config-new-example.bal

import ballerina.net.http;
import ballerina.config;

endpoint<http:Service> passthroughEP {
    port:9090
}

endpoint<http:Service> backendEP {
    port: getBackendPort()
}

endpoint<http:Client> backendClientEP {
    serviceUri: getBackendServiceURI()
}

@http:serviceConfig {
    basePath: config:getAsString("passthrough.basePath"),
    endpoints:[passthroughEP]
}
service<http:Service> passthrough {

    @http:resourceConfig {
        methods:["GET"],
        path:"/"
    }
    resource passthru (http:ServerConnector conn, http:Request request) {
        var response, _ = backendClientEP -> forward("/hello", request);
        _ = conn -> forward(response);
    }
}

@http:serviceConfig {
    basePath:"/hello",
    endpoints:[backendEP]
}
service<http:Service> hello {

    @http:resourceConfig {
        methods:["GET"],
        path:"/"
    }
    resource sayHello (http:ServerConnector conn, http:Request request) {
        http:Response response = {};
        response.setStringPayload("Hello World!!!");
        _ = conn -> respond(response);
    }
}

function getBackendPort() (int) {
      var port, err = <int>config:getAsString("hello.port");
      return err == null ? port : 8000;
}

function getBackendServiceURI() (string) {
      string url = config:getAsString("hello.host");
      int port = getBackendPort();
      return url + ":" +port;
}

Given below is a sample config file used to test this.

[passthrough]
host="http://localhost"
port=9090
basePath="/passthru"

[hello]
host="http://localhost"
port=8000
basePath="/hello"