Use-case 0002 — Log in

Initiating actor: Visitor

Other actors: —

A registered user authenticates so the system can treat subsequent actions as a Trader.

Scenario

1. Visitor chooses "Login" from the anonymous menu.
2. System prompts for username and password.
3. Visitor enters values.
4. System looks up the user:

SELECT id, password_hash
  FROM project.users
 WHERE username = $1;

5. If no row is returned, system responds "Invalid credentials." and scenario ends.
6. If a row is returned, system compares the stored hash against sha256 of the entered password. On mismatch it responds "Invalid credentials." and ends.
7. On match, system records the returned id and username in the session and displays the authenticated menu.

Alternate flow 4a — authenticated lookup with live balance

The system may combine identity lookup with live balance in a single query, for use cases that need both:

SELECT id, available_balance, invested_balance
  FROM project.users
 WHERE username = $1
   AND password_hash = encode(digest($2, 'sha256'), 'hex');