Example of choosing an upstream based on request body and proxying with subrequest

nginx.conf

#user  nobody;
worker_processes  1;

#error_log  logs/error.log;
#error_log  logs/error.log  notice;
#error_log  logs/error.log  info;

#pid        logs/nginx.pid;

load_module modules/ngx_http_js_module.so;
error_log /tmp/dev.log debug;


events {
    worker_connections  1024;
}


http {
    js_import script from script.js;
    include       mime.types;
    default_type  application/json;

    js_var $machine;
    js_var $nodename;
    js_var $redirectKey;
    log_format my_custom_log_format ‘$machine - $nodename’; 

    server {
      listen       4000;
      access_log /tmp/nginx-access.log my_custom_log_format;

      location /foo {
        js_content script.handleRequest;
      }

      location /upstream_one {
        js_content script.upstreamOne;
      }

      location /upstream_two {
        return 200 "this is upstream two";
      }
    }
}

script.js

async function handleRequest (r) {
  // Since this handler is invoked with `js_content`, r.requestText
  // is guaranteed to be present
  const body = r.requestText;

  // some logic  to process the body
  // the below variable is a stand-in for the result of that
  // processing
  const parsedRequestBody = {
    machine: "the machine",
    nodename: "the nodename",
    redirectKey: "upstream_one" // change this to see different upstreams
  };

  for (const key in parsedRequestBody) {
    r.variables[key] = parsedRequestBody[key];
  }

  // Here's the redirect. r.subrequest mirrors the headers
  // and body of the parent response to the target of the subrequest
  let upstreamResponse;
  if (parsedRequestBody.redirectKey === "upstream_one") {
    upstreamResponse = await r.subrequest("/upstream_one");
  } else {
    upstreamResponse = await r.subrequest("/upstream_two");
  }

  // The following code passes the response from the upstream back including
  // status, body, and headers

  // Doing this makes sure headers added in the upstream get passed back
  r.headersOut = Object.assign(r.headersOut, upstreamResponse.headersOut);
  
  // Mirror the upstream response status and body
  r.return(upstreamResponse.status, upstreamResponse.responseText);
}

// Just adding this as a njs handler so we can easily see what the upstream
// will receive in terms of body and headers
function upstreamOne(r) {
  r.error(`headers received in upstream_one: ${JSON.stringify(r.rawHeadersIn)}`);
  r.error(`body received in upstream_one: ${r.requestText}`);

  r.headersOut['upstream-one-added'] = "Upstream one added this header";

  r.return(200, "This is the body of the response from upstream_one");
}

export default { handleRequest, upstreamOne };