Pacemaker's XML schema has a version of its own, independent of the version of Pacemaker itself.

A versioned schema offers transparent backward and forward compatibility.

• It reflects the timeline of schema-backed features (introduction, changes to the syntax, possibly deprecation) through the versioned stable schema increments, while keeping schema versions used by default by older Pacemaker versions untouched.

• Pacemaker internally uses the latest stable schema version, and relies on supplemental transformations to promote cluster configurations based on older, incompatible schema versions into the desired form.

• It allows experimental features with a possibly unstable configuration interface to be developed using the special next version of the schema.

Each logical portion of the schema goes into its own RNG file, named like ${base}-${X}.${Y}.rng. ${base} identifies the portion of the schema (e.g. constraints, resources); ${X}.${Y} is the latest schema version that contained changes in this portion of the schema.

The complete, overall schema, pacemaker-${X}.${Y}.rng, is automatically generated from the other files via the Makefile.

Experimental features go into ${base}-next.rng where ${base} is the affected portion of the schema. If such a file does not already exist, create it by copying the most recent ${base}-${X}.${Y}.rng.

Pacemaker will not use the experimental schema by default; the cluster administrator must explicitly set the validate-with property appropriately to use it.

The current stable version is determined at runtime when crm_schema_init() scans the CRM_SCHEMA_DIRECTORY.

It will have the form pacemaker-${X}.${Y} and the highest ${X}.${Y} wins.

When the new syntax is a simple addition to the previous one, create a new entry, incrementing ${Y}.

When the new syntax is not a simple addition to the previous one, create a new entry, incrementing ${X} and setting ${Y} = 0.

An XSLT file is also required that converts an old syntax to the new one and must be named upgrade-${Xold}.${Yold}.xsl.

See xml/upgrade-1.3.xsl for an example.

Since xml/upgrade-2.10.xsl, rather self-descriptive approach is taken, separating metadata of the replacements and other modifications to perform from the actual executive parts, which is leveraged, e.g., with the on-the-fly overview as obtained with ./regression.sh -X test2to3. Also this was the first time particular key names of nvpairs, i.e. below the granularity of the schemas so far, received attention, and consequently, no longer expected names became systemically banned in the after-upgrade schemas, using <except> construct in the data type specification pertaining the affected XML path.

The implied complexity also resulted in establishing a new compound, stepwise transformation, alleviating the procedural burden from the core upgrade recipe. In particular, id-ref based syntactic simplification granted in the CIB format introduces nonnegligible internal "noise" because of the extra indirection encumbered with generally non-bijective character of such a scheme (context-dependent interpretation). To reduce this strain, a symmetric arrangement is introduced as a pair of enter/leave (pre-upgrade/post-upgrade) transformations where the latter is meant to eventually reversibly restore what the former intentionally simplified (normalized) for upgrade transformation's peruse. It's optional (even the post-upgrade counterpart is optional alone) and depends on whether the suitable files are found along the upgrade transformation itself: e.g., for upgrade-2.10.xsl, such files are upgrade-2.10-enter.xsl and upgrade-2.10-leave.xsl. Note that unfolding + refolding id-ref shortcuts is just a practically imposed individual case of how to reversibly make the configuration space tractable in the upgrade itself, allowing for more sophistication down the road.

1. Copy the most recent version of ${base}-*.rng to ${base}-${X}.${Y}.rng, such that the new file name increments the highest number of any schema file, not just the file being edited.
2. Commit the copy, e.g. "Low: xml: clone ${base} schema in preparation for changes". This way, the actual change will be obvious in the commit history.
3. Modify ${base}-${X}.${Y}.rng as required.
4. If required, add an XSLT file, and update xslt\_SCRIPTS in xml/Makefile.am.
5. Commit
6. make -C xml clean; make -C xml all to rebuild the schemas in the local source directory.
7. The CIB validity regression tests will break after the schema is updated. Run cts/cts-cli -s to make the referential outcomes reflect the transient changes made so far, git diff to ensure the these changes look sane, then commit them.
8. Similarly, with the new major version ${X}, it's advisable to refresh scheduler tests at some point, see the instructions in cts/README.md.

New features will not be available until the cluster administrator:

1. Updates all the nodes
2. Runs the equivalent of cibadmin --upgrade --force

From the source directory, run make -C xml diff to see the changes in the current schema (compared to the previous ones) and also the pending changes in pacemaker-next. Alternatively, if the intention is to grok the overall historical schema evolution, use make -C xml fulldiff.