Client Access Role (CAS) matures in Exchange 2010

When the Client Access Role (CAS) was first introduced with Exchange 2007 it was commonly misunderstood as the new “Front-end” server for Exchange. Not only was this not the case, we have come to understand over the last few years of supporting Exchange 2007 in production that the Exchange server and client architecture both maintain numerous dependencies on the CAS role that we never had with a Front-end Server. The CAS role also introduced administrators to a variety of new services including the Availability service, Autodiscover service, and the Calendar Concierge services.

While the CAS role had similarities with Front-end Servers it was quite different at its core. Rather than just act as a relay for communications between the Internet clients and the Exchange Back-end Server, the CAS server became an active participant in the presentation of data to internet protocol clients such as OWA. This was possible with the addition of a middle tier business logic layer. This middle tier handled processing for OWA, IMAP, POP, Web Services, Active Sync and other internet protocols that were not performed by Front-end Servers. With all of the new functionality introduced in Exchange 2007 via the CAS role, it may be hard to imagine that it is again the object of  significant architectural changes in Exchange 2010.  But it is.

The large fundamental change we see with Exchange 2010 is the change in data access paths for not just internet clients but also MAPI clients. Once a mailbox is moved to or created on an Exchange 2010 mailbox server, Outlook clients will connect to the CAS server rather than directly to their Mailbox Server. In fact all clients no matter what they are will need to go through a CAS server to get to the Exchange 2010 Mailbox Server data. The business logic layer has been extended to incorporate two new services: RPC Client Access Service and the Address Book Service. These services combined provide the functionality that resided on the Exchange 2007 and earlier Mailbox servers for Outlook (MAPI) clients. The RPC Client Access Service handles the data connections for RPC clients (Outlook) and the Address Book Service replaces the DSProxy interface as the Name Service Provider Interface (NSPI) endpoint.

One service that you will not find on the new Exchange 2010 CAS role is WebDAV. This means that any WebDAV clients will need to be replaced with the new Web Service Client versions to access a mailbox on Exchange 2010. This will have a direct impact on the Entourage client for MACs as the current version utilizes WebDAV. MAC users that use Entourage will need to upgrade to the Web Service version that is scheduled to be released the same time as Exchange 2010.

Much of the changes that we see in CAS role for Exchange 2010 is to support the new high availability features of Exchange Server. You may want to check out my previous blog “DAG-Nabbit” where I discussed some of the high availability features. You will find that this new architecture for CAS in Exchange 2010 improves failover times, connection scalability and support for SSL-ID load balancing. For further reading on the new CAS architecture check out this webcast by Ross Smith “TechNet Webcast: Exchange 2010 Architecture.”

-Rich Luckett

Related Courses

Configuring, Managing, and Troubleshooting Microsoft Exchange Server 2010 (M10135)

Designing and Deploying Messaging Solutions with Microsoft Exchange Server 2010 (M10233)

Microsoft Exchange Server 2007 (M5047, M5049, M5051)

Ultimate Exchange Server 2007 (M5050, M5053, M5054)

Author: Richard Luckett

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