gistfile1.js

var express = require('express');
var http = require('http');
var app = express();

app.configure(function(){
  app.set('port', process.env.PORT || 3000);
  app.use(express.logger('dev'));
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use(app.router);
  app.use(express.static(path.join(__dirname, 'public')));
});

var databaseId = function(req, res, next) {
  doSomeStuff(function(err, result) {
    if(err) return next(err);
    req.dbStuff = result;
    next();
  });
}

app.post('/some/url', databaseId, function(req, res, next) {
  // has access to req.dbStuff
});

app.get('/another/url', function(req, res) {
  // doesn't have access to req.dbStuff
});

http.createServer(app).listen(app.get('port'), function(){
  console.log("Express server listening on port " + app.get('port'));
});