title: Hitting a endpoint few times simultaneously could cause problems?
body: <p>Lets imagine you have a endpoint like this in your app(pseudo):  </p>
  
  <pre><code>app.get('/user/withdrawmoney',(req,res)=&gt;{
   DB.find(req.session.id,(user)=&gt;{
    if(user.balance &lt; 0){
       DB.update()...//Update the balance
      ...//Send user money
     }
   })
  
  })
  </code></pre>
  
  <p>would it not be possbible for the user to withdraw more than one time if the "timing" is right even if he has no balance?</p>