COM & .NET Component Services

Invoking Queued Components on the Client Side

A queued component client cannot create the component using CoCreateInstance( ) (or CreateObject( )/New for Visual Basic 6.0) because it would result with the normal synchronous mode of interaction. The client must create the component in a way that would make COM+ create a recorder for the calls instead.

Consider, for example, the system in Figure 8-5, which shows the component layout of an online retail shoe store. The customer interacts with a top-level Store component. The interaction with the customer must be fast and synchronous. The customer specifies shoe size, shipping method, email address, credit card number, and so on. The Store component saves the order information using the Order component and processes the order using the Shipment component.

Figure 8-5.  A simple online retail store system containing Store and Order COM+ components and a queued Shipment component

However, shipping the order (ordering the shoes from the vendor, updating inventory, interacting with the shipping company, etc.) can take a long time to complete. Processing and shipping the order should be done offline, using COM+ queued components.

The Shipment component exposes the IShipment interface, defined as:

interface IShipment: IDispatch
{
   [id(1)] HRESULT ShipOrder([in]DWORD dwOrderNumber);
}

The Shipment component prog-ID is MyApp.Shipment.

The first step ...