In an ideal world, your applications will be load balanced over several dedicated servers to prevent any load issues and downtime due to server crashes and have a failover system identical to the main system which can take over should your main system completely fail or become unreachable. This is usually very expensive and many companies have to make a choice between having a load balanced system or employing a failover mirrored system.
With a failover mirrored system, you would usually have 2 identical systems in 2 different physical locations with no common factor connecting them, so if the main system goes down, the backup system would take over.
Having a failover system is the best method for having a disaster recovery plan and is used widely in the Financial Services industry, indeed the FSA require many organisations to have a Diaster Recovery (DR) plan in case their data is damaged or lost. It allows for almost 0 loss of data and fairly high availability as well
The pitfalls of failover systems are that the speed of failover can be fairly slow depending on the method used, as its hard to divert web traffic from one server to the other in different physical locations and on different networks, it does not tend to be recommended for high availability web sites, however for office based applications where manual domain names resolution isn’t crucial it can result in much better availability. The cost is also a major issue as having an equivalent redundant system does mean you are paying for two systems but only using one at any one time.
Its recommended that failover systems are best for those who disaster recovery is the largest priority and where the retention and duplication of data is business critical.
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