$($PART I$)$

Pepito & Co$.$ is a new start$-$up e$-$business. Its business is to provide a B$2$C web platform

for trading. Its business is different from its major competitors, specifically on the

payment model which is described in the next paragraph.

In order to buy or sell goods on Pepito's site, all users must register with a positive

identity $($e$.$g$.,$ HKID or passport$)$ showing the name, gender and date of birth. Each user is

assigned a unique User ID and an initial password for web login$.$ Unlike other

competitors who normally require online credit card or PayPal payment, Pepito & Co$.$

partners with convenience stores to accept cash payments. A user stores "value" in his

own account simply by presenting the User ID and paying at the convenience stores by

cash or EPS before he starts purchasing. On the other hand, a user can withdraw the

balance from this own account for purchases with a fixed handling fee of $1\%$ on the

amount withdrawn. The minimum amount of withdrawal is HK$$100($i$.$e$.,$ minimum

HK $$101$ should be deducted from the account per withdrawal including the handling fee.

Here is a typical scenario of the business process:

a$)$ CoCo and Monique each opens an account with Pepito & Co$.$

b$)$ Monique stores HK $$500$ in her own account.

c$)$ Monique buys a box of macadamia nuts from CoCo, costing her HK $$150.$

d$)$ $$150$ is debited from Monique's account, and instantly credited into CoCo's account.

e$)$ CoCo withdraws $$150$ from her account, $$151\mathrm{.}5$ is debited from her account.

Question $7.$ Suggest one integrity constraint for each of the transaction types :

$(1)$ account opening

$(2)$ money withdrawal

$(3)$ trading or purchasing

$[6$ marks$]$

Question $8.$ With respect to the trading example, i$.$e$.$ transactions d$)$ and e$),$ describe

how ACID properties should be enforced.

$[12$ marks$]$