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Chapter 5 ■ Service Design Shared-Nothing, Geo-Replicated Clusters Cost & Complexity Replicated Systems Clustered & Virtualized Systems SPs & Line of Business 9 9 9 35 days 4 days 8 hours Figure 5-11. Availability versus cost Web & Cloud Services 9 50 mins eCommerce Telecoms Military 9 % 5 mins Why does the cost shoot up exponentially as availability increases by every decimal? To state it plainly, the cost of availability is expensive. The complexity of the infrastructure and supporting applications dictates the costing. The service design takes into account the agreed SLA levels for availability and designs the availability of services accordingly. If the customer wants to have 99.999% availability, the service provider needs to ensure that there is sufficient resilience to take into account all possible disruptions. For example, the service must have at least a couple of resilience layers. If the primary fails, the secondary takes over. If both the primary and secondary fail, then there is a tertiary layer that is ready to take over. The customer would keep using the services, as though nothing happened, all through the failures of the primary and secondary. In fact, as a backup to the tertiary, high availability designs ensure there is a disaster recovery site, with realtime replication, and it will automatically take over. I will discuss disaster recovery under the IT service continuity process later in this chapter. It is a common sight these days for servers to be set up in a clustered environment, network devices to have multiple failovers, and applications and data to have multiple copies of replication, lying in different parts of the globe, to cater to disasters for every kind. To architect such high-availability designs requires additional resources, storage requirements, people, and other add-ons. This is the reason for the inflated costs for every decimal point in availability. 5.7.5.1.1 Service Availability Service availability is the availability of a service measured end to end. When I say end to end, I mean all aspects of a service need to be available for it to be counted toward availability. If any of the aspects are unavailable, even the tiniest bit, it adds up toward unavailability of the service. 95
Chapter 5 ■ Service Design For example, let’s say you buy a webhost service, which is the core service, and it comes with e-mails as an enhancing service. If the e-mail service goes down and the web site stays up, it is still considered unavailable service. 5.7.5.1.2 Component Availability Interconnected components make up a service. Each component’s availability can be measured as well to ensure that the services offered are stable. Examples of components are servers, routers, load balancers, switches, and others. Component unavailability may not lead to service unavailability if the highavailability design is employed with resilience. If resilience is not factored in, due to various reasons starting from costs and requirements, component availability could very much lead to service availability. Let me further explain this with an example. Let’s assume in a high-availability design, a critical core switch is backed by an autofailover facility. If the switch fails, the traffic is automatically routed to a backup switch. The service is not affected, as the switchover takes place before users can notice it. This high-availability design is expensive. So, not all services and not all organizations opt for high-availability design. In a normal design, if the core switch fails, the concerned team is made aware, and they manually replace the core switch with a good one. The notification and replacement takes a certain amount of time, leading to service downtime. So, in the first instance, although the component (core switch) failed, the service was unaffected. The component availability was affected but the service availability was unaffected. In the normal design, the component took down the service with it. So, component availability affected the service availability. 5.7.5.2 Aspects of Service Availability There are a few terms and formulas that you need to learn as a part of this process. When was the last time you dealt with mathematical formulas, was it in college? You are required to remember these formulas and definitions from a Foundation examination perspective. 5.7.5.2.1 Availability Availability is defined in ITIL as the ability of a service, component, or CI to perform its agreed function when required. Agreed Service Time- Downtime %Availability = Agreed Service Time 96
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Become ITIL Foundation Certified in
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Contents at a Glance About the Auth
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■ Contents 1.8 Internal and Exter
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■ Contents ■Chapter ■ 4: Serv
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■ Contents 6.4 Service Transition
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■ Contents ■Chapter ■ 9: ITIL
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About the Technical Reviewer Sumit
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Preface When my first book, Communi
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CHAPTER 1 Service Management as a P
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Index A Access management and info
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■ INDEX definition, 108 objective
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■ INDEX J JAVA team, 190 K Key
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■ INDEX Service catalog managemen
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■ INDEX OR gate, 19 parts, 20 per
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