What is the purpose of the ConcurrencyStamp column in the AspNetUsers table in the new ASP.NET MVC 6 identity?

Question

What is the purpose of the ConcurrencyStamp column in the AspNetUsers table in the new ASP.NET MVC 6 identity?

This is the database schema of the AspNetUsers table:

It is also there in the AspNetRoles table:

As I remember it wasn't there in the ASP.NET MVC 5 identity.

What I've noticed so far is that it seems to have GUID values as it is defined with the following code:

/// <summary>
/// A random value that must change whenever a user is persisted to the store
/// </summary>
public virtual string ConcurrencyStamp { get; set; } = Guid.NewGuid().ToString();

But this documentation is not sufficient for me to understand in which situations it is used.

Answer 1

As the name state, it's used to prevent concurrency update conflict.

For example, there's a UserA named Peter in the database 2 admins open the editor page of UserA, want to update this user.

1. Admin_1 opened the page, and saw user called Peter.
2. Admin_2 opened the page, and saw user called Peter (obviously).
3. Admin_1 updated user name to Tom, and save data. Now UserA in the db named Tom.
4. Admin_2 updated user name to Thomas, and try to save it.

What would happen if there's no ConcurrencyStamp is Admin_1's update will be overwritten by Admin_2's update. But since we have ConcurrencyStamp, when Admin_1/Admin_2 loads the page, the stamp is loaded. When updating data this stamp will be changed too. So now step 5 would be system throw exception telling Admin_2 that this user has already been updated, since he ConcurrencyStamp is different from the one he loaded.