Blocking node from reading simultaneously from db

Question

Blocking node from reading simultaneously from db

I have a set of values in a mongoDB I need to only be read ones. So when I have read them I delete that line from the DB. But since node is async if I do getValue() twice in quick succession I get the same value, since the DB has not had time to delete the old one. Does anyone know a good way to fix this problem. I can think of a couple but nothing good.

I don’t have my code here so just wrote a quick example to show my problem.

Var getValue = function() {
  ReadfromDB(function(data){
       deleteRecord(); // show that what we read has been updated
   });
}

node.js
asynchronous

Answer 1

You could try something like this. The reading is placed into a promise that's done when the deleting is done as well. This can stack so if you do getValue() 20 times it still should execute in the correct order.

var getValue = function(){
    var currentPromise;
    var read = function(){
        return ReadfromDB().then(function(data){
           deleteRecord(); // show that what we read has been updated
        });
    }
    return function() {
        if(currentPromise){
            currentPromise = currentPromise.then(read)
        }else{
            currentPromise = read();
        }
    }
}

What you need to do outside this code is make sure ReadfromDB returns a promise object.

Answer 2

Sounds like a good use case for a closure!

var getValue = (function() {
  var called = false;
  return function() {
    if (!called) {
      called = true;
      ReadFromDB(function(data) {
        deleteRecord();
      });
    }
  };
}());

You could also use once to do exactly the same thing.

var once = require('once');

var getValue = once(function() {
  ReadFromDB(function(data) {
    deleteRecord();
  });
});