The new concept of the Database Availability Group (DAG) is exciting Exchange 2010 technology to bring low cost high availability without costly hardware SAN infrastructure.

Microsoft Exchange Server 2010 clients will connect to Client Access Servers, which will proxy the requests to the client.  No more LCR, SCR, or CCR…DAG (or Super CCR) uses low cost DAS storage to leverage a “Raid 5” striping of databases to multiple servers.  Client Access Servers (set in load balanced server farms), will provide primary HTTP and a new “distributed RPC endpoint” for Office 2010, Office 2007, Office 2003 emulation of a “standard exchange mailbox server” without needing to upgrade the clients.

Since clients connect to the CAS servers to proxy requests to the mailbox servers, failover from mailbox server to another in the DAG happen in less than 30 seconds in a failover or move command.

Some other notable highlights in Exchange 2010 database and HA architecture:

• Replication between databases will change from being a RPC method, to a TCP socket method which will increase performance on heavily loaded servers.  
• Replication can be locally or remote (cross-subnet).  You will need CAS servers at the DR site however if you lose the primary datacenter.
• You can have to 16 mailbox servers in a DAG.
• There will be no integration with Microsoft Online at the DAG level.   Microsoft Online cannot be used as DR site for a on-premise hosted mailbox.  Either it’s on-premise or hosted, not a mixture of the two.
• You still Windows Server 2008 Enterprise, as failover clustering feature is required.
• The concept of Storage groups are a depreciated.
• Jet is still the storage engine for Exchange 2010 databases.
• Exchange IO has been reduced 50% from 2007 to 2010 (and already a 70% IO reduction from Exchange 2003 to 2007).
• Single Instance Storage  is going away, as well as the per database table.  A new table is created for each mailbox, creating the scenario for 10,000+ messages in mailboxes due to the sequential read capability. 
• Server based PST files allows archiving with anywhere access. Helps for e-discovery, OWA searches, and compliance management.

Public folders are not covered by the new DAG changes, and the only way to replicate Public Folders in Exchange 2010 is using the same 10 year old Public folder replication methods we have used for years.  SCR replication of the public folder database for DR scenarios, possible in Exchange 2007, is depreciated in Exchange 2010.  Also, clients will continue to connect to public folder on mailbox servers in the DAG directly.  Public Folders will not take part in the new Client Access Server 2010 model that is introduced with Exchange 2010 mailbox databases. Public folders are a legacy platform and significant changes won’t be introduced.