doc: Add details around xlator categories

Updates: bz#1193929
Change-Id: I6446bbdd8025f333a3e855032c0c7709bf4ae4cd
Signed-off-by: ShyamsundarR <srangana@redhat.com>

doc/developer-guide/xlator-classification.md

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+# xlator categories and expectations
+
+The purpose of the document is to define a category for various xlators
+and expectations around what each category means from a perspective of
+health and maintenance of a xlator.
+
+The need to do this is to ensure certain categories are kept in good
+health, and helps the community and contributors focus their efforts around the
+same.
+
+This document also provides implementation details for xlator developers to
+declare a category for any xlator.
+
+## Table of contents
+1. Audience
+2. Categories (and expectations of each category)
+3. Implementation and usage details
+
+## Audience
+
+This document is intended for the following community participants,
+- New xlator contributors
+- Existing xlator maintainers
+- Packaging and gluster management stack maintainers
+
+For a more user facing understanding it is recommended to read section [tbd](TBD)
+in the gluster documentation.
+
+## Categories
+1. Experimental (E)
+2. TechPreview  (TP)
+3. Maintained (M)
+4. Deprecated (D)
+5. Obsolete (O)
+
+### Experimental (E)
+
+Developed in the experimental branch, for exploring new features. These xlators
+are NEVER packaged as a part of releases, interested users and contributors can
+build and work with these from sources. In the future, these maybe available as
+an package based on a weekly build of the same.
+
+#### Quality expectations
+- Compiles or passes smoke tests
+- Does not break nightly experimental regressions
+  - NOTE: If a nightly is broken, then all patches that were merged are reverted
+  till the errant patch is found and subsequently fixed
+
+### TechPreview (TP)
+
+Xlators in master or release branches that are not deemed fit to be in
+production deployments, but are feature complete to invite feedback and host
+user data.
+
+These xlators will be worked upon with priority by maintainers/authors who are
+involved in making them more stable than xlators in the Experimental/Deprecated/
+Obsolete categories.
+
+There is no guarantee that these xlators will move to the Maintained state, and
+may just get Obsoleted based on feedback, or other project goals or technical
+alternatives.
+
+#### Quality expectations
+- Same as Maintained, minus
+  - Performance, Scale, other(?)
+  - *TBD* *NOTE* Need inputs, Intention is all quality goals as in Maintained,
+  other than the list above (which for now has scale and performance)
+
+### Maintained (M)
+
+These xltors are part of the core Gluster functionality and are maintained
+actively. These are part of master and release branches and are higher in
+priority of maintainers and other interested contributors.
+
+#### Quality expectations
+
+NOTE: A short note on what each of these mean are added here, details to follow.
+
+NOTE: Out of the gate all of the following are not mandated, consider the
+following a desirable state to reach as we progress on each
+
+- Bug backlog: Actively address bug backlog
+- Enhancement backlog: Actively maintain outstanding enhancement backlog (need
+        not be acted on, but should be visible to all)
+- Review backlog: Actively keep this below desired counts and states
+- Static code health:  Actively meet near-zero issues in this regard
+  - Coverity, spellcheck and other checks
+- Runtime code health: Actively meet defined coverage levels in this regard
+  - Coverage, others?
+  - Per-patch regressions
+  - Glusto runs
+  - Performance
+  - Scalability
+- Technical specifications: Implementation details should be documented and
+        updated at regular cadence (even per patch that change assumptions in
+        here)
+- User documentation: User facing details should be maintained to current
+        status in the documentation
+- Debuggability: Steps, tools, procedures should be documented and maintained
+        each release/patch as applicable
+- Troubleshooting: Steps, tools, procedures should be documented and maintained
+        each release/patch as applicable
+  - Steps/guides for self service
+  - Knowledge base for problems
+- Other common criteria that will apply: Required metrics/desired states to be
+        defined per criteria
+  - Monitoring, usability, statedump, and other such xlator expectations
+
+### Deprecated (D)
+
+Xlators on master or release branches that would be obsoleted and/or replaced
+with similar or other functionality in the next major release.
+
+#### Quality expectations
+- Retain status-quo when moved to this state, till it is moved to obsoleted
+- Provide migration steps if feature provided by the xlator is replaced with
+other xlators
+
+### Obsolete (O)
+
+Xlator/code still in tree, but not packaged or shipped or maintained in any
+form. This is noted as a category till the code is removed from the tree.
+
+These xlators and their corresponding code and test health will not be executed.
+
+#### Quality expectations
+- None
+
+## Implementation and usage details
+
+### How to specify an xlators category
+
+(TBD)
+
+### User experience using the categories
+
+#### Ability to use a category
+
+This section details which category of xlators can be used when and specifics
+around when each category is enabled.
+
+1. Maintained category xlators can be used by default, this implies, volumes
+created with these xlators enabled will throw no warnings, or need no user
+intervention to use the xlator.
+
+2. Tech Preview category xlators needs cluster configuration changes to allow
+these xlatorss to be used in volumes, further, logs will contain a message
+stating TP xlators are in use. Without the cluster configured to allow TP
+xlators, volumes created or edited to use such xlators would result in errors.
+  - (TBD) Cluster configuration option
+  - (TBD) Warning message
+  - (TBD) Code mechanics on how this is achieved
+
+3. Deprecated category xlators can be used by default, but will throw a warning
+in the logs that such are in use and will be deprecated in the future.
+  - (TBD) Warning message
+
+4. Obsolete category xlators will not be packaged and hence cannot be used from
+release builds.
+
+5. Experimental category xlators will not be packaged and hence cannot be used
+from release builds, if running experimental (weekly or other such) builds,
+these will throw a warning in the logs stating experimental xlators are in use.
+  - (TBD) Warning message
+
+#### Ability to query xlator category
+
+(TBD) Need to provide the ability to query xlator categories, or list xlators
+and their respective categories.
+
+#### User facing changes
+
+User facing changes that are expected due to this change include the following,
+- Cluster wide option to enable TP xlators, or more generically a category
+level of xlators
+- Errors in commands that fail due to invalid categories
+- Warning messages in logs to denote certain categories of xlators are in use
+- (TBD) Ability to query xlators and their respective categories