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High Availability Whitepapers  
 

High Availability
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What is High Availability?
The standard for availability is set by end users who determine service levels. Therefore, availability is more than just reliability; it means the computing resource is there (available) when users want it.

Since not all users are the same, their needs for access and response time differ. For instance, payroll clerks may only need access to the payroll service from 8 a.m. to 5 p.m. on business days, but a customer service representative (CSR) may need to use the order entry service 24 hours a day, 7 days a week. CSRs may always require two second response times while marketing analysts accept thirty seconds for retrieving and formatting product sales histories.

Payroll might tolerate eight second response times except at the end of a payroll period when it must be no longer than two seconds.

Even the same users may have different requirements for different services. A stock trader may need sub-second access to the trading service during market hours, but may be able to get by without access to customer history information for limited periods.

The service level expectations of different users determine the size of the computer system and policies for reacting to failures.

Highly available systems provide significant improvements in availability over standard systems by exploiting advanced technology, people skills and processes. The complexity of these systems, however drives up the costs to acquire, maintain, administer, manage and use.

But downtime also has a cost, and it can be significant. The goal is to make sure that the investment in high availability costs less than the risk of unavailability.


High Availability vs Fault-Tolerant Systems
It's important to know the difference. Highly available systems are constructed from the same underlying components as standard systems, and failures may cause some limited interruptions in service. Fault-tolerant systems employ very high levels of redundancy and specialized hardware and software to achieve complete availability. Highly available systems are substantially less expensive than these proprietary non-stop systems. Use of one or the other is a decision made on the basis of balancing cost vs. risk of outage.

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