repeated check to determine host status

If the platform has a bad network stability, judging from pacemaker and
corosync, the host status would swing between up and down. It would
result in host recovery, which is not expected.

Repeated check is more reliable than once check to determine host
status.

Change-Id: I61a4a05f912c764d77bca87f5a297b6da42b9ae7
Implements: bp retry-check-when-host-failure

specs/wallaby/approved/repeated-check-to-determine-host-status.rst

@@ -0,0 +1,128 @@
+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+repeated check to determine host status
+==========================================
+
+https://blueprints.launchpad.net/masakari-monitors/+spec/retry-check-when-host-failure
+
+
+Problem description
+===================
+
+If the platform has a bad network stability, judging from pacemaker and
+corosync, the host status would swing between up and down. If the host
+monitor react quickly in this condition, it would result in host recovery,
+which is not expected.
+
+Proposed change
+===============
+
+It is imprecise to determine host status by once check. repeated checks
+is more reliable.
+
+While the host-monitor keeps a sequence of one host' latest status. The host
+status is 'down' only if the sequence of its latest status is consistently
+'down'.
+
+The length of the sequence is determined by the ``monitoring_samples``
+configuration, which has a default int value 1. It means the host status is
+'down' only if the once check status of the host is 'down'.
+
+Meanwhile, the default value of the configuration ``monitoring_interval``
+is suggested to set to 60 seconds, just the same as before.
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+
+* suzhengwei <sugar-2008@163.com>
+
+Work Items
+----------
+
+None
+
+Dependencies
+============
+
+None
+
+Testing
+=======
+
+Unit tests are needed.
+
+Documentation Impact
+====================
+
+Update user documentations.
+
+References
+==========
+
+None
+
+History
+=======
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Wallaby
+     - Introduced