Some SQL, some .NET, and whatever else


Cluster terminology – What “Active/Active” actually means

As a follow-up to my last entry (attempting to clear up some Windows Clustering terminology), I've found an article that makes another distinction that I forgot to include - the difference between an active/passive and an active/active cluster:

The misconception of active/active clustering (a la

The understanding among those new to cluster seems to be that a/a vs. a/p is a licensing question, and then if you're licensed for it, you just turn it on. In reality, it really just describes whether you have clustered services living on only one node or split between both nodes (during normal operation - during a cluster failover, any cluster might be active/active for a short period of time. Or, I suppose, your cluster is active/active if your quorum drive lives on the opposite node from your clustered service). There's no load-balancing involved in clustering at all - at any time, only one node owns a particular resource, and only that node is responding to client requests for that resource.

In SQL Server 2012 AlwaysOn, the new high-availability feature, the SQL Server service is running on both cluster nodes, but client access (through the "Availability Group") is controlled by the cluster service. That means that all clients making a connection go first to the active server, and then the SQL Service there might send them to get their data from one of the other nodes (it's worth reiterating here that, in AlwaysOn, SQL Server isn't clustered, but the SQL services operate independently on each node).