[ClusterLabs] How to implement a fencing agent

[Ryan Thomas]

Thanks for the advice and information.

>> 1. The documentation encourages the use of the python fencing library.
>>    How does one install this?> Fencing library is basically this file
(sans the doubled extension):
> https://github.com/ClusterLabs/fence-agents/blob/v4.2.1/lib/fencing.py.py
<https://github.com/ClusterLabs/fence-agents/blob/v4.2.1/lib/fencing.py.py>
> that shall be assurably in predefined/run-time-mangled
> search path for Python modules (sys.path), so that you can then
> "from fencing import ...".  You can follow the native agents for
> how it is to be used.

My system did not have a fencing.py installed. However, I found that after
running "yum install fence-agents-common.x86_64", fencing.py could be found
in /usr/share/fence as you said.  Is "yum install
fence-agents-common.x86_64" the appropriate way to install the fencing.py
library? It seems like fencing.py being installed was a side-effect, rather
than explictly being installed. Also, although the fencing.py did appear,
it is not in the python path.  So "import fencing" fails with "ImportError:
No module named fencing".
  I can of course add /usr/share/fence to the python path.

>> 2. How to a “register” the agent so it shows up in the “pcs stonith
>>    list” output, and the “stonith_admin –list-installed” output?
>>    So, I can use my new agent?
> Living convention for this is that whatever in /usr/sbin
>  (symbolically in pacemaker context: RH_STONITH_DIR) named in way
>  that conforms "fence_*" glob pattern (symbolically RH_STONITH_PREFIX)
>  will qualify automatically.  Obviously, it shall be marked executable.

Following your advice, I ran "echo 'import fencing' > /usr/sbin/fence_foo"
and "chmod +x /usr/sbin/fence_foo".  After doing this the "stonith_admin
--list-installed" did list fence_foo.  However, the "pcs stonith list" >
did not, so there must be something more that needs to be done for the
fence agent to be found by pcs.

Thanks,
Ryan

On Thu, Aug 9, 2018 at 8:09 AM, Jan Pokorný <jpokorny at redhat.com> wrote:

> On 09/08/18 07:59 +0200, Ulrich Windl wrote:
> >>>> Ryan Thomas <developmentrjt at gmail.com> schrieb am 08.08.2018 um
> 23:26 in
> > Nachricht
> > <CAE_QAjkDx15bR=eb12jCxLX9e0BaGex+zjsTztWQnOugwsKK3w at mail.gmail.com>:
> >> I’m attempting to implement a fencing agent.
> >>
> >> The ClusterLabs/fence-agent github repo has some helpful information
> >> including fence-agents/doc/FenceAgentAPI.md, but I haven’t found the
> >> answers to a few basic questions.
> >
> > Personally I think a specification should not recommend any library to
> use,
> > but state in simple and precise words what the thing (=fencing agent)
> > is expected to do.
>
> +1 for a complete decoupling of API specification and referential
> library, it's pretty possible there are still some undocumented bits
> relied upon while only hardwired into the fencing library.  Note that
> pacemaker adds a thin configuration surface and decision logic on top
> of the base line set forth in the mentioned document.
>
> Also, mentioning Python below, there are certainly reasons why it can
> be more blessed than say shell scripting, though I stay vocal that
> transition from compiled fence agents was rather a regress than
> progress (not to speak about original Linux HA project going as far as
> to have ready-to-use agents preloaded in memory to achieve
> unprecedented robustness -- please correct me if I get these gory
> details about the past wrong).
>
> The problem with Python is that even for a trivial proxying of the
> operations towards 3rd party tooling (e.g. ipmitool), the costs of
> bringing the run-time up alone is unexpectedly I/O intensive (trying
> to open plethora files, preinitializing some objects that won't be
> used, anyway, etc.), with excessive complexity of the task/memory
> footprint proportion, and currently also carrying a risk of being
> affected/delayed with user customization of the Python deployment
> that gets loaded automatically unless suppressed [1].  That's not
> very aligned with the goal of preventing dangerous access to shared
> resources in timely and reliable fashion.
>
> Perhaps a reasonable tradeoff at this point would be to consider
> MicroPython, alternative lightweight implementation with a trimmed
> standard library: https://micropython.org/.  As a first step, fencing
> library would likely require some touches for compatibility, though.
>
> > Inspecting the source of SBD I found out that there is too much
> > undocumented magic implemented.
>
> >> 1. The documentation encourages the use of the python fencing library.
> >>    How does one install this?
>
> Fencing library is basically this file (sans the doubled extension):
> https://github.com/ClusterLabs/fence-agents/blob/v4.2.1/lib/fencing.py.py
> that shall be assurably in predefined/run-time-mangled
> search path for Python modules (sys.path), so that you can then
> "from fencing import ...".  You can follow the native agents for
> how it is to be used.
>
> For instance, EL family of distros will provide that file as
> /usr/share/fence/fencing.py by the means of fence-agents-common
> package.
>
> >> 2. How to a “register” the agent so it shows up in the “pcs stonith
> >>    list” output, and the “stonith_admin –list-installed” output?
> >>    So, I can use my new agent?
>
> Living convention for this is that whatever in /usr/sbin
> (symbolically in pacemaker context: RH_STONITH_DIR) named in way
> that conforms "fence_*" glob pattern (symbolically RH_STONITH_PREFIX)
> will qualify automatically.  Obviously, it shall be marked executable.
>
>
> Hope this helps.
>
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1600893
>
> --
> Nazdar,
> Jan (Poki)
>
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