Redirect request url of backend doesn't work

[wolframite]

Environment info:

• KrakenD version: Version: 2.7.2
• System info: docker
• Hardware specs: 8 CPUs / 12GB RAM
• Backend technology: Java
• Additional environment information:
  I'm running krakend in docker-compose based on the community playground:

services:
  krakend:
    image: devopsfaith/krakend:2.7
    restart: unless-stopped
    volumes:
      - ./config/krakend:/etc/krakend
    ports:
      - "8080:8080"
    command: ["run", "-d", "-c", "/etc/krakend/krakend.json"]

Describe what are you trying to do:
I'm trying to implement the following behind the /seq endpoint

Call /info if X-Auth exists, fetch customerId and insert into /cart/{resp0_customerId}.json

1. Happy path: If an X-Auth header is present, call the /info backend, extract the customerId and insert it into the /cart/{resp0_customerId}.json backend using the sequential proxy. This part works
2. Invalid header: In case the /info backend returns a 401 because the X-Auth header is invalid, I pass-through the status code and interrupt the sequence. This part also works
3. In case the X-Auth header is not set at all, I want to skip both the /info and the /cart/{resp0_customerId}.json backend and forward the request to /cart instead. I'm simulating the /cart backend without the parameter with a benchmark which also returns json. This is the part where I need help. I tried to use CEL to suppress the execution of the /info backend, I tried Lua to redirect, but I can't figure out how to make it work.

Is this mix of conditions and sequence possible in KrakenD?

Your configuration file:

{
  "$schema": "https://www.krakend.io/schema/v3.json",
  "version": 3,
  "name": "KrakenD Enterprise API Gateway",
  "port": 8080,
  "host": ["http://fake_api"],
  "timeout": "3000ms",
  "debug_endpoint": true,
  "endpoints": [
    {
      "@comment": "Call /info if X-Auth exists, fetch customerId and insert into /cart/{resp0_customerId}.json :: Redirect to /cart if X-Auth is missing. Pass through 401 from /info if X-Auth is invalid",
      "endpoint": "/seq",
      "input_headers": ["X-Auth"],
      "backend": [
        {
          "host": ["http://benchmark:8080"],
          "url_pattern": "/info",
          "allow": ["customerId"],
          "extra_config": {
            "backend/http": {
              "return_error_code": true
            },
            "modifier/lua-backend": {
              "allow_open_libs": true,
              "pre": "local req = request.load(); if string.len(req:headers('X-Auth')) == 0 then req:url('http://benchmark:8080/cart') end;",
              "post": "print('post'); local r = response.load(); print(r:body());"
            }
          }
        },
        {
          "url_pattern": "/cart/{resp0_customerId}.json",
          "extra_config": {
            "validation/cel": [
              {
                "check_expr": "req_headers['X-Auth'] != []"
              }
            ]
          }
        }
      ],
      "extra_config": {
        "proxy": {
          "sequential": true
        }
      }
    }
  ],
  "extra_config": {
    "telemetry/logging": {
      "level": "DEBUG",
      "syslog": false,
      "stdout": true
    },
    "security/cors": {
      "allow_origins": ["*"],
      "allow_methods": ["POST", "GET"],
      "allow_headers": ["Origin", "Authorization", "Content-Type"],
      "expose_headers": ["Content-Length"],
      "max_age": "12h"
    }
  }
}

Configuration check output:
Result of krakend check -dtc krakend.json --lint command

/etc/krakend # krakend check -dtc krakend.json --lint
Parsing configuration file: krakend.json
Global settings
	Name: KrakenD Enterprise API Gateway
	Version: 3
	Address:
	Port: 8080
	Default backend hosts: [http://fake_api]
3 global component configuration(s):
- security/cors
- telemetry/logging
- telemetry/metrics
1 API endpoint(s):
	- GET /seq
	Timeout: 3s
	1 endpoint component configuration(s):
	- proxy
	Connecting to 2 backend(s):
		[+] GET /info
		Timeout: 3s
		Hosts: [http://benchmark:8080]
		2 backend component configuration(s):
		- backend/http
		- modifier/lua-backend

		[+] GET /cart/{{.Resp0_customerId}}.json
		Timeout: 3s
		Hosts: [http://fake_api]
		1 backend component configuration(s):
		- validation/cel

0 async agent(s):
Syntax OK!

Commands used:
How did you start the software?

via docker-compose

Logs:
That's the logs for the happy path under 1.

krakend-1  | post
krakend-1  |
krakend-1  |  2024/11/04 - 19:37:41.096 ▶ DEBUG [BACKEND: /cart/{{.Resp0_customerId}}.json][CEL][pre] Evaluator #0 result: true
krakend-1  | [GIN] 2024/11/04 - 19:37:41 | 200 |   24.532445ms |      172.18.0.1 | GET      "/seq"

That's the logs for 2. with the invalid header:

krakend-1  |  2024/11/04 - 19:39:06.659 ▶ ERROR [ENDPOINT: /seq]
krakend-1  | [GIN] 2024/11/04 - 19:39:06 | 401 |    1.890536ms |      172.18.0.1 | GET      "/seq"
krakend-1  | Error #01:

That's the logs for 3. when the X-Auth header is missing:

krakend-1  | post
krakend-1  |
krakend-1  |  2024/11/04 - 19:39:57.966 ▶ INFO [BACKEND: /cart/{{.Resp0_customerId}}.json][CEL][pre] Evaluator #0 failed: no such key: X-Auth
krakend-1  |  2024/11/04 - 19:39:57.966 ▶ ERROR [ENDPOINT: /seq] Error #0: request aborted by evaluator #0
krakend-1  | [GIN] 2024/11/04 - 19:39:57 | 200 |   21.706592ms |      172.18.0.1 | GET      "/seq"
krakend-1  | Error #01: request aborted by evaluator #0

Additional comments:
Even when I set the encoding of the 1st backend to no-op, I don't get any response, only an empty object:

HTTP/1.1 200 OK
Content-Length: 2
Content-Type: application/json; charset=utf-8
Date: Mon, 04 Nov 2024 19:39:57 GMT
Vary: Origin
X-Krakend: Version 2.7.2
X-Krakend-Completed: false

{}