mirror of https://github.com/schoebel/mars
arch-guide: new section explaining HA
This commit is contained in:
parent
4a6a430d27
commit
1c5b66b939
|
@ -1151,6 +1151,141 @@ reach
|
|||
.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Section
|
||||
What is HA = High Availability
|
||||
\begin_inset CommandInset label
|
||||
LatexCommand label
|
||||
name "sec:What-is-HA"
|
||||
|
||||
\end_inset
|
||||
|
||||
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
HA is defined by a single number, denoting the
|
||||
\emph on
|
||||
minimum percentage of uptime
|
||||
\emph default
|
||||
of a certain system from a user's perspective.
|
||||
Some examples:
|
||||
\end_layout
|
||||
|
||||
\begin_layout Itemize
|
||||
99% availability: a total downtime of more than 87.6 hours per year is not
|
||||
acceptable.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Itemize
|
||||
99.9% availability: a total downtime of more than 8.76 hours per year is not
|
||||
acceptable.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Itemize
|
||||
99.99% availability: a total downtime of more than 52.56 minutes per year
|
||||
is not acceptable.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
HA is a
|
||||
\series bold
|
||||
requirement
|
||||
\series default
|
||||
.
|
||||
Requirements are characterizations of the
|
||||
\series bold
|
||||
problem space
|
||||
\series default
|
||||
.
|
||||
In software engineering, requirements are
|
||||
\emph on
|
||||
strictly separated
|
||||
\emph default
|
||||
from any measures, how a requirement can be met (solution space).
|
||||
In general, there may be
|
||||
\emph on
|
||||
several
|
||||
\emph default
|
||||
solutions for achieving a certain HA percentage.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
Hint for managers: some of the potential solutions for the same HA percentage
|
||||
may be much more
|
||||
\series bold
|
||||
expensive
|
||||
\series default
|
||||
than others, sometimes by
|
||||
\emph on
|
||||
factors
|
||||
\emph default
|
||||
.
|
||||
We will see some examples later.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/MatieresCorrosives.png
|
||||
lyxscale 50
|
||||
scale 17
|
||||
|
||||
\end_inset
|
||||
|
||||
Some people are arguing
|
||||
\emph on
|
||||
incorrectly
|
||||
\emph default
|
||||
, by claiming that
|
||||
\emph on
|
||||
any
|
||||
\emph default
|
||||
HA solution needs to be built up by
|
||||
\emph on
|
||||
redundancy of each and every single component
|
||||
\emph default
|
||||
.
|
||||
This confuses requirements with solutions.
|
||||
It is wrong in general, because even a certain degree of redundancy cannot
|
||||
guarantee a certain HA percentage, for example when certain components
|
||||
are not reliable enough.
|
||||
See also section
|
||||
\begin_inset CommandInset ref
|
||||
LatexCommand nameref
|
||||
reference "sub:Detailed-explanation"
|
||||
plural "false"
|
||||
caps "false"
|
||||
noprefix "false"
|
||||
|
||||
\end_inset
|
||||
|
||||
for a counter-example, where addition of more redundancy does not help.
|
||||
Of course, higher degrees of HA are
|
||||
\emph on
|
||||
typically
|
||||
\emph default
|
||||
built using certain types and degrees of redundancy, including variants
|
||||
like geo-redundancy.
|
||||
In general, however, there might be other means for achieving HA, like
|
||||
extremely quick automatic repair methods, self-healing systems, etc.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Section
|
||||
What is
|
||||
\emph on
|
||||
|
|
Loading…
Reference in New Issue