Software Engineering for Internet Applications - Student Community

give their site a unique look and feel. Eve expected that laws andaccounting procedures regarding sales tax would change. So sheencapsulated the looking up of sales tax by state, the figuring out ifthat state charges tax on shipping, and the multiplication of tax rateby price into an Oracle PL/SQL function:create or replace function ec_tax(v_price IN number, v_shipping IN number,v_order_id IN integer)return numberIStaxes ec_sales_tax_by_state%ROWTYPE;tax_exempt_p ec_orders.tax_exempt_p%TYPE;BEGINSELECT tax_exempt_p INTO tax_exempt_pFROM ec_ordersWHERE order_id = v_order_id;IF tax_exempt_p = 't' THENreturn 0;END IF;SELECT t.* into taxesFROM ec_orders o, ec_addresses a,ec_sales_tax_by_state tWHERE o.shipping_address=a.address_idAND a.usps_abbrev=t.usps_abbrev(+)AND o.order_id=v_order_id;IF nvl(taxes.shipping_p,'f') = 'f' THENreturn nvl(taxes.tax_rate,0) * v_price;ELSEreturn nvl(taxes.tax_rate,0) * (v_price + v_shipping);END IF;END;The Web script or other PL/SQL procedure that calls this functionneed only know the proposed cost of an item, the proposed shippingcost, and the order ID to which this item might be added (these arethe three arguments to ec_tax). That sales taxes for each state arestored in the ec_sales_tax_by_state table, for example, ishidden from the rest of the application. If an organization thatadopted this software decided to switch to using third-party softwarefor calculating tax, that organization would need only to change thisone function rather than wading through hundreds of Web scriptslooking for tax-related code.know how many programs you have to examine and modify after apersonnel change. With magic numbers in the code it is tough toknow if rules are being enforced consistently site-wide.Where should you store parameters such as these? Except for thedatabase username and password, an obvious answer would seemto be "in the database." There are a bunch of keys (the parameternames) and a bunch of values (the parameters). This is the veryproblem for which a database management system is ideal.-- use Oracle's unique key generatorcreate sequence config_param_seq start with 1;create table config_param_keys (config_param_key_id integer primary key,key_name varchar(4000) not null,param_comment varchar(4000));create table config_param_values (config_param_key_idnot nullreferences config_param_keys,value_index integer default 1 not null,param_value varchar(4000) not null);-- we use the Oracle operator "nextval" to get the next-- value from the sequence generatorinsert into config_param_keysvalues(config_param_seq.nextval, 'view_source_link_p', 'damn6.171 instructor is making me do this');-- we use the Oracle operator "currval" to get the last-- value from the sequence generator (so that rows-- inserted in this transaction will all have-- the same ID)insert into config_param_valuesvalues(config_param_seq.currval, 1, 't');commit;insert into config_param_keysvalues(config_param_seq.nextval, 'redirect','dropping the /wtr/ directory');insert into config_param_valuesvalues204145