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Chapter 1: Introduction - Pg. 1

1 Chapter 1. Introduction Organizations invest in information systems because the value they receive (through automation of business processes) is greater than the cost. Depending upon the business processes supported, certain availability, response times, and throughput must be provided for an application in order for it to be useful. An unreliable credit card authorization system would be useless. Meeting the service levels required for each application, however, comes at a potentially significant cost in terms of additional redundancy, capacity, and complexity. By consolidating services onto System z9 and zSeries mainframe hosts, critical mass can be achieved wherein investments to provide higher availability for all applications are more easily justified. For the purposes of this book, high availability is the ability of an application to continue to provide its services in spite of component failures. High availability is achieved through choosing highly reliable system components (such as System z9 and zSeries mainframes) and understanding and avoiding single points of failure : places in the underlying infrastructure where the failure of a single component can impact the availability of an entire application (or set of applications). Note: In some cases, the cost of eliminating a single point of failure exceeds the expected cost of outages (estimated as the cost of a single outage multiplied by the probability of occurrence). For example, it may be too expensive to justify either of the following: Providing redundant high-speed wide area network connectivity for certain small locations Rewriting an application so that it can be distributed across multiple hosts (to prevent a host failure from impacting application availability) In such cases, the organization chooses to assume the risk of outage. Redundancy, by itself, does not necessarily provide higher availability. It is also necessary to design and implement the system using technologies that can take advantage of the redundancy. For example: Dynamic routing protocols (such as OSPF) enable networks to take advantage of redundant connectivity to route traffic around failed links or nodes. © Copyright IBM Corp. 2006. All rights reserved. 1