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+# Email Draft
+
+**To:** Vivekananda Boudia <vivekananda.boudia@insa-lyon.fr>, Pierre Francois <pierre.francois@insa-lyon.fr>, Maxence Younsi <maxence.younsi@insa-lyon.fr>
+**CC:** netmod@ietf.org
+**Subject:** Feedback on "YANG Libraries Feature Comparison" (IETF 126) — from the yangkit author
+
+---
+
+Hi Vivekananda, Pierre, and Maxence,
+
+Thank you for the "YANG Libraries Feature Comparison" presentation at IETF 126 NETMOD. As the author and maintainer of yangkit, I'm delighted to see this kind of systematic, cross-implementation comparison — it is exactly the type of work that benefits the entire YANG ecosystem.
+
+I'd like to offer some clarifications on a few of the yangkit evaluation results, and more broadly, to suggest that the community could benefit from developing a shared set of test criteria together.
+
+### 1. Anydata Validation — rated "Partially Supported"
+
+As the original proposer of the `anydata` statement during the YANG 1.1 revision (the proposal was accepted into RFC 7950), I'd like to clarify its intended semantics.
+
+RFC 7950 Section 7.10 distinguishes `anydata` from `anyxml`: anydata represents "an unknown set of nodes that can be modeled with YANG," meaning its content must conform to some YANG model — we just don't know which one at compile time. Yangkit requires an explicit schema context (via `AnydataValidationOptions`) before validating anydata payload, which directly reflects this semantics.
+
+There is an important distinction between "preserving anydata payload as opaque data" and "validating anydata payload against a schema." Both are useful capabilities, but they serve different purposes. It would be valuable for the community to discuss and agree on what "anydata validation support" should mean in the context of a feature comparison.
+
+The same applies to the "Anydata inside structures" rating.
+
+### 2. XPath / Data Extraction — rated "Partially Supported"
+
+Yangkit implements a complete XPath 1.0 engine (based on Jaxen) with all YANG-specific extension functions defined in RFC 7950: `current()`, `deref()`, `derived-from()`, `derived-from-or-self()`, `enum-value()`, `re-match()`, and `bit-is-set()`.
+
+In RFC 7950, XPath is used for `when`, `must`, `leafref path`, and `instance-identifier` — all of which evaluate over the data tree. YANG XPath paths are data node paths that skip schema-only constructs like `choice` and `case`, making them fundamentally different from schema paths.
+
+It's also worth noting that RFC 7950 defines anydata as an opaque, indivisible node in the data tree — it has no addressable child nodes. XPath path steps cannot enter anydata internals regardless of implementation; this is inherent to the data model defined by the specification.
+
+I'd be interested to understand which specific test scenarios yangkit did not pass, so we can determine whether the gap is in RFC-mandated functionality or in capabilities beyond the specification scope.
+
+### 3. Add New Schema Node — rated "Not Supported"
+
+yangkit-model supports dynamically adding, removing, and modifying schema nodes via APIs such as `SchemaNodeContainer.addSchemaNodeChild()` — this is a design-time capability for model editing and compilation.
+
+For data validation (yangkit-data), the schema is treated as immutable, which is a deliberate design choice: data validation requires a stable reference point. If a new schema is needed at runtime, the approach is to re-run model parsing and build a new schema context.
+
+These are two distinct capabilities — model editing and data validation — and it may be useful to evaluate them separately.
+
+### 4. Update Existing Data Node — rated "Not Supported"
+
+Yangkit supports updating data node values through the `LeafData.setValue()` API. After modification, calling `validate()` re-validates the data tree. This decoupled design enables efficient batch updates — multiple nodes can be modified without triggering validation on each intermediate (potentially invalid) state, with a single validation pass after all changes are complete.
+
+I noticed the slides annotate this item with "can easily be changed in code source," which suggests the capability is recognized. I'd be happy to provide more guidance on the intended API usage patterns if that would be helpful.
+
+### 5. Schema Comparison — rated "Implementable"
+
+yang-comparator (https://github.com/yang-central/yang-comparator) is a tool built on yangkit that performs semantic-level schema tree comparison. It compiles two versions of YANG modules into their respective schema trees and performs structural diff at the schema level — node additions/deletions/modifications, type changes, constraint changes, and so on.
+
+While its output format may not yet fully align with `draft-ietf-netmod-yang-schema-comparison`, the core schema comparison capability is implemented and has been used in production. I'd welcome discussion on what output format or criteria the comparison should use.
+
+---
+
+I want to emphasize that this feedback is meant to be constructive, and I think your work points to something the community needs: **a shared, open set of test cases and evaluation criteria for YANG libraries.** Different libraries make different design choices, and having an agreed-upon test specification would make comparisons fairer and more useful for everyone.
+
+Would you be open to collaborating on this? I'd be happy to contribute — whether that means helping refine test cases, providing guidance on yangkit APIs, or working together on a community-driven test specification that the NETMOD WG could adopt.
