engine2.js

// example middleware
engine.use(function *(req, res, next) {
  // make changes to request
  // eg: check cache hit
  yield next;
  // read the result
  // eg: populate cache
});

// cache middleware
engine.use(function *(req, res, next) {
  // check cache hit
  let idString = generateIdForReq(req)
  let cacheResult = yield cacheGet(idString)
  if (cacheResult) res.result = cacheResult
  yield next;
  // populate cache
  if (!cacheResult) {
    yield cachePut(idString, res.result)
  }
});

// perf measuring middleware
engine.use(function *(req, res, next) {
  var start = new Date();
  // wait for request result to bubble back up
  yield next;
  // log elapsed time
  var ms = new Date() - start;
  console.log('%s %s %s - %sms', req.method, req.params, req.result, ms);
});