mirror of https://github.com/schoebel/mars
arch-guide: rework advice
This commit is contained in:
Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
06646e8fa2
commit
c5a60faa6f
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@ -29298,6 +29298,31 @@ external experts
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\noindent
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status open
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\begin_layout Plain Layout
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\series bold
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Pitfall
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\begin_inset Quotes eld
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\end_inset
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false experts
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\begin_inset Quotes erd
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\end_layout
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\end_inset
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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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@ -29317,7 +29342,23 @@ experts
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\begin_inset Quotes erd
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\end_inset
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.
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: the internet.
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Quotation
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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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On the internet, you can find a lot of so-called
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\begin_inset Quotes eld
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\end_inset
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@ -29358,7 +29399,7 @@ real
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\emph on
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anyone
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\emph default
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can post falsely generalized
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can post incorrectly generalized
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\begin_inset Quotes eld
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\end_inset
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@ -29392,7 +29433,151 @@ proofs
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information bubbles
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\series default
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.
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\end_layout
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\begin_layout Quotation
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\begin_inset Flex Custom Color Box 1
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status open
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\begin_layout Plain Layout
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\begin_inset Argument 1
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status open
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\begin_layout Plain Layout
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\series bold
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Superfluous load balancers
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\end_layout
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\end_inset
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Good examples are HTTP or other IP-based load balancers placed in front
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of VMs.
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Almost always, this is an
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\series bold
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expensive ill-design
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\series default
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.
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\end_layout
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\begin_layout Plain Layout
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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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Notice: as long as
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\emph on
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multiple
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\emph default
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VM instances are hosted on
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\emph on
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one
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\emph default
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hypervisor iron, load balancers are most likely completely useless
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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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Reason: on SMP servers, there
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\emph on
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already exists
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\emph default
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a
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\begin_inset Quotes eld
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\end_inset
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load balancer
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\begin_inset Quotes erd
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\end_inset
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.
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The kernel and its
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\series bold
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process scheduler
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\series default
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can do even better than any external load balancer, by better distribution
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of physical CPUs to processes, and by exploitation of
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\series bold
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shared memory
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\series default
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, for example shared filesystem kernel caches, such as the Dentry Cache,
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and the fscache / Page Cache.
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Exceptions would only occur when there were per-VM global bottlenecks,
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such as interdependent processes.
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For instance, it is easy to
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\emph on
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misconfigure
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\emph default
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Apache logfiles to become such a bottleneck.
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Just fix such misconfigurations, before claiming that SMP scalability would
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be limited.
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\end_layout
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\end_inset
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.
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Instead, just assign more physical resources to a single VM.
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Only when the application load is
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\emph on
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really
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\emph default
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so high that 1 VM would fill up a hypervisor
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\emph on
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completely
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\emph default
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, only then a load balancer
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\emph on
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might
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\emph default
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be potentially useful.
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However,
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\emph on
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first
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\emph default
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check that there are enough RAM and SMP hardware threads.
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Only when state-of-the-art multi-socket CPUs with
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\begin_inset Formula $\approx128$
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\end_inset
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or more CPU threads would be insufficient for a very high connection rate,
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and after tuning measures like PHP OpCache were not sufficient, a load
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balancer or another means for load distribution
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\emph on
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could
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\emph default
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become necessary.
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\end_layout
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\begin_layout Plain Layout
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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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Even then, there are often more intelligent alternative solutions, like
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wide-area
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\emph on
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distributed
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\emph default
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\series bold
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input traffic partitioning
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\series default
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to geo-distributed servers, in place of a central load balancer acting
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as a SPOF in a single datacenter.
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For example, source-IP based routing can partition global traffic into
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per-continent datacenters, drastically reducing application traffic latencies.
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In essence, this is coarse granularity sharding at global level.
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Quotation
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@ -29404,7 +29589,411 @@ information bubbles
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\end_inset
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Real knowledge originates from evaluated sources, such as
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In a nutshell: compared to the scalability of sharding, load balancers
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would be
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\series bold
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only suitable for small-scale scalability
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\series default
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.
