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arch-guide: rework explanation of architecture
This commit is contained in:
Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
abb1b3cb43
commit
41ce72f667
@ -409,6 +409,32 @@ Datacenter architects have no easy job.
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There are some architectural laws to know and some rules to follow.
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\end_layout
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\begin_layout Standard
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As a responsible manager, you will make architectural decisions, even if
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you are
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\emph on
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not aware
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\emph default
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of them.
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Bad decisions, even if you are not aware of its consequences, can endanger
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major products, and increase cost 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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Once you have commited to a certain architecture, it will be
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\emph on
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extremely cumbersome
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\emph default
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to modify it later.
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Thus you need to get it right from start.
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Typically, you will have
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\series bold
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only one shot
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\series default
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.
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\end_layout
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\begin_layout Standard
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First, we need to take a look at the most general possibilities how storage
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can be architecturally designed:
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@ -443,7 +469,7 @@ too simple
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\begin_inset Quotes erd
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\end_inset
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to provide both enterprise grade reliability, and scalability.
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to provide enterprise grade reliability, and scalability, and maintainability.
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In the past, this was sometimes true.
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\end_layout
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@ -461,8 +487,25 @@ LV Football
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football-user-manual.pdf
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\family default
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.
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We will later review what level of reliability and scalability can be achieved
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with each of the fundamental models mentioned here.
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When Football is combined with a
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\family typewriter
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FlexibleSharding
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\family default
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architecture (see section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "subsec:FlexibleSharding"
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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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), practically the same flexibility as promised by
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\family typewriter
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BigCluster
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\family default
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is possible.
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\end_layout
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\begin_layout Section
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@ -504,23 +547,47 @@ discipline
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\end_layout
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\begin_layout Standard
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Throughout this paper, the term
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Throughout this document, the term
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\begin_inset Quotes eld
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\end_inset
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\series bold
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architecture
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\series default
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\begin_inset Quotes erd
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\end_inset
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is strictly separated from
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(without preceding
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\begin_inset Quotes eld
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\end_inset
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implementations
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software
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\begin_inset Quotes erd
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\end_inset
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.
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) is strictly separated from
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\begin_inset Quotes eld
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\end_inset
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\series bold
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implementation
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\series default
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\begin_inset Quotes erd
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\end_inset
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(without preceding
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\begin_inset Quotes eld
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\end_inset
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software
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\begin_inset Quotes erd
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\end_inset
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).
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Any of
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\begin_inset Quotes eld
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\end_inset
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@ -537,7 +604,66 @@ implementation
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\begin_inset Quotes erd
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\end_inset
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can relate to both hard- and software in general.
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can relate to both hard- and software.
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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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A certain architecture may have multiple implementations.
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An implementation is based on a
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\emph on
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set
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\emph default
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of
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\series bold
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technologies
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\series default
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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Architectures are serving as aids for
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\series bold
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classification of solutions
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\series default
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.
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An implementation is a solution which has
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\emph on
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materialized
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\emph default
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(in contrast to solutions which exist only on paper).
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Complex solutions / implementations are typically matching only one architectur
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e.
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Thus the relationship between architectures and solutions / implementations
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is typically
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\begin_inset Formula $1:n$
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\end_inset
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, while the relationship between solutions / implementations and technologies
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is
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\begin_inset Formula $n:m$
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\end_inset
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in general.
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In case of a very simple solution, it may
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\emph on
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exceptionally
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\emph default
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match multiple architectures, but this is not typical for classification
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schemes.
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\end_layout
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\end_inset
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.
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\end_layout
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\begin_layout Standard
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@ -549,7 +675,99 @@ implementation
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\end_inset
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Confusion of
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Unfortunately, certain technologies are not suitable for certain architectures.
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There may be
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\series bold
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restrictions
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\series default
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.
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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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Because of
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\series bold
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hidden restrictions
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\series default
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which may show up later, you should not start with implementations or technolog
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ies.
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Always start top-down with architectural considerations, while trying to
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identify potential restrictions
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\emph on
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as early as possible
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\emph default
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.
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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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Motivation: the biggest
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\series bold
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potential for good solutions
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\series default
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is at architectural level.
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Exchanging a single component or a technology is typically much easier
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than changing a whole architecture, once it has been implemented.
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Often, changing an architecture is close to impossible.
