As you may or may not have heard, Microsoft has announced the next version of their messaging suite, Microsoft Exchange 2010, will be available later this year! The new version of Exchange hosts many improvements and feature additions on top of Exchange 2007. One of the most exciting ones announced, was a feature called Database Availability Groups, or DAG’s. In this four part series, we’ll go over the concepts of DAG’s, and how to get one working and test it out.
So, what is a DAG? A DAG is the evolution of the CCR and SCR functionality that was introduced in Exchange 2007. CCR allowed you to keep two copies of your database’s in a cluster, protecting against both server failure, and a corruption of the database. SCR allowed you to add site resiliency to your Exchange design, by replicating data to your disaster recovery site, and activating it if needed. CCR and SCR have now been rolled into the DAG feature. The best part about it, is most of the legwork, as well as the activation of the data, is automatic! It still uses the concept of log shipping for the replication, although its been much improved. Let’s get into a little bit of how it works.
The first thing you need to know, is in Exchange 2010, the storage architecture is different than that of Exchange 2007. There are no more storage groups to start. Transaction logs, checkpoint files, are all based off of the mailbox database now. Microsoft was moving away from the storage groups, especially when you consider the requirement for any type of replication in Exchange 2007 required a maximum of one database per storage group. The next big change is that database’s are no longer objects of a server, but objects of the Exchange organization itself. What exactly does that mean, well, take a look of this screen shot in the Exchange 2010 management console:
If you notice, I am under the Organization Configuration node, not the server configuration node. The picture shows two database’s, each hosted on different servers. If you notice on the bottom half of the screen, the console lists the Database Copies that make up this particular database. A DAG consists of multiple copies of a set of database’s, that can be activated as the active copy at any time. You can have up to 16 servers in a DAG, meaning you can have up to 16 separate copies of one database! For example, you could have two servers in your main datacenter, each with a copy of one database for high availability in your main site, and then a third copy of the database in your disaster recovery site, in case you lost your main datacenter. Members of a DAG do not have to be members of the same AD site, like stretched 2008 CCR clusters did. Each of these can be activated at any time, automatically if you have a failure, or manually by running some commands.
The last major point, is that no client connects directly to a mailbox server anymore, including outlook. Outlook clients connect to a Client Access Server, just like a POP or IMAP client does to connect to it’s mailbox. This allows for incredible quick failovers (30 seconds or less), of the Outlook client to a new copy of the database.
So now that we have an idea of the high level concept, let’s take a look at actually setting up one. Here is my lab environment. I have two separate AD sites. New York has two subnets, 10.1.1.0/24 and 172.16.1.0/24, and London has two subnets, 192.168.1.0/24 and 172.17.1.0/24. In NY, production traffic will occur over the 10.1.1.0/24 network, and replication and heartbeat over the 172.16.1.0/24. In London, production is 192.168.1.0/24, and replication and heartbeat 172.17.1.0/24. Now, since these are two separate sites, both replication networks need to be able to contact each other. This means both networks need to be routable to each other, which in our case they are. You can use a stretched VLAN, but is a much more complicated scenario, for no true benefit. In each site, I have a single Domain Controller, that is also a Client Access Server and Hub Transport server, as well as one machine with just the Mailbox Role installed. It should be noted, one of the coolest feature of the DAG, is that the mailbox role does not have to be installed by itself for it to be part of a DAG. You can have any combination of roles installed, and it will still work EXACTLY the same. Below is a Visio of the setup:
All of the Exchange 2010 is installed, as you do NO customization during the install, all is done after. This means you do not have to re-install Exchange if you decide down the rode to make it part of a DAG. Let’s take a look at the network configuration. First, the NY server.
I have two NIC’s, one labeled “Client” and one labeled “Replication”. The client NIC, is configured as normal, with an IP, Subnet Mask, Gateway, all the regular stuff. The replication NIC should only be configured with an IP and subnet, NO DEFAULT GATEWAY:
Now, select the advanced button, and select the DNS tab. At the bottom, un-select the box to “Register this connection’s address in DNS”:
Next select the WINS tab and select the radio button to disable NetBIOS over TCP/IP:
After this, select OK to save your settings and return to the Network Connections screen. Select Advanced->Adapters and Bindings. Make sure your production or “Client” NIC is listed above “Replication”:
Now, you may be wondering about the default gateway missing on the heartbeat network. If you add a default gateway on two different NIC’s, windows provides you with a warning:
Hmm, seems like this most certainly pertains to us. Also, DAG’s still use the Windows Failover Clustering feature of Windows Server 2008. Have a configuration with a default gateway on the replication or heartbeat NIC is not supported, as very odd behavior can be exhibited. So, then the question is asked, well the network’s are routed, how do we tell the replication NIC on one node, how to get to the replication networks of the other nodes? For this, we add static routes to the individual server’s routing tables. Tim McMichael had a great article about this, and you can read it here.
So, on the NY node, we want it to contact the LN node’s replication network of 172.17.1.0/24 on its replication network of 172.16.1.0/24. The gateway on the NY side is 172.16.1.254, so we run the following command:
route add 172.17.1.0 MASK 255.255.255.0 172.16.1.254 –p
The –p makes it consistent across reboots. We can check if it was successful with the route print command:
So now, all replication and heartbeat traffic should pass through the specific replication NIC, over the replication network, to the replication NIC of the London node. Repeat this step for ALL your replication networks, on all nodes. For the London node, with a gateway on the London replication network of 172.17.1.254, the command back to NY would be:
Route Print 172.16.1.0 MASK 255.255.255.0 172.17.1.254 –p
Okay, that does it for part 1 of this series. We went over the basic concepts of the DAG, and how to set up the networking for it. In the next section, we’ll go over how to create the DAG, and add nodes to it. Stay tuned.