While designing large SharePoint farms, many considerations need to be assessed.
Below are some decisions that have implications on the farm logical architecture:
A SharePoint farm can publish many different vanity URLs each having its own web applications, so different URLs and even purpose shouldn’t be the driving force for having multiple farms. However, in some scenarios, the business requirements are totally different between different web sites and it is expected to have different administration teams for the different farms. Many corporations currently have different SharePoint farms for different purposes, while others have multiple SharePoint farms that are geographically dispersed to cater for regional requirements.
Another example of cases where is more than one SharePoint farm is when you have an authoring farm and a publishing farm.
The selection of the number of farms and their roles is a blend of a business decision and a technical decision and need to be confirmed in case of large farms.
2. What are the required service applications?
The following list includes the service applications for SharePoint 2010:
Depending on the business needs, a farm can have a subset of these service applications. For example, Access service, Visio service, Office Web App or project server might not be needed on a specific farm as there is no need for them.
Thus, it is important to identify the required service applications and plan for them.
3. How are the service applications deployed? What is the service applications architecture?
Technet has an article on services architecture planning at http://technet.microsoft.com/en-us/library/cc560988.aspx. This article discusses service application design considerations andoptions.
The most important decisions to make at the moment are the follows:
The following service applications can be shared across farms:
Business Connectivity Services (BCS)
Thus, it is an important decision to decide on whether to share them across farms. In this case, you can have these services reside on one farm (enterprise services farm) and publish them so that other farms can consume them. Moreover, you can decide on having dedicated farms for specific purpose (ex. Search farm that serves all search requests).
Note: It is most common to share User Profile, Managed Metadata, BCS and Search across farms.
Note: Not all service applications are recommended for use in WAN environments. The recommended service applications in WAN are search and managed metadata.
After deciding on the farm that a service application will run on, we can decide to group them in service application groups. The criteria for grouping are their use by the various web applications that you have in your farm