Database Availability Group (DAG) is the new Exchange 2010 high availability feature. This feature provides data availability together with service availability. DAG now is the only built-in way to protect data in Exchange 2010.

This article is composed of an introductory section where we look at the key facts of Database Availability Groups and a walk through the implementation of DAG. The introductory sections where authored after researching various TechNet articles and here I am reproducing salient information taken from TechNet. This information was restructured for Administrators to have a central reference point rather than having to go through many TechNet articles. So credits for the article introduction go to TechNet.

In Exchange 2007, we have Local Continuous Replication (LCR), Cluster Continuous Replication (CCR), Single Copy Clusters (SCC) and Standby Continuous Replication (SCR). Exchange 2010 combined on-site data replication (CCR) and off-site data replication (SCR) to produce one method to protect mailbox databases.

DAG is a group of up to 16 mailbox servers that host a set of databases and provide automatic database-level recovery from failures that affect individual servers or databases. Those mailbox servers can be geographically dispersed to replicate mailbox databases across sites. Any server in a DAG can host a copy of a mailbox database from any other server in the DAG.

Storage groups no longer exist in Exchange 2010. The mailbox database name is unique within an Exchange 2010 organization, they are now global objects and, as a result, the primary management interfaces for Exchange databases has moved within the Exchange Management Console from the Mailbox node under Server Configuration to the Mailbox node under Organization Configuration. Also continuous replication now operates at the database level because of Storage Group removal from Exchange 2010.

In Exchange 2007 the Microsoft Exchange Replication service on the passive node connects to the share on the active node and copies, or pulls, the log files using the Server Message Block (SMB) protocol. In Exchange 2010 SMB is no longer used for Log shipping and seeding. Instead, Exchange 2010 continuous replication uses a single administrator-defined TCP port, by default DAG uses port 64327. Also, Log shipping no longer uses a pull model where the passive copy pulls the closed log files from the active copy; now the active copy pushes the log files to each configured passive copy.

Another good enhancement is that seeding is no longer restricted to using only the active copy of the database. Passive copies of mailbox databases can now be specified as sources for database copy seeding and reseeding. In addition, Exchange 2010 includes built-in options for network encryption and compression for the data stream.

There are two editions of Exchange 2010, standard and enterprise editions. Both editions include DAGs, but standard edition is limited to 5 databases per server while the enterprise edition can host up to 100 databases per server. Note that if you want to use DAG with failover clustering, you have to install Exchange 2010 on the enterprise editions of Windows Server 2008. And all DAG members should run the same operating system, either Windows Server 2008 on all members or Windows Server 2008 R2 on all members.

Creating and Configuring DAG

There are specific networking requirements that must be met for each DAG and for each DAG member. Each DAG has a single MAPI network, which is used by other servers (e.g., other Exchange 2010 servers, directory servers, witness servers, etc.) to communicate with the DAG member, and zero or more Replication networks, which are networks that are dedicated to log shipping and seeding. However, unlike previous Exchange versions, database availability group configuration is supported using single network.

An IP address (either IPv4 or both IPv4 and IPv6) must be assigned to the DAG. This IP address must be on the subnet intended for the MAPI network.

You can assign static IP addresses to the DAG by using the DatabaseAvailabilityGroupIpAddresses parameter. If you use the Exchange Management Console (EMC) to create the DAG, or if you use the New-DatabaseAvailabilityGroup cmdlet without the DatabaseAvailabilityGroupIpAddresses parameter, the task will configure the DAG to use Dynamic Host Configuration Protocol (DHCP) to obtain the necessary IP addresses. If you don't want the DAG to use DHCP, you can use the Set-DatabaseAvailabilityGroup cmdlet to configure one or more IP addresses for the DAG after it has been created.

Now we will create the DAG. In EMC | Organization Configuration | Mailbox. Click the Database Availability Groups tab, right-click and select New Database Availability Group:

Type a name for the DAG. Remember that the DAG must have a unique name inside the Exchange organization and it can consist of up to 15 characters. I will select the server that hosts the Hub Transport and Client Access server roles as a witness server, and define C:\DAG1-WS as the witness directory

Click Next to start creating the DAG:

After DAG has been created, we can run the command "Get-DatabaseAvailabilityGroup DAG1 | fl" to see the default properties of the DAG:

Note that the DAG has no IP addresses configured. I don't have DHCP in my test environment, so we have to configure an IP address for the DAG. To do so, we will use the command:
Set-DatabaseAvailabilityGroup DAG1 -DatabaseAvailabilityGroupIpAddresses 20.20.0.6

Now you can add servers to the DAG. In EMC | Organization Configuration | Mailbox, click the Database Availability Groups tab, right-click the DAG you want to manage, and then click Manage Database Availability Group Membership:

Click Add then select the servers you want to add. I will chose one server then add the second server using Exchange Management Shell.

Click Manage to add the server as a member to the DAG

Now we will configure the DAG networks to allow the replication on one subnet other than the MAPI network subnet. From the Database Availability Groups tab, select the DAG. At the bottom pane we can then configure the network properties for the selected DAG.

I will add an IPv4 subnet and remove the IPv6 subnet. Also make sure that the Enable replication check box is selected to allow the replication to happen over this network

Next we will disable the replication on the MAPI network. Open the properties of the second network which is configured with the IP of your internal network, uncheck Enable replication check box

Now we will add the second server using the command:
Add-DatabaseAvailabilityGroupServer -Identity DAG1 -MailboxServer Ex14Mbx2 -Verbose

The -Verbose parameter instructs the command to provide detailed information about the operation.

To check the memebers after we have two members in the DAG, we can use the command:
Get-DatabaseAvailabilityGroup