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High Availability Whitepapers  

High Availability

Application Design for Availability
Applications do not necessarily inherit the underlying availability of the computing platform. In fact, appropriately designed applications can give users higher availability than just what the underlying components provide. The concepts in the previous section—redundancy, isolation and parallelism—can be used with each other and traded off to balance performance, availability, response and cost (technology and people).

Availability studies, using real-world situations, most often cite operational errors among the top two or three reasons for downtime. The most carefully designed high availability system will rapidly deteriorate without effective processes—usually expressed as operational policies and procedures—that constantly maintain availability characteristics.

Operational Processes
Highly available systems are generally more complex to administer and maintain than standard systems. They frequently consist of multiple nodes with non-obvious linkages. Seemingly innocent operational changes to one can seriously impact overall availability, sometimes in ways that do not show up until you have to replace a failed component.

Policies set the general framework in which the procedures are meaningful, and should cover issues like training; failure reaction; problem escalation; change control; application testing; database maintenance; software upgrades; hardware maintenance; component failure and so forth.

User Processes
Users are an important key to achieving high availability. The procedures they follow when the system goes down can make the difference between a straightforward recovery and a failed one.

They need to know and follow procedures that include how to react to slow response; and recognize and continue after a system failure; and recover after a connection failure.

No matter how much technology or how many procedures, you can't achieve high availability without well trained and motivated people to administer, maintain and use the systems.

Data Center
Data Center roles that impact availability include:

Data Center managers: besides setting the tone for the entire team and motivating them, these individuals set overall availability policies and procedures and monitor adherence to them.

Database administrators: because of the huge impact database administration has on availability, these individuals must be carefully trained, possess good performance monitoring skills and tools, and maintain adequate test environments.

System administrators: as front-line availability personnel, highly trained, highly motivated administrators can rapidly detect, contain, and recover from failures.

Vendor field support: they must be meticulous in their access to and maintenance of customer hardware and software. Often they will be working in the middle of a live, high availability system where one mistake will cause system failure Because different personnel will support the customer site, logging changes and leaving adequate information about them is critical to rapidly finding and fixing problems.

As the judges of availability, they must react properly when faced with ambiguous failure indications. Training is only part of the answer. Since highly available systems fail infrequently, it will be hard to remember what the instructor told them to do in any specific situation. Users need to know where operational procedures are kept and adhere to policies.

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