Understanding the right terminology…
Over the past 3 months, I've had several discussions around planning of messaging environments. These dicussions always lead to discussions around availability and the similar. But what I've discovered is that many people confuse some of the terminology which makes the planning phase more difficult. So I wanted to clarify some of these terms...
AVAILABILITY
Availability is the degree to which an application, service, or system is perceived by users to be available. Availability typically consists of redundancy and fault tolerance as a means to eliminate any single point of failure. A high-availability solution masks the effects of a hardware or software failure and maintains the availability of applications so that the perceived downtime for users is minimized. A good solution should be able to take appropriate action with little to no user involvement. Availability is not data protection and recovery, nor is it disaster recovery.
REDUNDANCY
Redundancy is a key part of availability. This is the use of multiple components, services, or systems to ensure that if one fails, another can carry the workload. Examples of redundancy include the use of multiple servers in a load-balanced environment to improve farm performance or to scale out to accommodate additional users. Redundancy may also be the use of identical backup components, such as power supplies or networking equipment, to provide continued functionality in the event of the failure of the primary component.
RECOVERABILITY
Recoverability is recovering from an outage for an application, service, system. This includes understanding the process to recover, the time needed to recover, how much data/productivity can be lost, etc. Basically, this is disaster recovery
SITE RESILIENCY
Site Resiliency is when a physical location or datacenter has experienced an issue that may impact user productivity or data. This might involve only one or a set of applications, systems, or services. In either case, if the current datacenter is not capable of providing the necessary resources to bring the resource(s) online at 100%, then you might need to fail over to an alternate location. Often failing over to an alternate site is a manual process.
So when considering an IT solution, be aware of the differences in these terms and that each may require a different solution.
Exchange Server 2010: High Availability and Site Resilience
https://technet.microsoft.com/en-us/library/dd638121.aspx
Exchange Server 2010: Disaster Recovery
https://technet.microsoft.com/en-us/library/dd876874.aspx
Exchange Server 2007: High Availability
https://technet.microsoft.com/en-us/library/bb124721(EXCHG.80).aspx
Exchange Server 2007: Disaster Recovery
https://technet.microsoft.com/en-us/library/aa998848(EXCHG.80).aspx
Doug