Availability refers to a level of service provided by applications, services, or systems. Highly available systems have minimal downtime, whether planned or unplanned. Availability is frequently expressed as the percentage of time that a service or system is available, for example, 99.9 percent for a service that is unavailable for 8.75 hours per year.

To improve availability, you have to implement fault tolerance mechanisms to mask or minimize the impact of failures of the components and dependencies of the service. Fault tolerance is achieved by implementing redundancy to single points of failure components. When planning for Microsoft Exchange availability, consider all components that are part of the messaging infrastructure. Some components could also be other services that have subcomponents.

The messaging service availability is determined by the availability of each component that is part of the infrastructure.

Microsoft Exchange Server 2007 Service Pack 1 includes features that can reduce costs and increase uptime:

• Local Continuous Replication (LCR): LCR is a single-server solution that uses built-in asynchronous log shipping technology to create and maintain a copy of a storage group on a second set of disks that are connected to the same server as the production storage group. LCR provides log shipping, log replay, and a quick manual switch to a secondary copy of the data.
  
• Cluster Continuous Replication (CCR): CCR is a clustered solution (referred to as a CCR environment) that uses built-in asynchronous log shipping technology to create and maintain a copy of each storage group on a second server in a failover cluster. CCR is designed to be either a one or two data center solution, providing both high availability and site resilience.
  
• Standby continuous replication (SCR):SCR is a new feature introduced in Exchange 2007 SP1. SCR is designed for scenarios that use or enable the use of standby recovery servers. SCR extends the existing continuous replication features and enables new data availability scenarios for Exchange 2007 Mailbox servers. SCR uses the same log shipping and replay technology used by LCR and CCR to provide added deployment options and configurations by providing the administrator with the ability to create additional storage group copies. SCR can be used to replicate data from stand-alone Mailbox servers and from clustered mailbox servers.
  
• Single-Copy Clusters (SCC): SCC is a clustered solution that uses a single copy of a storage group on storage that is shared between the nodes in the cluster. SCC clusters are basically the same as traditional clusters in early versions of Exchange, with some improvements in Exchange 2007.
  

CCR is the recommended technology for Hosted Messaging and Collaboration version 4.5.

A Comparison of the Different High Availability Solutions

The following table lists the difference between LCR, CCR and SCC in Exchange 2007.

	LCR	CCR	SCC
Failure or planned downtime of one node	No protection	Protected	Protected
Database corruption	Probable protected if database corruption is not replicated to the copy	Probable protected if database corruption is not replicated to the copy	No protection
Failed local storage system	Protected if second copy is on a different storage system	Protected	Protected if the failure is isolated to one node
Failed disk subsystem	Probable protected if second copy is on a different storage system	Protected	No protection
Data center failure	No protection	Probable protected if the second node is located in a second datacenter and the file share witness is accessible by it	No protection
Utilizes Windows clustering	No	Yes	Yes
Can use low cost storage systems	Yes	Yes	No
Storage requirements	Double	Double	Single
Backup databases without impacting server	Partial if the second copy is located on a different storage system the impact will be less. With CCR the primary server is not affected at all.	Yes	No

SCR is similar to LCR and CCR, but it has unique characteristics of its own:

• SCR supports multiple replication targets per storage group. LCR and CCR support only one replication target per storage group (the passive copy).
  
• SCR includes a built-in delay for replay activity, and it enables an administrator to specify an additional delay. This is useful in a variety of scenarios. For example, in the event of logical corruption of an active database, the built-in and additional administrator-configured delay could be used to prevent logical corruption of an SCR target database. LCR and CCR have no such delays.
  
• SCR is completely managed using the Exchange Management Shell. The Exchange Management Console can be used to manage many aspects of LCR and CCR, but it cannot be used to enable or manage any aspects of SCR.
  

For more information about Exchange 2007 SP1 high availability for data and service, see High Availability.