gistfile1.js

// populate simulated local database with one value
var cache = {
  'bob': 'DB BOB'
};

// simulated local DB
var db = {
  load: function(key, callback) {
    console.log('db.load', key);
    process.nextTick(function() {
      // simulate loading from local DB
      callback(undefined, cache[key]);
    });
  },
  save: function(key, value, callback) {
    console.log('db.save', key, value);
    // simulate saving to local DB
    cache[key] = value;
    process.nextTick(function() {
      callback();
    });
  }
};

// simulated remote resource
var remote = {
  load: function(key, callback) {
    console.log('remote.load', key);
    process.nextTick(function() {
      // simulate loading from remote data source
      callback(undefined, 'REMOTE ' + key.toUpperCase());
    });
  }
};

// create wrapper that has the logic to temporarily queue requests
function createQueueRequestHandler(fn) {
  var queues = {};
  return function(key, callback) {
    console.log('handler', key);
    if (key in queues) {
      return queues[key].push(callback);
    }
    queues[key] = [callback];
    fn(key, function(err, result) {
      var callbacks = queues[key];
      delete queues[key];
      callbacks.forEach(function(callback) {
        callback(err, result);
      });
    });
  };
}

// function that specifies logic for loading data
var localThanRemote = createQueueRequestHandler(function(key, callback) {
  db.load(key, function(err, result) {
    if (err || result) {
      return callback(err, result);
    }
    remote.load(key, function(err, result) {
      if (err) {
        return callback(err);
      }
      db.save(key, result, function(err) {
        return callback(err, result);
      });
    });
  });
});

// simulate calls
localThanRemote('alice', console.log);
localThanRemote('bob', console.log);
localThanRemote('alice', console.log);

// OUTPUT:
// handler alice
// db.load alice
// handler bob
// db.load bob
// handler alice
// remote.load alice
// undefined 'DB BOB'
// db.save alice REMOTE ALICE
// undefined 'REMOTE ALICE'
// undefined 'REMOTE ALICE'