Step 3: Define the Business Operations

Defining an application's business operations involves:

Identifying the operations that reflect the business processes the application is being designed to automate.
Defining and publishing the syntax for each business operation request.

Identify Operations that Reflect Business Processes

The DNA PurchaseOrder application defines business operations that reflect the process of selling books. These operations can be organized into four groups:

Account operations
Inventory operations
Line item operations
Purchase order operations

Tables 6-9 describe each of the operations defined by the DNA PurchaseOrder application.

Table 6. Account Operations Defined by the DNA PurchaseOrder Application

Operation	Description
Create	Creates a new account in the underlying Accounts database table. This operation is only available to application "Administrators."
CreateRecordset	Creates a recordset containing detailed account information based on user-supplied selection criteria.
DecrementBalance	Decrements the balance of an account by a user-defined amount.
Destroy	Deletes an account from the underlying Accounts database table. This operation is only available to application "Administrators."
IncrementBalance	Increments the balance of an account by a user-defined amount.
Modify	Changes an existing account in the underlying Accounts database table. This operation is only available to application "Administrators."

Table 7. Inventory Operations Defined by the DNA PurchaseOrder Application

Operation	Description
Create	Creates a new inventory item in the underlying Inventory database table. This operation is only available to application "Administrators."
CreateRecordset	Creates a recordset containing detailed inventory item information based on user-supplied selection criteria.
DecrementQOH	Decrements the quantities on hand of an inventory item by a user-defined number.
Destroy	Deletes an inventory item from the underlying Inventory database table. This operation is only available to application "Administrators."
IncrementQOH	Increments the quantities on hand of an inventory item by a user-defined number.
Modify	Changes an existing inventory item in the underlying Inventory database table. This operation is only available to application "Administrators."

Table 8. LineItem Operations Defined by the DNA PurchaseOrder Application

Operation	Description
Create	Creates a new line item in the underlying LineItems database table.
CreateRecordset	Creates a recordset containing detailed line item information based on user-supplied selection criteria.
Destroy	Deletes a line item from the underlying LineItems database table.
DestroyByPurchaseOrder	Deletes all of the line items associated with a particular purchase order.
Modify	Changes an existing line item in the underlying LineItems database table.

Table 9. PurchaseOrder Operations Defined by the DNA PurchaseOrder Application

Operation	Description
Cancel	Reverses the effects of creating a purchase order.
Create	Creates a new purchase order in the underlying PurchaseOrders database table.
CreateRecordset	Creates a recordset containing detailed purchase order information based on user-supplied selection criteria.
Destroy	Deletes a purchase order from the underlying PurchaseOrders database table.
Modify	Changes an existing purchase order in the underlying PurchaseOrders database table.

Define and Publish the Syntax for Each Business Operation Request

Once the various business operations have been identified, request syntaxes for each must be defined and published, so perspective clients will know how to properly format requests for service. Developers are free to define and publish their business operation request syntaxes however they see fit (for example, COM interfaces, XML-encoded MSMQ messages, and so on). The request syntaxes for the DNA PurchaseOrder application are defined using Component Object Model (COM) interfaces, and published using a COM type library, which contains all the information perspective clients need to issue properly formatted requests.

When defining request syntaxes for business operations, developers must take special care to ensure all of the data required to perform each individual business operation can be delivered with minimal trips across the network. Typically this involves supplying the required data as individual function arguments, or supplying the required data using a single ActiveX Data Objects (ADO) disconnected recordset, variant array, or persistable object. The DNA PurchaseOrder application uses the persistable object approach, in which "special" objects are used to represent the data required by individual business operations. When a business operation request is issued, the operation's corresponding special object marshals its internal state information to a byte stream, which is somehow transmitted across the network, where it is unmarshaled and used to initialize a local instance of the special object. The DNA PurchaseOrder application defines four COM interfaces that describe the data required to perform its various business operations:

IAccount—Used to marshal the data required to perform account operations.
IInventory—Used to marshal the data required to perform inventory operations.
ILineItem—Used to marshal the data required to perform line item operations.
IPurchaseOrder—Used to marshal the data required to perform purchase order operations.

Table 10. The IAccount Interface Is Used to Marshal the Data Required to Perform Account Operations.

