POST request to REST endpoint in on-premise system fails with "502: Bad Gateway"

[stephan_gutknecht]

I have a REST endpoint in an on-premise system which accepts POST request to create a project. The request looks as follows.

https://bdt-solutions-sandbox-ugkcpmuc-sandbox-dtv-service-cf-srv.cfapps.sap.hana.ondemand.com/dtv/c...

POST https://bdt-solutions-sandbox-ugkcpmuc-sandbox-dtv-service-cf-srv.cfapps.sap.hana.ondemand.com/dtv/c...
Content-Type: application/json
{
    "EXT_ID": "BDT_PROJECT2",
    "NAME": "BDT Project 2"
}

This is the result if I run the request (with CSRF token fetched beforehand via GET) in Postman.

[
    {
        "CLIENT": "",
        "ID": 559,
        "UUID": "NokfYuk+xB/hAAAACu8+5g==",
        "EXT_ID": "BDT_PROJECT2",
        "NAME": "BDT Project2",
        "NAMESPACE": "",
        "PROJ_MODE": "",
        "DEFAULT_PROJ": null,
        "VERSION_NUMBER": "",
        "EXECUTION_STATUS": "",
        "LIFECYCLE_STATUS": "01",
        "DATE_FORMAT": "",
        "TIME_FORMAT": "",
        "DECIMAL_NOTATION": "",
        "CREATED_BY": "GUTKNECHTS",
        "CREATED_ON": 20220302145554,
        "CHANGED_BY": "GUTKNECHTS",
        "CHANGED_ON": 20220302145554
    }
]

In CAP, I have setup a service model with an unbound action to run the POST request.

service DTVService {
...
  action createProject(EXT_ID : String, NAME : String ) returns {
      CLIENT: String;
      ID: Integer;
      UUID: String;
      EXT_ID: String;
      NAME: String;
      NAMESPACE: String;
      PROJ_MODE: String;
      DEFAULT_PROJ: Boolean;
      VERSION_NUMBER: String;
      EXECUTION_STATUS: String;
      LIFECYCLE_STATUS: String;
      DATE_FORMAT: String;
      TIME_FORMAT: String;
      DECIMAL_NOTATION: String;
      CREATED_BY: String;
      CREATED_ON: Timestamp;
      CHANGED_BY: String;
      CHANGED_ON: Timestamp;
  }
}

In the service implementation, I am executing the request with this coding. I am currently just using fixed parameters for the request to cancel out any issue with the parameter handover.

const cds = require("@sap/cds")
class DTVService extends cds.ApplicationService {
    init() {
...
        this.on("createProject", async (req) => {
            const dtvapi = await cds.connect.to("dtvapi")
            return dtvapi.post("/createproject", {EXT_ID:"BDT_PROJECT3", NAME:"BDT Project 3"})
        })
    }
}
module.exports = { DTVService }

However, the result after deploying the application (locally I do not get it to run) is a HTTP error 502 with "Bad Gateway". Any idea why this happens?

[gregorw]

You can use sap-cf-proxy or develop in BAS if you want to test it without deployment.