+
+Best regards,
+Frank
diff --git a/IETF126-feedback.md b/IETF126-feedback.md
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+# Feedback on IETF 126 YANG Libraries Feature Comparison
+
+Slides: https://datatracker.ietf.org/meeting/126/materials/slides-126-netmod-yang-libraries-feature-comparison-00
+Test repo: https://github.com/network-analytics/yang_tests
+Authors: Vivekananda Boudia, Pierre Francois, Maxence Younsi (INSA Lyon)
+
+---
+
+## 1. Anydata Validation — "Partially Supported" 评价有误
+
+**Slides 结论:** yangkit anydata validation 为 Partially Supported
+
+**反馈:**
+
+作为 anydata 语句在 YANG 1.1 修订过程中的最初提出者（提案被接纳进入 RFC 7950），需要澄清其设计意图。RFC 7950 Section 7.10 明确区分了 anydata 和 anyxml：
+- anydata: "an unknown set of nodes that **can be modeled with YANG**"
+- anyxml: 不需要符合任何 YANG 模型的任意数据
+
+Anydata 的内容**必须有对应的 YANG 模型**，只是在编译时不确定是哪个模型。如果没有 schema，anydata 在语义上就是无法验证的——这不是实现缺陷，而是 anydata 的本质。
+
+Yangkit 要求通过 `AnydataValidationOptions` 显式注册 payload schema 才能验证 anydata payload，这恰恰体现了 anydata 的核心语义。
+
+Libyang 在没有 schema 的情况下将 anydata payload 保留为 opaque 数据（类似 anyxml），这并不是 validation，而是 preservation。测试中的 "primitive values, unknown objects, arrays, null values" 场景如果不提供 schema，根本就不属于 anydata validation 的范畴。
+
+**测试用例证据:**
+- `testPrimitiveTypeAnydata()`: 测试者自己标注 TODO — "payload is primitive, invalid - in strict validation of yangkit, why is it succ?" — 测试者对预期行为不确定
+- `testNullAnydata()`: 测试者标注 TODO — "Heng - is this actually valid?" 引用 RFC 7951 Sec.5.5 — 预期结果存疑
+- 多个测试在不提供 schema 的情况下测试 anydata validation，这本身就偏离了 anydata 的语义
+
+**建议:** 重新审视 anydata validation 的"支持"标准定义——"不丢数据"和"能验证数据"是两个不同的能力，前者是 anyxml 的行为，后者才是 anydata 的行为。
+
+## 2. Anydata inside Structures — 同上
+
+此项 Partially Supported 评价是上述 anydata validation 问题在 structure 上下文中的延伸，反馈同上。
+
+`YangAnydataStructureTest.java` 中的 7 个测试用例（primitive, object-with/without-schema, null, array, null-object, structure-in-anydata）存在同样的问题。
+
+## 3. XPath / Data Extraction — "Partially Supported" 评价有误
+
+**Slides 结论:** yangkit XPath/data extraction 为 Partially Supported
+
+**反馈:**
+
+RFC 7950 中 XPath 的使用场景非常明确：when（条件性存在约束）、must（数据验证约束）、leafref path（叶节点引用）、instance-identifier（实例标识）。这些全部是在**数据树**上求值的。
+
+YANG XPath 表达式中的路径是 data node path，跳过 choice/case 等 schema-only 节点，这决定了 XPath 和 schema path 是两套不同的寻址体系。用 XPath 查询 schema 节点（如 libyang 的 `lys_find_xpath()`）是 libyang 提供的额外便利功能，不是 YANG 规范对 XPath 的要求。
+
+Yangkit 基于 Jaxen 实现了完整的 XPath 1.0 引擎，并扩展了 YANG 特有函数（current(), deref(), derived-from(), derived-from-or-self(), enum-value(), re-match(), bit-is-set()），完整覆盖了 RFC 7950 对 XPath 的所有要求。yangkit-xpath 模块的职责就是 RFC 7950 定义的 XPath 数据树求值，它完整实现了这个职责。
+
+至于 `evaluate()` 返回 List/String/Number/Boolean，这正是 XPath 1.0 规范定义的四种结果类型（node-set, string, number, boolean），不存在"结构化结果集"缺失的问题。
+
+**测试用例证据:**
+- `XPathExtractionTest.java` 中 4 个测试：
+  - `minimalXpathTest()` — 正常通过
+  - `structureXpathTest()` — 正常通过
+  - `nonExistentXpathTest()` — 正确返回空字符串
