Add incremental RST-to-MyST swap mechanism (#1857)

* feat: add swap_sources.py for incremental RST-to-MyST migration

Pre-build swap/restore script that renames md-{name}.md → {name}.md
before Sphinx builds and restores after. Includes state tracking,
exclude file generation, collision detection, and partial-failure
rollback. 10 tests cover all specified behaviors plus rollback path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat: add import_myst.py for importing MyST files from myst/* branches

Adds scripts/import_myst.py with import_page, git_show, list_myst_files,
list_rst_files, and do_import. Imported files are written as md-{name}.md
alongside existing RST files; importing is decoupled from swap activation.

Adds tests/test_import_myst.py covering single-page write, identical-skip,
warn-on-different-without-force, force-overwrite, and nested-path creation.
All 5 tests pass on Python 3.9.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat: add MyST swap exclude patterns and directive config to conf.py

🤖 Generated by [robots](https://vyos.io)

* feat: add swap-wrapped rendering targets to Makefile

🤖 Generated by [robots](https://vyos.io)

* feat: add swap pre/post build hooks for ReadTheDocs

🤖 Generated by [robots](https://vyos.io)

* feat: add empty _swap.txt, remove atexit from swap script

The atexit handler in --swap mode caused immediate restore on process
exit, breaking standalone usage. Makefile trap and RTD post_build
handle restore reliably.

🤖 Generated by [robots](https://vyos.io)

* feat: activate quick-start as MyST canary via swap mechanism

Imports docs/md-quick-start.md from origin/myst/current and adds
quick-start to docs/_swap.txt. Validates the swap pipeline end-to-end
on one page: import_myst pulls the MD via git show, swap_sources
renames md-quick-start.md to quick-start.md, sphinx-build renders
quick-start.html with zero MD-specific warnings, and restore reverses
the rename cleanly.

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* feat: activate 106 visual-validated canaries via swap

Imports 105 MD files (plus quick-start already present) from
origin/myst/current and adds them to docs/_swap.txt. The selection
is the BackstopJS visual-passers cohort: pages with <5% rendered
diff vs the live RST docs at docs.vyos.io/en/latest/, filtered to
those with an RST counterpart on current and no cmdincludemd usage
(template-format reconciliation pending).

Local sphinx-build with all 106 swapped: succeeded with 100
warnings (vs 95 baseline). The 5 new warnings are all undefined
cross-reference labels, not build failures:
- contributing/development.md (missing 'coding-guidelines')
- operation/upgrade-recovery.md (3 missing 'how_it_works' /
  'cancelling_recovery')
- vpp/configuration/dataplane/{buffers,memory,unix}.md (missing
  'vpp_config_dataplane_*' labels)

Source list: ~/.claude/projects/-Users-vybot-GitHub-vyos-documentation/docs/2026-04-29-myst-conversion-audit/visual-passers-under-5pct.txt
BackstopJS report: claude/gifted-hertz-74b9f9 worktree
(visual-compare/), 2026-04-23 vs vyos--1838.org.readthedocs.build.

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* fix: re-import 4 canary md-*.md files with xref label fixes

Re-imports the dash-form-corrected versions of:
- contributing/md-development.md (added (coding-guidelines)= anchor)
- operation/md-upgrade-recovery.md (3 ref renames: how_it_works /
  cancelling_recovery -> dash form)
- vpp/configuration/dataplane/md-buffers.md (vpp_config_dataplane_physmem
  -> vpp-config-dataplane-physmem)
- vpp/configuration/dataplane/md-unix.md
  (vpp_config_dataplane_interface_rx_mode
  -> vpp-config-dataplane-interface-rx-mode)

Source: origin/myst/current commit 59fbe3ea. Verified locally: clean
swap-build no longer reports any of the 5 target labels (1 of 6 —
vpp-config-hugepages — remains because system.md isn't in the canary
swap list; that anchor lives there).

