Bank plugin example: records, commands and events

#[record(kind = 1, record_len = 64,
         uk(id = 0, fields = [account_id]))]
pub struct Account {
    #[field(index = 1, immutable = true)]
    pub account_id: u64,
    #[field(index = 2)]
    pub total_credit: Decimal<6>,
    #[field(index = 3)]
    pub total_debit: Decimal<6>,
}

#[command(kind = 1)]
pub struct Deposit {
    #[field(index = 1)]
    pub account_id: u64,
    #[field(index = 2)]
    pub amount: Decimal<6>,
}

The model uses fixed-point decimals with six fractional digits and a unique key on account ID. The excerpt is partial and does not compile on its own. Definitions are from the current Bank v1.1 schema.

Deposit as a single business transaction

For a deposit of 100.000000, the handler changes both the customer and platform accounts within the same command transaction:

Each side emits a BalanceChanged event reporting the account’s updated totals; events can be returned with the response, subject to configured size limits. Non-positive amounts, deposits into the platform account and arithmetic overflow are rejected, leaving both accounts unchanged.

let account_id = command.account_id();
let amount = command.amount();

if account_id == PLATFORM_ACCOUNT_ID {
    return Err(BankV2Error::PlatformDeposit);
}
if amount <= ZERO_AMOUNT {
    return Err(BankV2Error::NonPositiveAmount);
}

// Both changes happen in the same transaction.
// adjust_account uses checked_add internally.
let (account_credit, account_debit) =
    Self::adjust_account(account_id, tx, amount, ZERO_AMOUNT, true)?;
let (platform_credit, platform_debit) =
    Self::adjust_account(
        PLATFORM_ACCOUNT_ID, tx, ZERO_AMOUNT, amount, false,
    )?;

Self::emit_balance_changed(tx, account_id, account_credit, account_debit);
Self::emit_balance_changed(
    tx, PLATFORM_ACCOUNT_ID, platform_credit, platform_debit,
);
Ok(())