+  - `anydataXpathTest()` — TODO: "'value' returns '' instead of 'router1'" — 测试期望 XPath 能穿透 anydata payload 内部寻址，但这违反了 RFC 7950 对 anydata 的定义
+
+关于 `anydataXpathTest()` 需要特别说明：RFC 7950 Section 7.10 将 anydata 定义为数据树中的一个不可分割的不透明节点，它没有可寻址的子节点。无论是否有 schema，XPath 都无法用路径步（path step）进入 anydata 内部——这不是实现缺陷，而是 RFC 7950 数据模型的本质决定的。任何符合规范的 YANG library 都不应该支持这种寻址方式。如果某个库允许 XPath 穿透 anydata 内部，那反而是对规范的偏离。
+
+**建议:** 如果测试期望的是 schema 树上的 XPath 查询能力或 anydata 内部寻址能力，这些都超出了 RFC 7950 对 YANG XPath 的定义，不应作为 YANG library 的 XPath 支持评价标准。
+
+## 4. Add New Schema Node — "Not Supported" 评价不公平
+
+**Slides 结论:** yangkit 不支持动态添加 schema 节点
+
+**反馈:**
+
+首先，yangkit-model **支持动态增删改 schema 节点**，但这是在**设计态**（模型编辑/编译阶段）工作的能力，`SchemaNodeContainer.addSchemaNodeChild()` 等 API 就是为此设计的。
+
+其次，对于 yangkit-data（数据验证）来说，基于确定的 schema 来验证数据正确性是基本前提。在运行时改变 schema 意味着在验证过程中移动标准线，这在数据验证场景下是不可接受的。如果需要使用新的 schema，正确的做法是重新运行一次模型解析，构建新的 schema context，而不是在已有 context 上动态修改。
+
+这个测试实际上混淆了两个不同的职责：
+- **模型编辑/编译**（设计态）：yangkit-model 支持，可以动态增删改节点
+- **数据验证**（运行态）：yangkit-data 基于不可变的 schema 进行验证，这是正确的设计
+
+**建议:** 如果测试的目的是评估"模型编辑能力"，应当在设计态下测试 yangkit-model 的 API，而非在数据验证的上下文中评价。
+
+## 5. Update Existing Data Node — "Not Supported" 评价有误
+
+**Slides 结论:** yangkit 不支持更新现有数据节点（slides 注明 "can easily be changed in code source"）
+
+**反馈:**
+
+Yangkit 完全支持更新数据节点值。`LeafData.setValue()` 方法可以直接修改节点值，修改后调用 `validate()` 即可重新验证。这是 yangkit 的设计理念：**数据操作和验证是解耦的**，用户可以自由修改数据树，然后在需要时统一触发验证。
+
+这种设计比"每次 setValue 自动触发验证"更灵活——批量修改多个节点时，不需要每改一个就验证一次（中间状态可能本来就不合法），而是在所有修改完成后统一验证一次。
+
+**测试用例证据:**
+`updateValueTest()` 中的代码尝试通过 `container.addChild(newLeafNode)` 传入同名新节点来"更新"值——**测试者根本没有使用 `LeafData.setValue()` API**。正确的做法是：
+```java
+leafData.setValue(newValue);
+dataDocument.validate();
+```
+这是 API 误用，不是功能缺失。
+
+Slides 自己也标注了 "can easily be changed in code source"，说明测试者也认识到这不是能力缺失，评为 "Not Supported" 与自身注释矛盾。
+
+## 6. Schema Comparison — "Implementable" 评价有误
+
+**Slides 结论:** yangkit 的 schema comparison 为 Implementable（尚未实现）
+
+**反馈:**
+
+Yang-comparator（https://github.com/yang-central/yang-comparator）是基于 yangkit schema 树的语义级比较工具，而非文本级比较。它通过 yangkit-model 编译两个版本的 YANG 模块，构建各自的 schema 树，然后在 schema 树层面进行语义级 diff（节点增删改、类型变更、约束变更等）。
+
+正是因为这个 schema comparison 能力，yangkit 被瑞士电信（Swisscom）选中用于 YANG 模型版本管理，也是 yangkit 作者参与 IETF YANG Module Versioning Design Team 的原因之一。
+
+**测试用例证据:**
+测试仓库中包含 `YangkitSchemaComparator.java`——测试者**自己使用 yangkit API 从零实现了一个 schema comparator**，包括：
+- 节点增删改检测（indexEffectiveSchemaTree + compareNode）
+- 类型变更比较
+- 约束变更评估（config, mandatory, status, default, min-elements, max-elements, presence, when, must）
+- 向后兼容性评估
+
+这个事实本身就证明了：
+1. yangkit 的 schema 内省能力足够强大，足以构建完整的 schema comparison
+2. 测试者没有使用已有的 yang-comparator，而是自己重写了一个
+3. 评价 yangkit 的 schema comparison 为 "Implementable"（暗示尚未实现）与测试者自己的代码自相矛盾
+
+Yang-comparator 已经是一个成熟的、经过生产环境验证的 schema comparison 实现。至于 `draft-ietf-netmod-yang-schema-comparison` 定义的标准化输出格式，yang-comparator 的输出格式可能与之不完全一致，但 schema comparison 的核心能力是具备的。适配特定输出格式是工程层面的工作，不影响"是否支持 schema comparison"的评价。
