MSMQ can be used to simplify the development of traditional publish and subscribe applications. These applications are unique because although clients do subscribe to receive a particular type of data, they do not have to continuously request the data. Similarly, although a server application must continuously send updated data of a specified type to the client, the server does not have to continuously try to resend the data if the client if offline or unavailable. Traditional publish and subscribe applications include newswire services and financial or stock-market-data subscription services.
The following figure shows a publish and subscribe scenario that keeps sales staff up-to-date on the status of available goods. In this scenario, an MSMQ application could also be used to take orders, and the two systems could be integrated. However, to simplify the details of the publish and subscribe scenario, this scenario covers only the dissemination of information.
Figure 1.12
In Figure 1.12, the mobile independent clients retrieve data as they work in the field. The regional servers retrieve from the main server only those data types to which the MSMQ independent clients subscribe. Each MSMQ independent client then receives only the data it has subscribed to receive. This reduces both inter-site communication and mobile user's cellular phone use.
The following MSMQ features are critical to the success of traditional publish and subscribe applications: