The single-tiered logical architecture for multiple hosts is a set of servers that each run the services particular to a component product. For example, the Messaging Server host is installed and configured to run all the Messaging Server services, the Calendar Server host is installed and configured to run all the Calendar Server services, and so on. This architecture might also be configured for high availability.
The distinction of the single-tiered logical architecture is that end users communicate directly to the data stores, and not through proxies or other agents. For example, in Messaging Server, users would not be routed through MMPs or MTAs. The single-tiered logical architecture might have standalone MTA routers for routing mail between servers, or in and out of the corporate network, but end users submit mail to the MTA on their message stores. No MMPs are involved in intranet connection to the message stores.
The same idea applies to both Calendar Server and Instant Messaging. In the single-tiered logical architecture, no front-end processes are located on separate machines.
Figure 5–2 represents the single-tiered logical architecture for multiple hosts.
In the preceding figure, end-user client programs, such as Outlook and Messenger Express, form the User Tier. Tier 1 is a set of four servers. One server runs the Calendar Server processes, the second runs the Messaging Server processes, the third runs the Instant Messaging processes, and the fourth runs the Directory Server process. If you are deploying Communications Express, the Messaging Server host also includes a web server, either Web Server or Application Server, (for Webmail).