mirror of https://github.com/schoebel/mars
user-manual: rework scripting advice
This commit is contained in:
Thomas Schoebel-Theuer
Thomas Schoebel-Theuer
parent
72c1ad5d78
commit
2cde907480
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@ -31718,6 +31718,193 @@ Same as the
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firmly built in.
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\end_layout
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\begin_layout Section
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Scripting Advice
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:Scripting-HOWTO"
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\end_inset
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\end_layout
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\begin_layout Standard
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Both the
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\series bold
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asynchronous communication model
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\series default
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of MARS including the Lamport clock, and the
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\series bold
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state model
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\series default
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(cf section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:The-State-of"
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\end_inset
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) is something you
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\emph on
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definitely
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\emph default
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should have in mind when you want to do some scripting.
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Here is some advice:
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\end_layout
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\begin_layout Itemize
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Don't access anything on
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\family typewriter
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/mars/
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\family default
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directly, except for debugging purposes.
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Use
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\family typewriter
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marsadm
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\family default
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.
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\end_layout
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\begin_layout Itemize
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Avoid running scripts in parallel, other than for inspection / monitoring
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purposes.
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When you give two
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\family typewriter
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marsadm
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\family default
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commands in parallel (whether on the same host, or on different hosts belonging
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to the same cluster), it is possible to produce a mess.
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\family typewriter
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marsadm
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\family default
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has no internal locking.
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There is no cluster-wide locking at all, because if would cause trouble
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during long-distance network outages.
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Unfortunately, some systems like Pacemaker are violating this in many cases
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(depending on their configuration).
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Best is if you have a dedicated / more or less centralized
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\series bold
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control machine
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\series default
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which controls masses of your georedundant working servers.
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This reduces the risk of running interfering actions in parallel.
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Of course, you need backup machines for your control machines, and in different
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locations.
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Not obeying this advice can easily lead to problems such as complex races
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which are very difficult to solve in long-distance distributed systems,
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even in general (not limited to MARS).
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\end_layout
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\begin_layout Itemize
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\family typewriter
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marsadm wait-cluster
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\family default
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is your friend.
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Whenever your (near-)central script has to switch between different hosts
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\family typewriter
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A
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\family default
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and
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\family typewriter
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B
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\family default
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(of the same cluster), use it in the following way:
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\begin_inset Newline newline
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\end_inset
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\family typewriter
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ssh A
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\begin_inset Quotes eld
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\end_inset
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marsadm action1
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\begin_inset Quotes erd
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\end_inset
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; ssh B
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\begin_inset Quotes eld
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\end_inset
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marsadm wait-cluster; marsadm action2
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\begin_inset Quotes erd
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\end_inset
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\begin_inset Newline newline
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\end_inset
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\family default
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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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Don't ignore this advice! Interference is almost
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\emph on
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sure
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\emph default
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! As a rule of thumb, precede almost any action command with some appropriate
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waiting command!
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\end_layout
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\begin_layout Itemize
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Further friends are any
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\family typewriter
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marsadm wait-*
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\family default
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commands, such as
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\family typewriter
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wait-umount
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\family default
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.
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\end_layout
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\begin_layout Itemize
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In some places, busy-wait loops might be needed, e.g.
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for waiting until a specific resource is
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\family typewriter
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UpToDate
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\family default
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or matches some other condition.
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Examples of waiting conditions can be found under
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\family typewriter
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github.com/schoebel/test-suite
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\family default
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in subdirectory
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\family typewriter
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mars/modules/
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\family default
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, specifically
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\family typewriter
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02_predicates.sh
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\family default
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or similar.
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\end_layout
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\begin_layout Itemize
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In case of network problems, some command may hang (forever), if you don't
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set the
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\family typewriter
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--timeout=
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\family default
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option.
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Don't forget the check the return state of any failed / timeouted commands,
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and to take appropriate measures!
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\end_layout
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\begin_layout Itemize
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Test your scripts in failure scenarios!
