently in real-world KONG 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

This quick fix is part of the DodaTech infrastructure engineering series. Learn more at DodaTech tutorials.