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However, small-scale scalability is much easier to achieve via hardware-based
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SMP = Symmetric MultiProcessing, at least in
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\emph on
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most
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\emph 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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Personally, I have never seen a situation where a load balancer was really
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necessary.
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In all example cases, they were superfluous.
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In a few cases, they were even counter-productive.
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\end_layout
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\end_inset
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cases.
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\end_layout
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\begin_layout Quotation
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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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Never start a design with a load balancer
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\emph on
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by default
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\emph default
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.
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Only use load balancers when there is
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\emph on
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well-founded strong evidence
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\emph default
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that other scalability measures won't suffice.
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In particular, it needs to be very clear that sharding is really impossible,
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which in turn implies that there exists only 1 big customer, and that its
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data cannot be partitioned at all.
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\end_layout
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\begin_layout Quotation
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\begin_inset Flex Custom Color Box 3
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status open
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\begin_layout Plain Layout
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\begin_inset Argument 1
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status open
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\begin_layout Plain Layout
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\series bold
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Cost explosion by superfluous load balancers
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\end_layout
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\end_inset
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Unnecessary load balancers are causing
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\series bold
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follow-up cost by increased complexity
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\series default
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.
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In addition to the load balancer and its administration,
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\emph on
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multiple
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\emph default
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servers and/or VMs need to be set up and administered.
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\end_layout
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\begin_layout Plain Layout
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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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If you just need a redirection mechanism, read sections
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "sec:Location-transparency"
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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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and
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "sec:Where-implement-Location-Transparency"
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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 Plain Layout
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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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For example, the traffic from BGP = Border Gateway Protocol is executed
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by your
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\series bold
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ordinary network routers
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\series default
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, without additional hardware, and they can distribute sharded traffic to
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wide-area geo-locations.
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In comparison, load balancers are just restricted
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\series bold
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overkill
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\series default
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.
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\end_layout
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\begin_layout Plain Layout
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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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Never accept a system design with a
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\emph on
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mandatory
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\emph default
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load balancer.
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It will likely imply a BigCluster-like
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\emph on
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architecture
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\emph default
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, though typically only
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\emph on
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implemented
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\emph default
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as a SmallCluster.
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\end_layout
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\end_inset
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\begin_inset Flex Custom Color Box 2
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status open
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\begin_layout Plain Layout
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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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Mandatory load balancers are often
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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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There are some rare potential exceptions, like
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\series bold
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game servers
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\series default
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rendering scenes in
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\series bold
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realtime
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\series default
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, consuming
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\emph on
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massive
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\emph default
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CPU and/or GPU power in relation to network bandwidth.
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Even there, sharding is often a better alternative.
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In contrast, ordinary video streaming typically consumes very low CPU power,
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because file streaming is executed by kernel
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\family typewriter
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sendpage()
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\family default
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and partly offloaded to DMA hardware acceleration.
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\end_layout
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\end_inset
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creating some
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\begin_inset Formula $O(n^{2})$
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\end_inset
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behaviour, showing up somewhere, often unexpectedly.
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Even when reduced to
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\begin_inset Formula $O(n)$
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\end_inset
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, load balancers are close to the
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\series bold
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opposite of sharding
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\series default
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at
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\emph on
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concept level
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\emph default
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, because they try to
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\emph on
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distribute
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\emph default
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an
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\emph on
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unpartitioned load
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\emph default
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to servers needing
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\series bold
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shared data
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\series default
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similar to DSM (see section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "subsec:Explanations-from-DSM"
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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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), instead of first
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\emph on
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partitioning the data
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\emph default
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and thus also partitioning the corresponding traffic.
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Read section
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\begin_inset CommandInset ref
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LatexCommand nameref
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reference "subsec:Error-Propagation-to"
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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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about typical
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\emph on
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real
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\emph default
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scalability and reliability.
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When this doesn't help, read 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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where the load balancer was a major
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\emph on
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source(!)
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\emph default
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of massive scalability problems.