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* fix: re-add 4 canary md-*.md files deleted by 242b334a

Commit 242b334a accidentally staged deletions instead of modifications
because the working tree had unprefixed *.md files left over from an
incomplete swap-restore cycle. Re-imports the same 4 files from
origin/myst/current with the xref label fixes applied:

- contributing/md-development.md       — (coding-guidelines)= anchor
- operation/md-upgrade-recovery.md     — how_it_works → how-it-works,
                                         cancelling_recovery → cancelling-recovery
- vpp/configuration/dataplane/md-buffers.md — vpp_config_dataplane_physmem
                                              → vpp-config-dataplane-physmem
- vpp/configuration/dataplane/md-unix.md   — vpp_config_dataplane_interface_rx_mode
                                              → vpp-config-dataplane-interface-rx-mode

Source: origin/myst/current commit 59fbe3ea.

🤖 Generated by [robots](https://vyos.io)

* fix: resolve remaining xref label gaps in swap-active build

Three small additions clear the cross-reference warnings tied to
underscore-vs-dash label form mismatches and the vpp-config-hugepages
reference that previously needed system.md in the canary set.

- system.rst: add .. _vpp-config-hugepages: alongside the existing
  underscore label so memory.md references resolve regardless of
  whether system.md is swap-active.
- md-lcp.md: add (vpp_config_dataplane_lcp_ignore-kernel-routes)=
  alongside dash form (carries upstream from myst/current 079fa786).
- md-memory.md: add (vpp_config_dataplane_memory)= alongside dash
  form (also from myst/current 079fa786).

Local clean swap-build with 106 canaries:
  before: 305 warnings, 8 undefined-label entries in our scope
  after:  300 warnings, 0 undefined-label entries in our scope

Remaining undefined-label warnings (release-notes, prepare_commit)
are in documentation.rst and unrelated to the canary swap mechanism.

🤖 Generated by [robots](https://vyos.io)

* fix: re-add md-lcp.md and md-memory.md (deleted by 870c9e7e)

Same disaster pattern as 242b334a: a swap-restore cycle left
unprefixed *.md files in the working tree, and the subsequent
git add staged deletions instead of modifications. Restoring the
two affected md-*.md files from origin/myst/current 079fa786
(which has the dual underscore+dash anchors needed for the
swap-active build).

🤖 Generated by [robots](https://vyos.io)

* feat: expand canaries to 114; refresh 3 with cfgcmd body fix

Adds 8 new visual-validated canaries from the post-cfgcmd-fix
BackstopJS run (2026-04-29):

- configuration/policy/as-path-list
- configuration/policy/community-list
- configuration/policy/extcommunity-list
- configuration/policy/large-community-list
- configuration/policy/local-route
- configuration/policy/prefix-list
- configuration/service/salt-minion
- configuration/system/updates

Refreshes 3 existing canaries whose MD content changed via the
cfgcmd/opcmd single-line body fix on myst/current fc19ab5c:

- configuration/firewall/global-options
- configuration/firewall/groups
- configuration/policy/route

All 11 sourced from origin/myst/current. Net: 106 -> 114 canaries.

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* fix: re-import md-cloud-init.md (block 3 fix from myst/current)

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* feat(swap): import .md files and webp transition from myst/current

Selective import from origin/myst/current (cf9c9b34):
- Add/update 255 .md files (full MyST conversion plus webp ref updates)
- Delete 175 PNG/JPG from docs/_static/images (webp twins already present)
- Delete 5 autotest topology.png (webp twins already present)

Preserved on swap (untouched):
- All .rst files (incremental swap pattern)
- conf.py, _ext/, _include/*.txt, .gitignore
- 115 canary md-*.md files
- 7 superpowers/specs/*.md design docs
- Logos vyos-logo.png / vyos-logo-icon.png (referenced by conf.py)

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* chore(swap): remove canary md-*.md files and docs/superpowers

- Remove 115 canary md-*.md files (incremental swap helpers no longer needed)
- Remove 8 files under docs/superpowers (project planning/design docs that
  shouldn't ship in the documentation tree)

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* docs: address Copilot review feedback on imported MyST pages

Fix issues flagged by Copilot review on PR #1857 (the same content lives
in myst/current as the canonical source):

Real bugs:
- site-2-site-cisco.md: replace curly quote (U+2019) with ASCII apostrophe
- rsa-keys.md: fix typo "key-pair nam>>" → "key-pair name>"
- vmware.md: lowercase admonition directive (:::{NOTE} → :::{note})
- vpp/configuration/nat/index.md: remove blank line inside {include} fence

Grammar:
- vpp/configuration/interfaces/loopback.md: "bounded" → "bound"
- vpp/configuration/sflow.md: "VyOS support" → "VyOS supports"
- vpp/requirements.md: "bypass" → "bypasses"
- vpp/configuration/dataplane/interface.md: "configures" → "configure"

CI linter (IP addresses):
- nmp.md: wrap 8.8.8.8 example with stop/start_vyoslinter
- lac-lns.md: wrap LNS config block (contains 8.8.8.8)
- wan-load-balancing.md: wrap whole file (illustrative non-RFC IPs)
- policy/examples.md: replace 192.0.1.1 with RFC 5737 192.0.2.1

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* fix(swap): address Copilot review feedback on swap infrastructure

Category D — drop obsolete canary mechanism settings:
- conf.py: remove '**/md-*.md' from exclude_patterns (no canaries left)
- Makefile: replace malformed '*/_build/*' with '$(BUILDDIR)/**' and drop
  the '*/md-*' ignore (canary files no longer exist)

Category C — script robustness:
- import_myst.py:
  * list_myst_files() now raises SystemExit on git ls-tree failure instead
    of silently returning [] (would have masked typo'd --source refs)
  * list_rst_files() skips _build/ when scanning for .rst stems
  * import_page() rejects stems containing '..' or absolute paths and
    re-checks that the resolved destination stays under docs_dir
  * --dry-run uses a separate "would_import" counter; summary line now
    distinguishes dry-run from actual imports
- swap_sources.py:
  * parse_swap_list() reads with explicit encoding='utf-8'
  * do_restore() validates state file version + entry shape before
    renaming files; raises with actionable message on corruption
  * State file reads/writes use explicit encoding='utf-8' throughout

_swap.txt:
- Wrap long comment line to satisfy 80-character doc-linter limit

🤖 Generated by [robots](https://vyos.io)

* refactor(swap): rename imported .md files to md- prefix for swap mechanism

Restore the canary file naming convention that swap_sources.py expects:
the imported MyST pages now live as docs/<dir>/md-<name>.md alongside
the existing docs/<dir>/<name>.rst, so swap_sources.py --swap can rename
them into place at build time.

- 254 .md files renamed (every page with a matching .rst counterpart)
- 2 MyST-only pages left at their final names (no .rst exists, no swap
  needed): docs/copyright.md, docs/automation/terraform/terraformvyos.md

All 114 stems listed in docs/_swap.txt now have a corresponding
md-<name>.md source file ready to swap in.

🤖 Generated by [robots](https://vyos.io)

* docs: address CodeRabbit review feedback on imported MyST pages

Fix issues flagged by CodeRabbit on PR #1857. All issues are pre-existing
in the upstream RST docs and inherited by the MyST conversion.

Real bugs:
- inter-vrf-routing-vrf-lite.md: invalid IPv6 next-hop "2001:db8::*" →
  "2001:db8::1"
- ipsec-pa-route-based.md: vendor mislabel "Cisco" → "Palo Alto"
  (header on line 39 and "Monitoring on Cisco side" section heading)
- bgp-ipv6-unnumbered.md: AS number mismatch between configuration and
  verification output for both routers (Router A: 65020 → 64496;
  Router B: 65021 → 64499)
- qos.md: class 30 used "match ADDRESS20" instead of ADDRESS30 — broke
  the documented pattern (classes 10/20/30 → ADDRESS10/20/30)

Security:
- OpenVPN_with_LDAP.md: redact full PEM private key material from the
  three "set pki ... private key '...'" lines and from the embedded
  OpenVPN client <key> block; replace with <REDACTED> / ...REDACTED...
  placeholders. Public certificates retained.

🤖 Generated by [robots](https://vyos.io)

* feat(swap): default to serving MyST for all swapped pages

Replace the previously-curated 114-stem _swap.txt with the full set of
254 imported md-prefixed pages, so MD is served by default at build
time. To revert any specific page back to RST, remove its stem from
_swap.txt (or comment it out).

🤖 Generated by [robots](https://vyos.io)

* fix(ext): handle RST fallback in CmdInclude when _renderer absent

`cmdincludemd` is in `myst_fence_as_directive`, so MyST routes
fence blocks through `render_fence → render_restructuredtext →
MockRSTParser`. In that path `self.state` is a plain docutils Body
with no `_renderer`, crashing the build.

Fall back to `nested_parse` when `_renderer` is unavailable so the
directive works in both MyST and RST/MockRSTParser contexts.

🤖 Generated by [robots](https://vyos.io)

* feat(conf): copy .md sources into HTML output for plain-text serving

Adds a build-finished hook that mirrors every .md file from the Sphinx
source tree into the HTML output directory verbatim, making unrendered
MyST sources accessible alongside HTML renders at the same URL path.

🤖 Generated by [robots](https://vyos.io)

* docs: address review feedback from PR #1857

Fix conversion artifacts, typos, grammar errors, and technical
inaccuracies flagged by automated code review (Copilot + CodeRabbit).

Infrastructure: add root-level md-*.md exclusion to conf.py,
fix sphinx-autobuild ignore globs in Makefile.

Content: fix curly quotes, invalid Go panic() calls, shell quoting
in cURL examples, incorrect firewall command paths, typos across
22 documentation files, remove duplicate sections.

🤖 Generated by [robots](https://vyos.io)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 549 insertions, 0 deletions

