If I told you that before I cross the street, I stop, look, and listen, perhaps you’d think I was sensible. If I told you I just listen, you know I’d probably have a run-in with a silent electric car. Yet if I mentioned that I look left, right, down, and up (in case there’s something falling in the street), maybe you’d think I was being paranoid about being hit by a meteor or space junk?
With messaging services, how much redundancy is sensible and productive? Can you have too much redundancy? With Exchange Server 2007, Five Things About Exchange Server 2007 You Probably Don’t Know (part 2), we looked at going from one Exchange server to four servers for minimal redundancy. Exchange Server 2010 gives us some extra freedoms beyond its predecessor.
During a recent First Look at Exchange Server 2010 session, among the many questions were “How are consolidated redundant servers accomplished?” and “I understand resiliency, but how do we make the HT and CAS redundant?” Let’s look at a few aspects of each of these questions.
Like Exchange Server 2007, the 2010 version has Edge Transport, Unified Messaging, and of course the triumvirate Hub Transport, Client Access, and Mailbox roles. Single or multiple instances of these roles could be deployed in Exchange Server 2010 environments, and with the exception of the Edge Transport role, the other roles can be consolidated and combined onto a single Windows Server 2008 (R1 SP2 or R2) based server. But certain rules have changed in Exchange 2010 compared with the past.
Exchange Server 2010 no longer requires the clustered mailbox roles (active and passive) be hosted by dedicated servers, unable to host other roles as the 2007 version had. This opens up many possibilities. While the Hub Transport (HT) and Client Access (CA or CAS) server roles could be consolidated using Exchange 2007, when using Exchange 2010, the Mailbox (MB) role could also be consolidated with CA and HT roles, and perhaps Unified Messaging (UM) as well, as long as there is sufficient capacity on the server. Yes, even with clustering for mailbox resiliency.
One of the ramifications of this new flexibility is that Exchange Server 2010 allows deployments of two servers instead of four as a minimal redundant configuration. There are several new features which make this possible.
1. Beside Exchange ActiveSync (EAS), Outlook Web App/Access (OWA), POP3, IMAP4, Web Services, and RPC/HTTP proxy access, even local Messaging Application Programming Interface (MAPI) access via the Remote Procedure Call (RPC) goes through the Client Access (CA) role in Exchange Server 2010. Contrast this with Exchange 2007 allowing local MAPI/RPC clients to connect directly with Mailbox (MB) role servers.
1. Mailbox Databases in Exchange 2010 are no longer configured under the auspices of each server which hosts the MB role. Instead, the databases are configured within a Database Availability Group (DAG). The servers with the MB role then are configured to participate in the DAG.
1. Mailbox resiliency is supported by allowing the servers added to a DAG to replicate the databases in the DAG. The mailboxes in those databases are therefore stored redundantly. For two or more servers with the MB role within the same Active Directory (AD) site, this is similar to Exchange 2007’s Cluster Continuous Replication (CCR). Between AD sites, a delay can be added, reminiscent of Exchange 2007’s Standby Continuous Replication (SCR).
1. Transport resiliency is supported by utilizing Shadow Redundancy which allows multiple transport servers to retain a given message until it has been delivered to one of the MB servers in the DAG. Then the transport servers move the message from the regular queues to a shadow queue. Once the message has been replicated to all DAG replicas, the transport servers remove their copies of the message to their transport dumpster.
When these features are combined with a failover cluster to support the storage redundancy, and a network load balancing cluster to support the network protocol redundancy, some of Exchange Server 2010’s advantages can be realized. With redundant MB, HT, CA, and UM roles in just two or more servers, architects and administrators deploying and maintaining Exchange Server 2010 have much more control over scalability and availability at potentially much lower cost.