Property/method	Data type	Description
(P) ID	Long	Uniquely identifies each account.
(P) FirstName	String	The first name of the account owner.
(P) LastName	String	The last name of the account owner.
(P) AddressLine1	String	The address of the account owner.
(P) AddressLine2	String	The address of the account owner.
(P) City	String	The city of the account owner.
(P) State	String	The state of the account owner.
(P) Phone	String	The phone number of the account owner.
(P) Email	String	The e-mail address of the account owner.
(P) Balance	Currency	The balance of the account.
(P) Limit	Currency	The credit limit of the account.
(M) MarshalToPropertyBag	Byte()	Marshals the object into a property bag and returns the bag's contents as an array of bytes.
(P) Valid	Boolean	Reflects whether the object has any invalid properties.

Table 11. The IInventory Interface Is Used to Marshal the Data Required to Perform Inventory Operations.

Property/method	Data type	Description
(P) ID	Long	Uniquely identifies each inventory item.
(P) Description	String	A description of the item.
(P) Price	Currency	The price of the item.
(P) QOH	Long	The quantity on hand.
(M) MarshalToPropertyBag	Byte()	Marshals the object into a property bag and returns the bag's contents as an array of bytes.
(P) Valid	Boolean	Reflects whether the object has any invalid properties.

Table 12. The ILineItem Interface Is Used to Marshal the Data Required to Perform Line Item Operations.

Property/method	Data type	Description
(P) ID	Long	Uniquely identifies each line item.
(P) PurchaseOrderID	Long	The unique identifier of the purchase order to which the line item belongs.
(P) Item	IInventory	The inventory item being purchased.
(P) Price	Currency	The price for which the inventory item is being sold.
(P) Quantity	Long	The number of inventory items being purchased.
(M) MarshalToPropertyBag	Byte()	Marshals the object into a property bag and returns the bag's contents as an array of bytes.
(P) Valid	Boolean	Reflects whether the object has any invalid properties.

Table 13. The IPurchaseOrder Interface Is Used to Marshal the Data Required to Perform Purchase Order Operations.

Property/method	Data type	Description
(P) ID	Long	Uniquely identifies each purchase order.
(P) AccountID	Long	The account to which the order should be charged.
(P) OrderDate	Date	The data the purchase was made.
(P) SubTotal	Currency	The sum of (the quantity of each line item x the price of each line item) the purchases.
(P) ShippingHandling	Currency	The cost to ship the order.
(P) TaxRate	Double	The percentage of tax to be levied.
(P) Tax	Currency	The sum of (SubTotal + ShippingHandling) x TaxRate.
(P) Total	Currency	The sum of (SubTotal + ShippingHandling + Tax).
(P) ShipToFirstName	String	The first name of the order recipient.
(P) ShipToLastName	String	The last name of the order recipient.
(P) ShipToAddressLine1	String	The address of the order recipient.
(P) ShipToAddressLine2	String	The address of the order recipient.
(P) ShipToCity	String	The city of the order recipient.
(P) ShipToState	String	The state of the order recipient.
(P) ShipToPhone	String	The phone number of the order recipient.
(P) ShipToEmail	String	The e-mail address of the order recipient.
(P) LineItems	ILineItems	The ILineItems of the object that manages the purchase order's individual line items.
(M) MarshalToPropertyBag	Byte()	Marshals the object into a property bag and returns the bag's contents as an array of bytes.
(P) Valid	Boolean	Reflects whether the object has any invalid properties.

The DNA PurchaseOrder application also defines the ILineItems interface, which provides an easy way to manage the various individual line items associated with a particular purchase order.

Table 14. The ILineItems Interface Is Used to Manage the Various Individual Line Items Associated with a Particular Purchase Order.

Property/method	Data type	Description
(M) Add	Boolean	Adds a line item to the purchase order.
(P) Count	Long	Returns the number of line items associated with the purchase order.
(P) Item	ILineItem	Returns line item #n from the purchase order.
(M) Remove	Boolean	Removes line item #n from the purchase order.

To define the COM interfaces that clients will use to request the business operations defined by the DNA PurchaseOrder application

Start Visual Basic if it's not already running.
From the File menu, select New Project.
Select ActiveX DLL from the New Project window and click OK.
From the Project menu, select Project1 Properties, type POInterfaces in the Project Name text box, and click OK.
From the Project menu, select References, place a check mark next to Microsoft ActiveX Data Access Objects 2.1 Library in the Available References list box, and click OK.
Create eight additional class modules (for a total of nine) to contain the interface definitions, and name them IAccount, IAccountExec, IInventory, IInventoryExec, ILineItem, ILineItemExec, ILineItems, IPurchaseOrder, and IPurchaseOrderExec interface definitions.
Define each interface using the code in Listings 6-14.

Once all of the interfaces have been defined, save your work, and compile the ActiveX DLL into POInterfaces.dll. POInterfaces.dll will contain all the request syntaxes for the various business operations defined by the DNA PurchaseOrder application.