gistfile1.js

import finished from 'on-finished';


let counts;

function reset() {
    counts = {
        /*
        Synthesized properties...
        http_200: 0,
        http_300: 0,
        http_400: 0,
        http_500: 0,
        ...
        */
        'total_requests': 0,
        'current_requests': 0,
        'errors': 0
    };
}

function range(n) {
     return ((n / 100) | 0) * 100;
}

function inc(counter) {
     let count = counts[counter];
     if (!count || count === Number.MAX_SAFE_INTEGER) {
          count = 1;
     } else {
          count += 1;
     }
     counts[counter] = count;
}

function dec(counter) {
     if (counter in counts) {
          counts[counter] -= 1;
     }
}

export default function ({ interval = 5 * 1000 }) {
    let app, server;
    
    setInterval(() => {
        if (app) {
            app.emit('stats', counts);
            reset();
        }     
    }, interval).unref();

    reset();
    
    return function (req, res, next) {
          { app } = req;
          
          if (!server) {
              // Add instrumentation to get response times. There
              // is a caveat that we won't record the very first request,
              // but I'm ok with that tradeoff since we guarantee accurate
              // request times due to not relying on middleware ordering.
              { socket: { server } } = req;
              server.on('request', function (req, res) {
                  let start = Date.now();
                  finished(res, () => {
                      let duration = Date.now() - start;
                      // do something with duration
                  });
              });
          }
          
          inc('total_requests');
          inc('current_requests');
          
          finished(res, (err, res) => {
               if (err) {
                    inc('errors');
               }
               inc('http_' + range(res.statusCode | 0));
               dec('current_requests');
          });
          
          next();
     }
     
}