Editing Availability (section)

=== Series vs Parallel components ===
[[File:Series vs parallel components.png|alt=series vs parallel components|thumb|397x397px|series vs parallel components]]



Let's say a series component is composed of components A, B and C. Then following formula applies: 

Availability of series component = (availability of component A) x (availability of component B) x (availability of component C) <ref name=":0">{{Cite book |title=System Sustainment: Acquisition And Engineering Processes For The Sustainment Of Critical And Legacy Systems |year=2022 |isbn=9789811256868 |last1=Sandborn |first1=Peter |last2=Lucyshyn |first2=William |publisher=World Scientific }}</ref><ref name=":1">{{Cite book |title=Reliability and Availability Engineering: Modeling, Analysis, and Applications |year=2017 |isbn=978-1107099500 |last1=Trivedi |first1=Kishor S. |last2=Bobbio |first2=Andrea |publisher=Cambridge University Press }}</ref>

Therefore, combined availability of multiple components in a series is always lower than the availability of individual components. 

On the other hand, following formula applies to parallel components:

Availability of parallel components = 1 - (1 - availability of component A) X (1 - availability of component B) X (1 - availability of component C) <ref name=":0" /><ref name=":1" />
[[File:System availability chart.png|alt=10 hosts, each having 50% availability. But if they are used in parallel and fail independently, they can provide high availability.|thumb|10 hosts, each having 50% availability. But if they are used in parallel and fail independently, they can provide high availability.]]
In corollary, if you have N parallel components each having X availability, then:

Availability of parallel components = 1 - (1 - X)^ N <ref name=":1" />

Using parallel components can exponentially increase the availability of overall system. <ref name=":0" />  For example if each of your hosts has only 50% availability, by using 10 of hosts in parallel, you can achieve 99.9023% availability. <ref name=":1" />

Note that redundancy doesn’t always lead to higher availability. In fact, redundancy increases complexity which in turn reduces availability. According to Marc Brooker, to take advantage of redundancy, ensure that:<ref>{{Cite book |title=Understanding Distributed Systems, Second Edition: What every developer should know about large distributed applications |isbn=978-1838430214 |last1=Vitillo |first1=Roberto |date=23 February 2022 |publisher=Roberto Vitillo }}</ref>

# You achieve a net-positive improvement in the overall availability of your system
# Your redundant components fail independently
# Your system can reliably detect healthy redundant components
# Your system can reliably scale out and scale-in redundant components.