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\end_layout
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\begin_layout Plain Layout
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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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\series bold
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Sharding
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\series default
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architectures typically don't need any load balancers, although they are
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\series bold
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massively scalable
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\emph on
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horizontally
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\series default
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\emph default
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.
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Typically, they rely on the scalability of DNS, and of IP routing.
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Notice: when DNS would reach its scalability limit, then the internet as
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such would not scale anymore.
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\end_layout
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\begin_layout Plain Layout
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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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In comparison, a load balancer is a SPOB = Single Point Of
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\series bold
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Bottleneck
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\series default
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, where the traffic must physically
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\series bold
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flow through
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\series default
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(thereby increasing hops and latencies), instead of dynamic wide-area routing.
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\end_layout
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\end_inset
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\end_layout
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\begin_layout Quotation
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\begin_inset Flex Custom Color Box 3
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status open
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\begin_layout Plain Layout
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\begin_inset Argument 1
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status open
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\begin_layout Plain Layout
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\series bold
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Load balancers vs sharding
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\end_layout
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\end_inset
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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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As a manger, if you
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\begin_inset Quotes eld
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\end_inset
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buy
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\begin_inset Quotes erd
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\end_inset
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a
|
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\emph on
|
||||
mandatory
|
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\emph default
|
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load balancer, there is a high risk for
|
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\series bold
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architecturally hindering long-term scalability
|
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\series default
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by sharding.
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\end_layout
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|
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\begin_layout Plain Layout
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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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|
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\end_inset
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|
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Check whether people are
|
||||
\emph on
|
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really
|
||||
\emph default
|
||||
experts, when they want to solve suspected(!) scalability problems via
|
||||
mandatory load balancers.
|
||||
It is just poor system design, often inducing DSM problems, and producing
|
||||
unnecessary follow-up cost.
|
||||
Unfortunately, load balancers are systematically promoted by
|
||||
\series bold
|
||||
internet information bubbles
|
||||
\series default
|
||||
.
|
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\end_layout
|
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|
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\end_inset
|
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|
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|
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\end_layout
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|
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\begin_layout Quotation
|
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\noindent
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\begin_inset Graphics
|
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filename images/lightbulb_brightlit_benj_.png
|
||||
lyxscale 12
|
||||
scale 7
|
||||
|
||||
\end_inset
|
||||
|
||||
Real knowledge originates from evaluated sources, such as
|
||||
\series bold
|
||||
scientific publications
|
||||
\series default
|
||||
|
|
@ -29444,6 +30033,10 @@ multiple
|
|||
\emph default
|
||||
ways for obtaining such information, such as measurements, simulation,
|
||||
etc.
|
||||
In addition, real experts are able to do well-founded measurements and
|
||||
deriving forecasts from them.
|
||||
Later, when it works, their forecasts were roughly correct.
|
||||
Check the quality of forecasts afterwards!
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
|
|
@ -29524,8 +30117,15 @@ dangerous mindset
|
|||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
As a responsible manager, how can you detect dangerous partly knowledge?
|
||||
Good indicators are wrong usage of the term
|
||||
As a responsible manager,
|
||||
\series bold
|
||||
how can you detect
|
||||
\series default
|
||||
dangerous partly knowledge?
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Good indicators are wrong usage of the term
|
||||
\begin_inset Quotes eld
|
||||
\end_inset
|
||||
|
||||
|
|
@ -29596,6 +30196,15 @@ risk
|
|||
of getting a non-optimum, or possibly even a bad / dangerous solution.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Another good indicator is advocacy of load balancers.
|
||||
See above boxes about the size of their real application area and their
|
||||
real value.
|
||||
Do not confuse people's belief with deep knowledge.
|
||||
The latter also requires theoretical background, in addition to practical
|
||||
experience.
|
||||
\end_layout
|
||||
|
||||
\begin_layout Standard
|
||||
Not everything which works in a garage, or in a student pool, or in the
|
||||
testlab (whether it's yours or from a commercial storage vendor), or in
|
||||
|
|
|
|||
Loading…
Reference in New Issue