oslo.middleware: EnableByFiles Healthcheck plugin

Change-Id: If6451773fb17dd99fa71f1ae79c5ae007e349932

doc/source/index.rst

@@ -7,6 +7,15 @@
  Oslo Design Specifications
 ============================
 
+Dalmatian
+=========
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+   specs/dalmatian/*
+
 Bobcat
 ========
 

specs/dalmatian/enable_by_files_healthcheck_plugin.rst

@@ -0,0 +1,139 @@
+===================================
+ Enable By Files Healthcheck Plugin
+===================================
+
+https://blueprints.launchpad.net/oslo.middleware/+spec/enable-by-files-healthcheck
+
+A generic healthcheck plugin to verify specific file path is available.
+
+Problem description
+===================
+
+The filesystem backend of glance can be used to mount NFS share as local
+filesystem, so it does not required to store any special configs at
+glance side. Glance does not care about NFS server address or NFS share
+path at all, it just assumes that each image is stored in the local
+filesystem. The downside of this assumption is that glance is not
+aware whether NFS server is connected/available or not, NFS share
+is mounted or not and just keeps performing add/delete operations
+on local filesystem directory which later might causes problem
+in synchronization when NFS is back online.
+
+Proposed change
+===============
+
+We are planning to add new plugin `enable_by_files` to `healthcheck`
+wsgi middleware which can be used by all openstack components to check
+if desired path is not present then report `503 <REASON>` error or
+`200 OK` if everything is OK.
+
+.. code-block:: ini
+
+  [app:healthcheck]
+  paste.app_factory = oslo_middleware:Healthcheck.app_factory
+  backends = enable_by_files (optional, default: empty)
+  # used by the 'enable_by_files' backend
+  enable_by_file_paths = /var/lib/glance/images,/var/lib/glance/cache (optional, default: empty)
+
+  # Use this composite for keystone auth with caching and cache management
+  [composite:glance-api-keystone+cachemanagement]
+  paste.composite_factory = glance.api:root_app_factory
+  /: api-keystone+cachemanagement
+  /healthcheck: healthcheck
+
+The middleware will return "200 OK" if everything is OK,
+or "503 <REASON>" if not with the reason of why this API should not be used.
+
+"backends" will the name of a stevedore extentions in the namespace
+"oslo.middleware.healthcheck".
+
+In addition we will also add a check to verify if both backends i.e.
+`disable_by_file` or `enable_by_files` are mentioned then we will
+raise appropriate exception which will exit the process with
+failure.
+
+Alternatives
+------------
+
+None
+
+Impact on Existing APIs
+-----------------------
+
+This new healthcheck plugin is exactly opposite of existing plugin
+`disable_by_file`. So operator needs to make sure that `disable_by_file`
+plugin should not be configured along with `enable_by_files` plugin.
+
+Security impact
+---------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Configuration Impact
+--------------------
+
+The middleware needs to be configured as shown in `Proposed Change`
+section.
+
+Developer Impact
+----------------
+
+None
+
+Testing Impact
+--------------
+
+Middleware will be covered by the unittest
+And also have a tempest test for each services that have integrated it.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  Abhishek Kekane (abhishek-kekane)
+
+Other contributors:
+  None
+
+Milestones
+----------
+
+Target Milestone for completion:
+
+* Dalmatian-3
+
+Work Items
+----------
+
+* Write the `enable_by_files` healthcheck plugin
+* Update the applications to use it
+
+Documentation Impact
+====================
+
+N/A
+
+Dependencies
+============
+
+N/A
+
+References
+==========
+
+* Glance NFS improvement spec - https://review.opendev.org/917284
+* EnableByFiles plugin PoC - https://review.opendev.org/919666
+
+.. note::
+
+  This work is licensed under a Creative Commons Attribution 3.0
+  Unported License.
+  http://creativecommons.org/licenses/by/3.0/legalcode