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Chapter 5 ■ Service Design<br />
Shared-Nothing,<br />
Geo-Replicated<br />
Clusters<br />
Cost & Complexity<br />
Replicated Systems<br />
Clustered &<br />
Virtualized Systems<br />
SPs &<br />
Line of<br />
Business<br />
9 9 9<br />
35 days 4 days 8 hours<br />
Figure 5-11. Availability versus cost<br />
Web &<br />
Cloud<br />
Services<br />
9<br />
50 mins<br />
eCommerce<br />
Tele<strong>com</strong>s<br />
Military<br />
9 %<br />
5 mins<br />
Why does the cost shoot up exponentially as availability increases by every decimal?<br />
To state it plainly, the cost of availability is expensive. The <strong>com</strong>plexity of the infrastructure<br />
and supporting applications dictates the costing. The service design takes into account<br />
the agreed SLA levels for availability and designs the availability of services accordingly. If<br />
the customer wants to have 99.999% availability, the service provider needs to ensure that<br />
there is sufficient resilience to take into account all possible disruptions. For example, the<br />
service must have at least a couple of resilience layers. If the primary fails, the secondary<br />
takes over. If both the primary and secondary fail, then there is a tertiary layer that is<br />
ready to take over. The customer would keep using the services, as though nothing<br />
happened, all through the failures of the primary and secondary. In fact, as a backup to<br />
the tertiary, high availability designs ensure there is a disaster recovery site, with realtime<br />
replication, and it will automatically take over. I will discuss disaster recovery under<br />
the IT service continuity process later in this chapter.<br />
It is a <strong>com</strong>mon sight these days for servers to be set up in a clustered environment,<br />
network devices to have multiple failovers, and applications and data to have multiple<br />
copies of replication, lying in different parts of the globe, to cater to disasters for every<br />
kind. To architect such high-availability designs requires additional resources, storage<br />
requirements, people, and other add-ons. This is the reason for the inflated costs for<br />
every decimal point in availability.<br />
5.7.5.1.1 Service Availability<br />
Service availability is the availability of a service measured end to end. When I say end<br />
to end, I mean all aspects of a service need to be available for it to be counted toward<br />
availability. If any of the aspects are unavailable, even the tiniest bit, it adds up toward<br />
unavailability of the service.<br />
95
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