Question

I am developing an app where users can post. I want to alert users when a new post is submitted that they have not seen. The whole process is basically the way facebook is set up. However, I was wondering how I keep track of posts users have seen already.

How does facebook accomplish that?

Answer 1

Whenever a user views a post (makes a request that returns said post), add an entry to the views table. Here’s a simple diagram

That way you’d know which user viewed what. Additionally you can add a timestamp field to keep track of when the user viewed the post.

So if you you wanna know who viewed the post 666 you can simply

SELECT DISTINCT u.user_id
FROM users u
INNER JOIN views v ON u.user_id = v.user_id
WHERE v.post_id = 666

You can also check if a user viewed a certain post so you can decided whether to alert him or not

SELECT DISTINCT u.user_id
FROM users u
INNER JOIN views v ON u.user_id = v.user_id
WHERE v.post_id = 666 AND v.user_id = 1000

And you can work a simple logic

if (lastQueryReturnedAResult){
   //User had seen the post
}else{
   alertUser();
   viewPostInDifferentColor();
}

Note: This isn’t really how Facebook does it, as this method doesn’t scale pretty well. But this is the basic idea.