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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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Starting with a particular implementation and/or with a particular technology
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in mind, and not sufficiently reasoning abouts its fundamental architecture,
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and/or
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\series bold
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not seriously considering alternative architectures
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\series default
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, is a major source of
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\series bold
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costly ill-designs
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\series default
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.
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An example may be found in section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "subsec:Example-Failures-of"
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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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.
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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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Confusion of
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\begin_inset Quotes eld
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\end_inset
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@ -565,22 +783,17 @@ implementation
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\begin_inset Quotes erd
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\end_inset
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is a major source of ill-designs, which then often cause major product
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flaws and/or operational problems.
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Be sure to understand the difference.
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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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and/or
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\begin_inset Quotes eld
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\end_inset
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Another source of costly ill-designs is starting with a particular implementatio
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n in mind, and not sufficiently reasoning abouts its fundamental architecture.
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technology
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\begin_inset Quotes erd
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\end_inset
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is another major source of ill-designs, which then often cause major product
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flaws and/or operational problems.
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Be sure to understand the differences.
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\end_layout
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\begin_layout Standard
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@ -592,7 +805,7 @@ n in mind, and not sufficiently reasoning abouts its fundamental architecture.
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\end_inset
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Recommended best practice is to (1) look at the
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Recommended best practice is to (1) look at the
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\series bold
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problem space
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\series default
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@ -604,12 +817,28 @@ set
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\series bold
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architectural solution classes
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\series default
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, and (3) look at the
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, and (3) look at each of the
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\series bold
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mappings
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\series default
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between them.
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This means: start with
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between problem space and solution space.
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The
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\emph on
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complexity
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\emph default
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of such a mapping is a first hint.
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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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In somewhat more detail: start with
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\series bold
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architectural requirements
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\series default
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@ -636,27 +865,22 @@ set
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after
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\emph default
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the former has been understood.
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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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Starting with a particular single solution in mind is almost a
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Selection of components and technologies should be the
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\emph on
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guarantee
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last
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\emph default
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for a non-optimum solution, or even a failed project, or even a disaster
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at company level when
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step during the first iteration of this method.
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Then do a
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\series bold
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enterprise-critical mass data
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quality check
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\series default
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is involved.
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at
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\emph on
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concept
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\emph default
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level.
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Often, this review will disguise some problems / limitations etc, which
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should be treated by further iterations, restarting top-down again.
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\end_layout
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\begin_layout Standard
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@ -668,7 +892,59 @@ enterprise-critical mass data
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\end_inset
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Nevertheless, don't think in waterfall models.
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During this iterative concept work, you should
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\series bold
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validate
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\series default
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your solution(s) several times, e.g.
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for
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\series bold
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compatibility
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\series default
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(no conflicts caused by restrictions, etc).
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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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Starting at the bottom with a particular single solution in mind, and/or
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presuming a certain technology, is almost a
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\emph on
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guarantee
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\emph default
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for a non-optimum solution, or even a failed project, or even a disaster
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at company level when
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\series bold
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enterprise-critical mass data
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\series default
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is involved.
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Always consider a
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\emph on
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set of
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\emph default
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candidate architectures, and for each of them, a
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\emph on
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set of
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\emph default
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solutions / technologies.
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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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Nevertheless, don't think in waterfall models.
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Always work
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\series bold
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iteratively
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@ -677,10 +953,198 @@ iteratively
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\series bold
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evolutionary
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\series default
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, but nevertheless obey the principle that any bug in an architectural ill-desig
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n cannot be fixed by the best implementation of the world.
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by
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\emph on
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re-considering architecture
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\emph default
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whenever you find problems / contradictions induced by restrictions, similar
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to the
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\series bold
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spiral model
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\series default
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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See
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\begin_inset Flex URL
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status open
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\begin_layout Plain Layout
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https://en.wikipedia.org/wiki/Spiral_model
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\end_layout
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\end_inset
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.
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\end_layout
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\end_inset
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.
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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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Be cautious when transferring
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\emph on
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software
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\emph default
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development methods to storage architectures, where operations involves
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masses of hardware.
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You need to find a balance between extreme waterfall-like and agile
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\begin_inset Foot
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status open
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\begin_layout Plain Layout
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Purely agile methods are less suited for quality ensurance of storage architectu
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res, because they are tempting people to start with simple approaches before
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the problem domain has been fully understood, increasing the
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\series bold
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||||
risk of architectural ill-designs
|
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\series default
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.