diff --git a/docs/contributing/md-development.md b/docs/contributing/md-development.md
new file mode 100644
index 00000000..8d2cec40
--- /dev/null
+++ b/docs/contributing/md-development.md
@@ -0,0 +1,549 @@
+---
+lastproofread: '2025-12-12'
+---
+
+(development)=
+
+# Development
+
+Learn how to contribute to VyOS.
+
+(architecture-overview)=
+
+## Architecture overview
+
+VyOS source code is hosted on GitHub in the VyOS organization:
+<https://github.com/vyos>
+
+VyOS is composed of multiple modules spread across different
+repositories. Some modules contain forks of upstream
+packages and are periodically synced.
+VyOS consolidates most packages into the
+[vyos-1x](https://github.com/vyos/vyos-1x)
+repository while maintaining a consistent structure.
+The base code is being rewritten
+from Perl and Bash to Python using an XML-based CLI interface definition.
+
+VyOS ISO build scripts are hosted in the
+[vyos-build](https://github.com/vyos/vyos-build) repository. See the
+`vyos-build` repository
+[README.md file](https://github.com/vyos/vyos-build/blob/current/README.md)
+for more information on building VyOS ISO images.
+
+## Contributing code
+
+:::{warning}
+You must sign the {doc}`Contributor License Agreement<cla>`
+for your contributions to be accepted.
+:::
+
+VyOS is open-source and welcomes patches.
+All submissions must adhere to these guidelines:
+
+- Each commit addresses a single issue or feature.
+- Each commit message references a [Phabricator](https://vyos.dev/) task ID
+  (for example, `T1234`).
+- Each commit is associated with a username and email address
+  to identify the author (see [Configure your Git identity](configure-your-git-identity)).
+- Only submit bugfixes in packages other than <https://github.com/vyos/vyos-1x>.
+- Commits follow the [coding guidelines](coding-guidelines) outlined below.
+
+### Determining package ownership
+
+To determine which VyOS package contains a file you want to modify, use Debian's
+`dpkg -S` command on your running VyOS installation.
+
+### Submitting your code
+
+Fork the repository and submit a GitHub pull request. This is the preferred way
+to contribute changes to VyOS.
+
+To fork a VyOS repository:
+
+1. Append `/fork` to the repository URL on GitHub. For example, to fork
+   `vyos-1x`, use: <https://github.com/vyos/vyos-1x/fork>
+
+2. Clone your fork or add it as a remote to your local repository:
+
+   - Clone: `git clone https://github.com/<user>/vyos-1x.git`
+   - Add remote: `git remote add myfork https://github.com/<user>/vyos-1x.git`
+
+(configure-your-git-identity)=
+
+3. Configure your Git identity:
+
+   ```none
+   git config --global user.name "J. Random Hacker"
+   git config --global user.email "jrhacker@example.net"
+   ```
+
+4. Make your changes and add files to the Git index:
+
+   - Single file: `git add myfile`
+   - Directory: `git add somedir/*`
+
+5. Commit your changes with a meaningful headline and [Phabricator](https://vyos.dev/) reference:
+
+   `git commit`
+
+6. Push to your fork and create a GitHub pull request:
+
+   `git push`
+
+Alternatively, you can export commits as patches and send them to
+[maintainers@vyos.net](mailto:maintainers@vyos.net) or attach them directly to the [Phabricator](https://vyos.dev/) task:
+
+- Export last commit: `git format-patch`
+- Export last two commits: `git format-patch -2`
+
+## Commit messages
+
+For guidance on writing commit messages, review the file history
+with `git log path/to/file.txt`.
+
+Every change must be associated with a task number (prefixed with **T**) and
+a component. If no bug report or feature request exists for your changes,
+create a [Phabricator](https://vyos.dev/) task first. Reference the task ID in your commit message:
+
+- `ddclient: T1030: auto create runtime directories`
+- `Jenkins: add current Git commit ID to build description`
+
+If your pull request lacks a [Phabricator](https://vyos.dev/) reference, maintainers will request
+that you amend the commit message.
+
+### Writing good commit messages
+
+Follow the format described in
+the [Git documentation](https://git-scm.com/book/ch5-2.html)
+and [Chris Beams' guide](https://chris.beams.io/posts/git-commit/).
+
+Commit message format:
+
+1. **Summary line** (50 characters recommended, 80 maximum): Include the
+   component
+   prefix and [Phabricator](https://vyos.dev/) reference (for example, `snmp: T1111:` or
+   `ethernet: T2222:`). Concatenate multiple components with colons
+   (for example, `snmp: ethernet: T3333`).
+2. **Blank line**: Separate the summary from the body.
+   This blank line is critical.
+
+4) **Message body** with details:
+
+   - Describe what changed, why, and how. This helps with `git bisect`.
+   - Wrap text at 72 characters for readability with `git log` on an 80x25
+     terminal.
+   - Reference previous commits when applicable:
+     `After commit abcd12ef ("snmp: this is a headline")
+     a Python import statement is missing, throwing the following exception:
+     ABCDEF`
+
+5) **Cherry-pick option**: Always use the `-x` option when back-porting or
+   forward-porting commits:
+
+   `git cherry-pick -x <commit>`
+
+   This appends `(cherry picked from commit <ID>)` to the commit message,
+   making bisecting easier.
+
+6) **Single responsibility**: Each commit must be self-contained. Do not fix
+   multiple bugs in a single commit. Use `git add --patch` to stage only
+   the parts related to one issue.
+
+Constraints:
+
+- Bugfixes are only accepted for packages other than
+  <https://github.com/vyos/vyos-1x>.
+  New functionality must use the new XML/Python interface, not old-style
+  templates (`node.def` files and Perl/Bash code).
+
+## Coding guidelines
+
+VyOS maintains consistent coding standards to help contributors navigate the
+codebase and understand its logic.
+
+### Formatting
+
+- **Python**: Use 4 spaces per indentation level. Tabs **must not** be used.
+- **XML**: Use 2 spaces per indentation level. Tabs **must not** be used.
+
+Use tools like VIM extensions (xmllint) to enforce correct indentation. Add this
+to your `.vimrc` file:
+
+```none
+au FileType xml setlocal equalprg=xmllint\ --format\ --recover\ -\ 2>/dev/null
+```
+
+Then use `gg=G` in command mode to run the linter.
+
+### Text generation
+
+Use a template processor for generating config files:
+
+- **Jinja2** is the default template processor for VyOS code.
+- Built-in string formatting **may** be used for simple line-oriented formats
+  (for example, iptables rules) where every line is self-contained.
+- Template processors **must** be used for structured, multi-line formats
+  (for example, ISC DHCPd configuration).
+
+### Python code
+
+Configuration scripts and operation mode scripts written in Python3 should
+follow these guidelines:
+
+- Wrap lines at 80 characters. This improves readability when browsing
+  GitHub on mobile devices and reads well in side-by-side diffs.
+
+Structure your scripts with these functions:
+
+```python
+#!/usr/bin/env python3
+#
+# Copyright (C) 2020 VyOS maintainers and contributors
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 or later as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import sys
+
+from vyos.config import Config
+from vyos import ConfigError
+
+def get_config(config=None):
+    if config:
+        conf = config
+    else:
+        conf = Config()
+
+    # Base path to CLI nodes
+    base = ['...', '...']
+    # Convert the VyOS config to an abstract internal representation
+    config_data = conf.get_config_dict(base, key_mangling=('-', '_'), get_first_key=True)
+    return config_data
+
+def verify(config):
+    # Verify that configuration is valid
+    if invalid:
+        raise ConfigError("Descriptive message")
+
+def generate(config):
+    # Generate daemon configs
+    pass
+
+def apply(config):
+    # Apply the generated configs to the live system
+    pass
+
+try:
+    c = get_config()
+    verify(c)
+    generate(c)
+    apply(c)
+except ConfigError as e:
+    print(e)
+    sys.exit(1)
+```
+
+`get_config()`: This function converts a VyOS config object to an abstract
+internal representation. No other function may call the `vyos.config.Config`
+object directly. Limiting config reads to one function makes it easier to
+modify the config syntax in the future. Additionally, this design improves
+testability since you can construct an internal representation by hand rather
+than mocking the entire config subsystem.
+
+`verify()`: This function validates the internal representation. It must
+raise `ConfigError` with a descriptive message if the config is invalid. It
+**must not** make any changes to the system. This design enables future features
+like commit dry-run ("commit test" as in JunOS) where the system can abort a
+commit before making changes.
+
+`generate()`: This function generates config files for system components.
+
+`apply()`: This function applies the generated configuration to the live
+system. Prefer non-disruptive reload when possible. Disruptive operations like
+daemon restarts are acceptable only when:
+
+- The component does not support non-disruptive reload, or
+- The expected service degradation is minimal (for example, auxiliary services
+  like LLDPd)
+
+For high-impact services (VPN daemons, routing protocols), make effort to
+determine if changes can be applied non-disruptively before resorting to
+restarts.
+
+Never modify active configuration directly unless absolutely necessary. Instead,
+generate configuration files and apply them with a single command like service
+reload through systemd. For example, save iptables rules to a file and load them
+with `iptables-restore` rather than executing iptables commands one by one.
+
+The `apply()` and `generate()` functions may raise `ConfigError` if the
