Add spec for self-disabled pollster

Avoid loading pollster if no required environment

Change-Id: I5c1d6994edec171a5303244687564dd1f8466556

specs/kilo/self-disabled-pollster.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+======================
+Self-disabled Pollster
+======================
+
+https://blueprints.launchpad.net/ceilometer/+spec/self-disabled-pollster
+
+Avoid loading one pollster if required environment is not ready. Remove
+resources that cause continuous exception in polling time.
+
+
+Problem description
+===================
+
+Currently pollsters are used in following way.
+
+1. define via setup.cfg
+2. load each pollster in initialization
+3. pollster's get_samples get called in periodic way to collect metrics
+
+This fixed process is not flexible enough to handle various situation, like
+following cases:
+
+1. One pollster depends on specific environment, e.g compute node pollsters
+   need right hypervisor inspector, ipmi pollsters need ipmitool installed.
+   Loading pollster unconditionally produces extra exceptions or dummy samples
+   with minor performance drop. Check in deployment can not resolve all the
+   issues, as environment changes dynamically. The perfect solution is that
+   check in loading time and do not load the pollster if no required
+   environment.
+2. One pollster gets loaded and runs well, then some polling resource becomes
+   unavailable, so the pollster fails to produce samples and probably throws
+   exceptions with performance drop. We need detect such changes, then remove
+   this resource from polling. Also need put warning in log, so operator can
+   track and handle it.
+
+
+Proposed change
+===============
+
+Provides infrastructure to get pollster specific response then take different
+action.
+
+For case 1, need provide a function in pollster constructor, to raise specific
+exception if required hardware is missing. When loading extensions, add
+on_load_failure_callback, which checks and suppresses the exception to avoid
+loading this extension. Probably need one configuration list to indicate which
+pollster can be skipped.
+
+For case 2, we need pollsters throw 2 different exceptions in run time:
+
+* transient failure: treat like existing ones
+* permanent failure: Not poll that resources from this pollster any more
+
+To achieve this, we have PollingTask to catch that permanent failure, then
+block polling for that resources.
+
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Pipeline impact
+---------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance/Scalability Impacts
+-------------------------------
+
+This BP improve performance due to avoid loading unnecessary pollster and
+remove unavailable resources to eliminate continuous exceptions.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+
+Primary assignee:
+  edwin-zhai
+
+Other contributors:
+  lianhao-lu
+
+Ongoing maintainer:
+  edwin-zhai
+
+Work Items
+----------
+
+* For loading time pollster
+
+  * Add environment check function in pollster constructor
+  * Add new on_load_failure_callback to avoid loading failed extension
+
+* For running time pollster
+
+  * Modify required pollster to throw permanent failure exception
+  * Modify PollingTask to skip failure resources
+
+
+Future lifecycle
+================
+
+Once this feature enabled, need test and bug fixing in next 2 releases to avoid
+regression
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Need unit test
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+None
+