Many small to medium-sized organizations start using SharePoint in a "two-tier" server farm topology. The two tiers consist of:Tier 1 - SharePoint Server with all web page serving and all Service Applications running on it Tier 2 - A SQL Server to store the SharePoint databases - the SQL Server could be dedicated to the farm or it might be shared with other non-SharePoint applications. This farm topology can frequently support companies with hundreds of employees. Of course, it depends a lot on the specifications of the hardware, but with late-model quad-core Xeons running on the two servers and 8 - 16 GBs of RAM on each one with RAID built with 15k RPM SAS drives in the SQL Server, this configuration with SharePoint Server 2010 can perform very well in many organizations that have less than 1000 users.At some point, an organization that started with this two-tier topology may want to scale out to the next level which is a three-tier topology. The three tiers would be:Tier 1 - SharePoint Server dedicated as a Web Front-End (WFE) with only the web application(s) and the search query service running on it Tier 2 - SharePoint Server dedicated as an Application Server with all of the other service applications running on it, but no web applications or query service Tier 3 - SQL Server for the databases Visually, this topology looks like this: There are many different reasons why a company might want to scale out to three-tiers from two. Some kind of performance improvement is frequently what drives it. However, it may not be the obvious one of desiring better page serving times for the end users. For instance, I frequently see companies do this to move the search crawling and index building process to a different server that is more tuned for its unique resource requirements and can do a more efficient job of crawling and indexing the company's content. Perhaps in the two-tier approach their crawl\index component can't get enough hardware resources to crawl through all of the content on a timely basis.One more point. Many organizations will also choose to add a second WFE when they scale out to a three-tier farm. (I don't show this in the diagram above). The second WFE will be configured exactly like the first one and some type of network load balancing (NLB) mechanism will be put in front of the WFEs to intelligently route user traffic to the two servers to balance out the load. In this scenario, the three-tier farm diagram above would be modified to add a second WFE and the total number of servers in the SharePoint farm would be four.
