API and Backend services part for Correlation Token example https://medium.com/@butaji/practical-microservices-correlation-tokens-75888baa5182

api.js

const getUserId = () => Math.floor((Math.random() * 1000000) + 1); //yeah, it should be real user id

const myLogger = (req, res, next) => {
  const token = req.method + "_" + req.path + "_" + getUserId() + "_" + new Date().getTime();
  console.log(token);
  req["app_token"] = token;
  next();
};

app.get('/products', (req, res) => {
  getProducts(req["app_token"], (d) => res.send(d));
});

const getProducts = (t, cb) => {
  request('http://localhost:3001/products?token=' + t, function (error, response, body) {
      if (!error && response.statusCode == 200) {
        cb(body);
      }
  });
}
//GET_/products_114582_1454235210155
//GET_/products_956499_1454235211210

backend.js

const myLogger = (req, res, next) => {
  console.log(req.query.token);
  next();
};

app.use(myLogger);
//GET_/products_114582_1454235210155
//GET_/products_956499_1454235211210