+daemon fails to start with the updated config. However, this is not a substitute
+for proper config validation in the `verify()` function. Make reasonable
+effort to verify that generated configuration is valid and will be accepted by
+the daemon, including cross-checks with other VyOS configuration subtrees when
+necessary.
+
+Exceptions like `VyOSError` (raised by `vyos.config.Config` on improper
+operations) should not be silenced or caught. While this may produce less
+polished error output for users, it generates better bug reports and helps
+maintainers debug issues.
+
+For reference implementations, see `ntp.py` or `interfaces-bonding.py` (for
+tag nodes) in the [vyos-1x](https://github.com/vyos/vyos-1x) repository.
+
+### Other considerations: `vyos-configd`
+
+All scripts now run under the config daemon and must conform to these
+requirements:
+
+1. The signature and first four lines of `get_config(...)` **must** be as
+   specified above.
+2. Each of `get_config`, `verify`, `apply`, and `generate` **must**
+   appear
+   with the correct signatures, even if they are a no-op.
+3. `Config` objects other than those in `get_config` **must not** appear.
+4. The legacy function `my_set` **must not** appear. Modifications to active
+   config **should not** appear in new code (alternative mechanisms may be used
+   if absolutely necessary).
+
+## XML for CLI definitions
+
+XML interface definitions define the VyOS CLI structure.
+Before VyOS `1.2` (crux), these
+files were created manually. After a redesign, new-style templates are
+automatically generated from XML input files.
+
+VyOS interface definitions come with a RelaxNG schema located in the
+[vyos-1x](https://github.com/vyos/vyos-1x/tree/current/schema)
+repository. This schema is a modified version from `VyConf` (VyOS `2.0`).
+VyOS `1.2.x`
+interface definitions are reusable in future VyOS versions with minimal changes.
+
+Schemas provide two benefits:
+
+- Complete grammar verification
+- Automatic validation against the schema
+
+The [build-command-templates](https://github.com/vyos/vyos-1x/blob/current/scripts/build-command-templates)
+script converts XML definitions to
+old-style templates and verifies them against the schema. A bad definition
+causes the package build to fail. While the XML format is verbose, no other
+format provides this level of verification. Specialized XML editors can help
+manage verbosity.
+
+Example XML interface definition:
+
+```xml
+<?xml version="1.0"?>
+<!-- Cron configuration -->
+<interfaceDefinition>
+  <node name="system">
+    <children>
+      <node name="task-scheduler">
+        <properties>
+          <help>Task scheduler settings</help>
+        </properties>
+        <children>
+          <tagNode name="task" owner="${vyos_conf_scripts_dir}/task_scheduler.py">
+            <properties>
+              <help>Scheduled task</help>
+              <valueHelp>
+                <format>&lt;string&gt;</format>
+                <description>Task name</description>
+              </valueHelp>
+              <priority>999</priority>
+            </properties>
+            <children>
+              <leafNode name="crontab-spec">
+                <properties>
+                  <help>UNIX crontab time specification string</help>
+                </properties>
+              </leafNode>
+              <leafNode name="interval">
+                <properties>
+                  <help>Execution interval</help>
+                  <valueHelp>
+                    <format>&lt;minutes&gt;</format>
+                    <description>Execution interval in minutes</description>
+                  </valueHelp>
+                  <valueHelp>
+                    <format>&lt;minutes&gt;m</format>
+                    <description>Execution interval in minutes</description>
+                  </valueHelp>
+                  <valueHelp>
+                    <format>&lt;hours&gt;h</format>
+                    <description>Execution interval in hours</description>
+                  </valueHelp>
+                  <valueHelp>
+                    <format>&lt;days&gt;d</format>
+                    <description>Execution interval in days</description>
+                  </valueHelp>
+                  <constraint>
+                    <regex>[1-9]([0-9]*)([mhd]{0,1})</regex>
+                  </constraint>
+                </properties>
+              </leafNode>
+              <node name="executable">
+                <properties>
+                  <help>Executable path and arguments</help>
+                </properties>
+                <children>
+                  <leafNode name="path">
+                    <properties>
+                      <help>Path to executable</help>
+                    </properties>
+                  </leafNode>
+                  <leafNode name="arguments">
+                    <properties>
