server.js

var express = require('express'),
  ejs = require('ejs'),
  fs  = require('fs');


var port = Number(process.env.PORT || 5000),
  app = express();

app.use('/assets', express.static(__dirname + '/assets'));
app.engine('html', ejs.renderFile);

app.get('/', function (req, res) {
  res.render('chart.html');
});

app.get('/chart', function (req, res) {
  res.render('chart.html');
});

app.get('/form', function(req, res) {
  res.render('form.html');
});

app.get('/datasheet', function (req, res) {
  var cacheFile = './cache.json';
  if(!fs.existsSync(cacheFile)){
    res.write(JSON.stringify({}));
    res.end();
  } else {
    cachedData = JSON.parse(fs.readFileSync(cacheFile, 'utf-8'));
    cacheModified = cachedData.lastModified;
    res.write(JSON.stringify(cachedData.data));
    res.end();
  }
});

app.get('/update_datasheet', function (req, res) {
  var spreadsheetKey = '0Ai29bxvOdvgjdDZuOEk0eWRGRUQ4ZmEwNnFEb3owVEE';
  var spreadsheetURL = 'http://spreadsheets.google.com/feeds/list/'+spreadsheetKey+'/od6/public/values?alt=json';
  var cacheFile = './cache.json';
  var http = require('http');
  // Get info about cache
  var cachedData = false;
  var cacheModified = 0;
  var lastModified  = Infinity; //getLastModified(spreadsheetKey);
  if(!fs.existsSync(cacheFile)){
    fs.writeFileSync(cacheFile, JSON.stringify({'lastModified' : 0, 'data':[]}));
  } else {
    cachedData = require(cacheFile);
    cacheModified = cachedData.lastModified;
  }

  var options = {
    method: 'HEAD',
    host: 'spreadsheets.google.com',
    port: 80,
    path: '/feeds/list/'+spreadsheetKey+'/od6/public/values?alt=json'
  };

  var modReq = http.request(options, function(modRes) {
    var mod = new Date(modRes.headers['last-modified']);
    lastModified = mod.getTime() / 1000;
    console.log('lastModified: ' + lastModified + ' cacheModified: ' + cacheModified);
    if(cacheModified >= lastModified) {
      console.log('Cache hit');
      res.write(JSON.stringify({data:cachedData.data,new:false}));
      res.end();
    } else {
      console.log('Cache miss');
      var sheetReq = http.get(spreadsheetURL,function(sheetResult) {
        var rawResp = '';
        var writeDat = {'lastModified':lastModified,'data':''};

        sheetResult.on('data',function(d){
          rawResp += d;
        });
        sheetResult.on('end', function () {
          // Format result
          var links = [];
          rawResp = JSON.parse(rawResp);
          var allRows = rawResp.feed.entry;
          for (var i = allRows.length - 1; i >= 0; i--) {
            links[i] = {};
            links[i].member = allRows[i].gsx$member.$t;
            links[i].memberband = allRows[i].gsx$memberband.$t;
            links[i].relationship = allRows[i].gsx$relationship.$t;
          };
          writeDat.data = links;

          // Write result to browser
          res.write(JSON.stringify({data:links,new:true}));

          // Write result to file
          fs.writeFile(cacheFile, JSON.stringify(writeDat), function (err) {
            if (err) throw err;
            console.log(new Date().toLocaleString() + ': Wrote ' + links.length +' rows');
          });
          res.end();
        });
      });
    }
  });
  modReq.end();
});

app.listen(port, function () {
  console.log('listening to on port %s', port);
});