Microsoft Exchange Server 2010 includes a new component called Active Manager that provides functionality that replaces the resource model and failover management features provided by integration with the Cluster service in previous versions of Exchange. Exchange no longer uses the cluster resource model for high availability. All Exchange cluster resources provided by exres.dll no longer exist, including the construct known as a clustered mailbox server. A Windows Failover Cluster is used by Exchange, but there are no cluster groups for Exchange, and there are no storage resources in the cluster. Thus, if you examine the cluster using cluster management tools, you’ll see only the core cluster resources (IP Address and Network Name, and if needed, quorum resource). Cluster nodes and networks will also exist, but those are managed by Exchange and not cluster or cluster tools.
Active Manager runs as a role on all Mailbox servers. On Mailbox servers that are not configured for high availability, there is a single Active Manager role: Standalone Active Manager. On servers that are members of a database availability group (DAG), there are two Active Manager roles: Primary Active Manager (PAM) and Standby Active Manager (SAM). PAM is the Active Manager in a DAG that decides which copies will be active and passive. PAM is responsible for getting topology change notifications and reacting to server failures. The DAG member that holds the PAM role is always the member that currently owns the cluster quorum resource (default cluster group). If the server that owns the cluster quorum resource fails, the PAM role automatically moves to a surviving server that takes ownership of the cluster quorum resource. In addition, if you need to take the server that hosts the cluster quorum resource offline for maintenance or an upgrade, you must first move the PAM to another server in the DAG. The PAM controls all movement of the active designations between a database’s copies (only one copy can be active at any specified time, and that copy may be mounted or dismounted). The PAM also performs the functions of the SAM role on the local system (detecting local database and local Information Store failures).
The SAM provides information on which server hosts the active copy of a mailbox database to other components of Exchange that are running an Active Manager client component (for example, RPC Client Access service or Hub Transport server). The SAM detects failures of local databases and the local Information Store. It reacts to failures by asking the PAM to initiate a failover (if the database is replicated). A SAM doesn’t determine the target of failover, nor does it update a database’s location state in the PAM. It will access the active database copy location state to answer queries for the active copy of the database that it receives.