= Use-case 0003 — Deposit virtual funds =

'''Initiating actor:''' Trader

'''Other actors:''' —

A logged-in Trader tops up their virtual cash balance. This is a simulation-only operation; no real money changes hands. The operation writes to two tables — the user row and the ledger — inside a single transaction.

== Scenario ==

 1. Trader chooses "Deposit virtual funds" from the authenticated menu.
 2. System prompts for an amount in USD.
 3. Trader enters an amount.
 4. System validates: amount must parse as a positive number.
 5. System opens a transaction and increments the balance:

{{{
BEGIN;

UPDATE project.users
   SET available_balance = available_balance + $1,
       updated_at        = now()
 WHERE id = $2;

INSERT INTO project.transactions (user_id, type, amount, currency, description)
VALUES ($2, 'deposit', $1, 'USD', 'Virtual deposit');

COMMIT;
}}}

 6. System confirms "Deposited X USD." and returns to the authenticated menu.

=== Alternate flow 4a — invalid input ===

If the amount is non-positive or non-numeric, system responds "Invalid amount." and scenario returns to step 2.

=== Verification query ===

To see the balance after the deposit, the Trader can trigger UC0006, or directly:

{{{
SELECT available_balance, invested_balance
  FROM project.users
 WHERE id = $1;
}}}