MOSS adopts a 3-tier model with:
1. Web servers at the front: web servers which can be network balanced to achieve addl performance and fault tolerance
2. Application servers in the middle: servers collection that provide shared services to portals and sites (SSP)
3. Database servers at the back where data and config is stored and can be clustered
The prominent services provided by the Application Servers include:
- Search
- Index
- Audience compilation
- User profiles database
- My Sites
- Business Data Catalogue
- Excel Services
Any of the above services can exist on any number of servers within the SSP.
Portal:
It is now called "Collaboration Portal", not "Portal" as in earlier version of SharePoint and it is just another site collection that is hosted on a IIS site(Web Application).Portals don't contain any application services such as Search, My Site etc. Now, everything has to come thru SSP. This means that we just need a web server to host a portal and a place to include the content database.
Different portals can live on their own hardware completely isolated from any other hardware other than the fact that it consumes services from a centrally managed SSP. Alternatively, you can put some of your portals on shared hardware and some on dedicated.
All three tiers (web, application, database) of the SharePoint model can be hosted on a single machine or scaled out to a huge collection of servers to meet the requirements.
Most organisations will want some level of fault tolerance and separation between server roles.
Medium-Farm Configuration:
Typically these kind of organisations will have at least 2 web servers running your portals and sites, at least one application server hosting all services (maybe a second for fault tolerance) and one database cluster.
Large-Farm Configuration:
Larger organisations may want to have separate web servers for each of their portals, sites and my sites. They may also go for multiple application servers as part of the SSP.
