In Exchange Server 2007 SP1, the EWS team added a new feature called EWS Request Proxying. It is sometimes also referred to as EWS Proxy or HTTP Proxy. Not much has been said about this feature, but if you care about performance, it is important to understand why it is there, why you want to avoid it if at all possible, and if you cannot avoid it, why it is important to have.
What is it?
EWS Request Proxying only comes into play in Exchange installations that have multiple Active Directory sites. If you have a single AD site, you can stop reading this post right now. Now that our readership has become smaller, let's continue. Active Directory sites are typically created to define a boundary of highly connected computers and devices. This implies that accessing resources cross-site is more (potentially much more) expensive than accessing resources within a single site. EWS lives on a Client Access Server (CAS) within a specific site. If the mailbox that EWS is trying to communicate with resides on a mailbox server in a different Active Directory site, the ensuing RPC calls between EWS and the mailbox server will be made across a potentially expensive cross-site link. Depending on the EWS operation that is being performed, there may be *many* resulting cross-site RPC calls.