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Starting an implementation
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\emph on
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too early
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\emph default
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on basis of an ill-design can easily
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\series bold
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lead into a dead end
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\series default
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.
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Agile methods are typically encouraging
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\series bold
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early deliverables
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\series default
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., which can be counter-productive.
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Example: it is clearly a bad idea to plan for an early deliverable for
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some petabytes of storage.
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Thus architects and managers are tempted to
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\emph on
|
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start small
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\emph default
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, e.g.
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a BigCluster architecture with only 3 storage servers.
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This type of
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\begin_inset Quotes eld
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\end_inset
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early deliverable
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\begin_inset Quotes erd
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\end_inset
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cannot detect any
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\series bold
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scalability problems
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\series default
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early enough, see section
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\begin_inset CommandInset ref
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||||
LatexCommand nameref
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reference "subsec:Example-Failures-of"
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plural "false"
|
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caps "false"
|
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noprefix "false"
|
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|
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\end_inset
|
||||
|
||||
.
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So you are in a
|
||||
\series bold
|
||||
dilemma
|
||||
\series default
|
||||
, whether you like it or not.
|
||||
Although you probably dislike it, the planning phase of big storage systems
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||||
is unfortunately more like a waterfall process, by its very nature.
|
||||
Thus
|
||||
\emph on
|
||||
workarounds
|
||||
\emph default
|
||||
for the shortcomings of a pure waterfall model are needed.
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||||
German readers may also check the V-model XT, as described in
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||||
\begin_inset Flex URL
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||||
status open
|
||||
|
||||
\begin_layout Plain Layout
|
||||
|
||||
https://de.wikipedia.org/wiki/V-Modell_(Entwicklungsstandard)
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||||
\end_layout
|
||||
|
||||
\end_inset
|
||||
|
||||
.
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||||
Unfortunately, the newer XT variant of the V-model is missing in the correspond
|
||||
ing English Wikipedia article (retrieved autumn 2019), misleading readers
|
||||
with unfortunate opinions like the V-model being too similar to a waterfall
|
||||
model.
|
||||
Notice that the newer XT variant of the V-model, as well as some other
|
||||
variants (e.g.
|
||||
lecture notes from Professor Jochen Ludewig / University of Stuttgart),
|
||||
have adopted many ideas from the agile community, such as rework in loops
|
||||
and cycles, and thus should not be classifed as
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
linear waterfall
|
||||
\begin_inset Quotes erd
|
||||
\end_inset
|
||||
|
||||
models.
|
||||
In particular,
|
||||
\series bold
|
||||
early quality ensurance of concepts and architectures
|
||||
\series default
|
||||
and
|
||||
\series bold
|
||||
rework of architecture as early as possible
|
||||
\series default
|
||||
is something you definitely should borrow from the V-model and its modern
|
||||
variants, even if you dislike V-models otherwise.
|
||||
\end_layout
|
||||
|
||||
\end_inset
|
||||
|
||||
methods.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
\noindent
|
||||
\begin_inset Graphics
|
||||
filename images/MatieresCorrosives.png
|
||||
lyxscale 50
|
||||
scale 17
|
||||
|
||||
\end_inset
|
||||
|
||||
Serious bugs in an
|
||||
\emph on
|
||||
architectural
|
||||
\emph default
|
||||
ill-design (examples see section
|
||||
\begin_inset CommandInset ref
|
||||
LatexCommand nameref
|
||||
reference "subsec:Example-Failures-of"
|
||||
plural "false"
|
||||
caps "false"
|
||||
noprefix "false"
|
||||
|
||||
\end_inset
|
||||
|
||||
) are typically very hard by causing serious limitation and/or impact, and
|
||||
cannot be fixed by the best implementation, or by the best technology of
|
||||
the world.
|
||||
Be sure to understand the fundamental difference between architecture and
|
||||
its (multiple / alternative) implemenations by their respective
|
||||
its (multiple / alternative) implementations, as well as multiple technologies,
|
||||
and their respective restrictions, as well as their
|
||||
\series bold
|
||||
reach
|
||||
\series default
|
||||
|
||||
Loading…
Reference in New Issue
Block a user