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\end_layout
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\begin_layout Chapter
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Troubleshooting
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\end_layout
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@ -32014,199 +32201,6 @@ name "chap:The-Macro-Processor"
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\end_layout
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\begin_layout Section
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Scripting HOWTO
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\begin_inset CommandInset label
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LatexCommand label
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name "sec:Scripting-HOWTO"
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\end_inset
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\end_layout
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\begin_layout Standard
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Both the
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\series bold
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asynchronous communication model
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\series default
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of MARS (cf section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:The-Lamport-Clock"
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\end_inset
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) including the Lamport clock, and the
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\series bold
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state model
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\series default
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(cf section
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\begin_inset CommandInset ref
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LatexCommand ref
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reference "sec:The-State-of"
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|
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\end_inset
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|
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) is something you
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\emph on
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definitely
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\emph default
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should have in mind when you want to do some scripting.
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Here is some further concrete advice:
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\end_layout
|
||||
|
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\begin_layout Itemize
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Don't access anything on
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\family typewriter
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/mars/
|
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\family default
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directly, except for debugging purposes.
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Use
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\family typewriter
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marsadm
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\family default
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.
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\end_layout
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\begin_layout Itemize
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Avoid running scripts in parallel, other than for inspection / monitoring
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purposes.
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When you give two
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\family typewriter
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marsadm
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\family default
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commands in parallel (whether on the same host, or on different hosts belonging
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to the same cluster), it is very likely to produce a mess.
|
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\family typewriter
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marsadm
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\family default
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has no internal locking.
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There is no cluster-wide locking at all.
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Unfortunately, some systems like Pacemaker are violating this in many cases
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(depending on their configuration).
|
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Best is if you have a dedicated / more or less centralized
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\series bold
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control machine
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\series default
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which controls masses of your georedundant working servers.
|
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This reduces the risk of running interfering actions in parallel.
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Of course, you need backup machines for your control machines, and in different
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locations.
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Not obeying this advice can easily lead to problems such as complex races
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which are very difficult to solve in long-distance distributed systems,
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even in general (not limited to MARS).
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\end_layout
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||||
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\begin_layout Itemize
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\family typewriter
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marsadm wait-cluster
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\family default
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is your friend.
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Whenever your (near-)central script has to switch between different hosts
|
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\family typewriter
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A
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\family default
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and
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\family typewriter
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B
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\family default
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(of the same cluster), use it in the following way:
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\begin_inset Newline newline
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\end_inset
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|
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\family typewriter
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ssh A
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\begin_inset Quotes eld
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\end_inset
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marsadm action1
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\begin_inset Quotes erd
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\end_inset
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; ssh B
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\begin_inset Quotes eld
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\end_inset
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marsadm wait-cluster; marsadm action2
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\begin_inset Quotes erd
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\end_inset
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\begin_inset Newline newline
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\end_inset
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\family default
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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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Don't ignore this advice! Interference is almost
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\emph on
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sure
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\emph default
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! As a rule of thumb, precede almost any action command with some appropriate
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waiting command!
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\end_layout
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\begin_layout Itemize
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Further friends are any
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\family typewriter
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marsadm wait-*
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\family default
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commands, such as
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\family typewriter
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wait-umount
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\family default
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.
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\end_layout
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\begin_layout Itemize
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In some places, busy-wait loops might be needed, e.g.
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for waiting until a specific resource is
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\family typewriter
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UpToDate
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\family default
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or matches some other condition.
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Examples of waiting conditions can be found under
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\family typewriter
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github.com/schoebel/test-suite
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\family default
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in subdirectory
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\family typewriter
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mars/modules/
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\family default
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, specifically
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\family typewriter
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02_predicates.sh
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\family default
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or similar.
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\end_layout
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\begin_layout Itemize
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In case of network problems, some command may hang (forever), if you don't
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set the
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\family typewriter
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--timeout=
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\family default
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option.
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Don't forget the check the return state of any failed / timeouted commands,
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and to take appropriate measures!
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\end_layout
|
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\begin_layout Itemize
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Test your scripts in failure scenarios!
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\end_layout
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\begin_layout Chapter
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||||
The Sysadmin Interface (
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\family typewriter
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|
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