Programming Microsoft Outlook and Microsoft Exchange, Second Edition (DV-MPS Programming)

Exchange Server provides an infrastructure with certain core services that enable you to focus on building value-added services rather than on re-creating existing services. This infrastructure complements current network topologies and protocols and, as you will see, guarantees that every message gets through to its destination. The following sections discuss some of the advantages of the Exchange Server messaging infrastructure.

Least-Cost Routing, Load Balancing, and Failover

Exchange Server provides technologies in its messaging engine that allow organizations to define different routes of communications between Exchange Servers. Costs can be assigned to these different routes, and the least costly route is always attempted first by the Exchange Server. If this route is unavailable, the Exchange Server will failover to the next least costly route. If you assign the same cost to two different routes, the Exchange Server will distribute the communications traffic evenly over both routes, thereby load balancing the connections.

Let's look at an example. Imagine there are three routes between an Exchange Server in New York and an Exchange Server in California, and the routes consist of one route over the Wide Area Network, another over a dial-up 28.8 modem, and the third over a satellite link. The administrator of the Exchange Server system can assign costs to each of these routes: the WAN route is assigned a cost of 20, the modem route is assigned a cost of 50, and the satellite route is assigned a cost of 70. Based on the cost of the routes, for communications, the Exchange Server would always attempt the WAN route first. If this route was down, the Exchange Server would failover to the next least costly route (the modem), and if that route was unavailable, it would attempt to connect over the satellite.

Now this is a simple example, but Exchange Server supports the building of very complex routing tables with associated costs that it automatically calculates. For example, consider a message that has to be routed through seven different Exchange Servers until it reaches its final destination. Each Exchange Server has three unique routes to the next server. Exchange Server would automatically find the least costly route of all of the supplied routes.

Delivery and Read Receipts

Exchange Server supports both delivery and read receipts when delivering information through the Exchange Server system. Delivery receipts are returned to an individual user or an application when an item has been delivered to its final destination. This destination can be another Exchange Server or messaging server over the Internet. Delivery receipts also report the time and date that an item was received by a particular system. You can take advantage of delivery receipts in your application by using them to trigger events when they are returned. For example, a workflow application can consolidate delivery receipts to track the status of message delivery to workflow participants. Figure 3-1 shows an example of a delivery receipt.

Read receipts are similar to delivery receipts, except that read receipts are sent to a user or application when the recipient actually opens the item, and delivery receipts are sent as soon as the item is delivered to the destination server. You might want to use read receipts in your application for time-sensitive items sent through the Exchange Server system. The application could track when the item is read, and if no action is taken after a certain amount of time, it could reroute the item to a different user or application. Figure 3-2 shows an example of a read receipt.

Figure 3-1 A delivery receipt sent back to a user looks like this. Applications can also send back delivery receipts.

Figure 3-2 Applications can use read receipts like this to track when users open items sent by the application.

Message Tracking

Exchange Server supports more than delivery and read receipts. When message tracking is enabled, Exchange Server keeps logs of the items that have entered the Exchange Server system from other systems. Exchange Server also logs where items were routed to, which Exchange Server components routed them, and when the items were delivered to their final destinations. Message tracking enables you to find an item's route based on specific criteria such as the sender of the item, the intended recipient, or even the component of Exchange Server that handled the message. This powerful tool allows you to trace any item in your application and determine whether or not it reached its destination. Figure 3-3 shows an example of tracing an item in the Exchange Server system.

Figure 3-3 Tracking items from Thomas Rizzo across the Exchange Server system.