+                      <help>Arguments passed to the executable</help>
+                    </properties>
+                  </leafNode>
+                </children>
+              </node>
+            </children>
+          </tagNode>
+        </children>
+      </node>
+    </children>
+  </node>
+</interfaceDefinition>
+```
+
+XML definitions are purely declarative and contain no logic. All logic for
+generating config files, restarting services, and related tasks is implemented
+in configuration scripts.
+
+### Template Processors
+
+XML interface definition files use the `.xml.in` file extension (implemented
+in {vytask}`T1843`). These files use the GCC preprocessor to reduce code
+duplication in common areas:
+
+- VIF (including VIF-S and VIF-C)
+- Address configuration
+- Description
+- Enabled/Disabled state
+
+Instead of repeating XML nodes, use include files with predefined features:
+
+- [IPv4, IPv6, and DHCP(v6)](https://github.com/vyos/vyos-1x/blob/current/interface-definitions/include/interface/address-ipv4-ipv6-dhcp.xml.i)
+  address assignment.
+- [IPv4 and IPv6](https://github.com/vyos/vyos-1x/blob/current/interface-definitions/include/interface/address-ipv4-ipv6.xml.i)
+  address assignment.
+- [VLAN (VIF)](https://github.com/vyos/vyos-1x/blob/current/interface-definitions/include/accel-ppp/vlan.xml.i)
+  definition.
+- [MAC address](https://github.com/vyos/vyos-1x/blob/current/interface-definitions/include/firewall/mac-address.xml.i)
+  assignment.
+
+The `.in` files are preprocessed and stored in the [interface-definitions](https://github.com/vyos/vyos-1x/tree/current/interface-definitions)
+folder. The [scripts/build-command-templates](https://github.com/vyos/vyos-1x/blob/current/scripts/build-command-templates)
+script then operates on this folder to generate all required CLI nodes.
+
+Example preprocessor output:
+
+```none
+$ make interface_definitions
+install -d -m 0755 build/interface-definitions
+install -d -m 0755 build/op-mode-definitions
+Generating build/interface-definitions/intel_qat.xml from interface-definitions/intel_qat.xml.in
+Generating build/interface-definitions/interfaces-bonding.xml from interface-definitions/interfaces-bonding.xml.in
+Generating build/interface-definitions/cron.xml from interface-definitions/cron.xml.in
+Generating build/interface-definitions/pppoe-server.xml from interface-definitions/pppoe-server.xml.in
+Generating build/interface-definitions/mdns-repeater.xml from interface-definitions/mdns-repeater.xml.in
+Generating build/interface-definitions/tftp-server.xml from interface-definitions/tftp-server.xml.in
+[...]
+```
+
+
+### Command Definition Guidelines
+
+#### Use of Numbers
+
+Avoid using numbers in command names unless the number is part of a protocol
+name or similar. For example, `protocols ospfv3` is appropriate,
+but `server-1` is questionable.
+
+#### Help Strings
+
+Follow these guidelines for consistent, readable help strings:
+
+##### Capitalization and Punctuation
+
+- Capitalize the first word of every help string.
+- Do not use a period at the end of help strings.
+
+This standard mirrors network device CLIs and improves aesthetics.
+
+Examples:
+
+- Good: "Frobnication algorithm"
+- Bad: "frobnication algorithm"
+- Bad: "Frobnication algorithm."
+- Incorrect: "frobnication algorithm."
+
+##### Abbreviations and Acronyms
+
+- Capitalize all abbreviations and acronyms.
+
+Examples:
+
+- Good: "TCP connection timeout"
+- Bad: "tcp connection timeout"
+- Bad: "Tcp connection timeout"
+- Capitalize acronyms to distinguish them from normal words.
+
+Examples:
+
+- Good: RADIUS (remote authentication for dial-in user services)
+- Bad: radius (unless referring to circular distance)
+- Follow accepted spelling conventions for mixed-case abbreviations. If it
+  contains "over" or "version", use lowercase. Follow RFC or standard spellings
+  when they exist.
+
+Examples:
+
+- Good: PPPoE, IPsec
+- Bad: PPPOE, IPSEC
+- Bad: pppoe, ipsec
+
+##### Verbs
+
+- Avoid verbs. If a verb can be omitted, omit it.
+
+Examples:
+
+- Good: "TCP connection timeout"
+- Bad: "Set TCP connection timeout"
+- When a verb is essential, use it. For example: "Disable IPv6 forwarding on
+  all interfaces" for `set system ipv6 disable-forwarding`.
+- Use infinitive form for necessary verbs.
+
+Examples:
+
+- Good: "Disable IPv6 forwarding"
+- Bad: "Disables IPv6 forwarding"
+
+## C++ Backend Code
+
+The VyOS CLI parser combines bash, bash-completion helpers, and the C++ backend
+library [vyatta-cfg](https://github.com/vyos/vyatta-cfg). This section
+references common CLI commands and their C/C++ entry points:
+
+`set`:
+
+- <https://github.com/vyos/vyatta-cfg/blob/0f42786a0b3/src/cstore/cstore.cpp#L352>
+- <https://github.com/vyos/vyatta-cfg/blob/0f42786a0b3/src/cstore/cstore.cpp#L2549>
+
+`commit`:
+
+- <https://github.com/vyos/vyatta-cfg/blob/0f42786a0b3/src/commit/commit-algorithm.cpp#L1252>
+
+