whats the correct way of depicting a login on a use case diagram

Question

In a use case diagram if someone needs to log in before completing an action should it be

 O
--- --> (login) ---<<extends>>-- (do action) 
 |          \    
/ \          \
            <<extends>>
               \
             (another action)

or

 O  
---  --> (do action)  
 |   \            
/ \   \       
      (login) 

or

 O  
--- --> (do action)  
 |          \    
/ \          \
            <<includes>>
               \
              (login)

or something different altogether?

Answer 1

Use case is for showing WHAT will be done by WHOM and you can organize these what's and whom's in groups (organizations and subsystems). Do not try to express inner structure or sequence of actions, there are other diagrams for that.

Use include only if one action of user can include another action, again of a user. When that another action is called by the system itself, don't show it!

The same about extends.

As for uses, it is not a standard stereotype for Use Case diagram.

Notice, you shouldn't show that after the registration the user is automatically logged in - it is not information for Use Case Diagram level. Leave it in some text document an put it later into the State Machine diagram.