mirror of https://github.com/schoebel/mars
arch-guide: new section explaining HA
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@ -1151,6 +1151,141 @@ reach
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.
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\end_layout
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\end_layout
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\begin_layout Section
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What is HA = High Availability
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:What-is-HA"
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\end_inset
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\end_layout
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\begin_layout Standard
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HA is defined by a single number, denoting the
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\emph on
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minimum percentage of uptime
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\emph default
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of a certain system from a user's perspective.
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Some examples:
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\end_layout
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\begin_layout Itemize
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99% availability: a total downtime of more than 87.6 hours per year is not
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acceptable.
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\end_layout
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\begin_layout Itemize
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99.9% availability: a total downtime of more than 8.76 hours per year is not
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acceptable.
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\end_layout
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\begin_layout Itemize
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99.99% availability: a total downtime of more than 52.56 minutes per year
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is not acceptable.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/lightbulb_brightlit_benj_.png
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lyxscale 12
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scale 7
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\end_inset
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HA is a
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\series bold
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requirement
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\series default
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.
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Requirements are characterizations of the
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\series bold
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problem space
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\series default
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.
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In software engineering, requirements are
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\emph on
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strictly separated
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\emph default
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from any measures, how a requirement can be met (solution space).
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In general, there may be
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\emph on
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several
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\emph default
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solutions for achieving a certain HA percentage.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/lightbulb_brightlit_benj_.png
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lyxscale 12
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scale 7
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\end_inset
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Hint for managers: some of the potential solutions for the same HA percentage
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may be much more
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\series bold
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expensive
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\series default
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than others, sometimes by
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\emph on
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factors
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\emph default
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.
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We will see some examples later.
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\end_layout
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\begin_layout Standard
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\noindent
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\begin_inset Graphics
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filename images/MatieresCorrosives.png
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lyxscale 50
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scale 17
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\end_inset
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Some people are arguing
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\emph on
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incorrectly
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\emph default
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, by claiming that
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\emph on
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any
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\emph default
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HA solution needs to be built up by
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\emph on
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redundancy of each and every single component
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\emph default
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.
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This confuses requirements with solutions.
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It is wrong in general, because even a certain degree of redundancy cannot
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guarantee a certain HA percentage, for example when certain components
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are not reliable enough.
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See also section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "sub:Detailed-explanation"
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plural "false"
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caps "false"
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noprefix "false"
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\end_inset
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for a counter-example, where addition of more redundancy does not help.
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Of course, higher degrees of HA are
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\emph on
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typically
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\emph default
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built using certain types and degrees of redundancy, including variants
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like geo-redundancy.
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In general, however, there might be other means for achieving HA, like
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extremely quick automatic repair methods, self-healing systems, etc.
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\end_layout
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\begin_layout Section
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\begin_layout Section
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What is
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What is
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\emph on
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\emph on
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