user_login_sq.puml

@startuml
title Login Sequence Diagram

actor User
participant "Web Browser" as Browser
participant "Web Server" as Server
database "Database" as DB

User -> Browser : Enters username and password
Browser -> Server : POST /login (username, password)
activate Server

Server -> DB : SELECT user WHERE username = ? AND password = ?
activate DB
DB --> Server : User record (or null)
deactivate DB

alt Credentials are valid
    Server -> Browser : HTTP 200 OK (Login Success)
    Browser -> User : Displays success message / redirects
else Credentials are invalid
    Server -> Browser : HTTP 401 Unauthorized (Login Failed)
    Browser -> User : Displays error message
end

deactivate Server

@enduml