Correct documentation for GET node API

	- Add new section on documentation to describe XPath
	- Correct doc description on GET node API

Issue-ID: CPS-1607
Signed-off-by: emaclee <lee.anjella.macabuhay@est.tech>
Change-Id: I23e4ccd1185a2d93fbb0b9e02e9daab949c0f952

1 files changed, 220 insertions, 0 deletions

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+.. This work is licensed under a Creative Commons Attribution 4.0 International License.
+.. http://creativecommons.org/licenses/by/4.0
+.. Copyright (C) 2023 Nordix Foundation
+
+.. DO NOT CHANGE THIS LABEL FOR RELEASE NOTES - EVEN THOUGH IT GIVES A WARNING
+.. _xpath:
+
+
+XPath
+########
+
+.. toctree::
+   :maxdepth: 1
+
+Introduction
+============
+
+In CPS , Xpath is a parameter used in several APIs, which allows us to retrieve JSON and XML data efficiently.
+The XPath syntax provides us with the ability to navigate through the hierarchical structure of data used in CPS easily via specification of node names, values ,attributes etc.
+
+Sample Yang Model
+=================
+
+.. code-block::
+
+  module stores {
+      yang-version 1.1;
+      namespace "org:onap:ccsdk:sample";
+
+      prefix book-store;
+
+      revision "2020-09-15" {
+          description
+            "Sample Model";
+      }
+      container shops {
+
+          container bookstore {
+
+              leaf bookstore-name {
+                  type string;
+              }
+
+              leaf name {
+                  type string;
+              }
+
+              list categories {
+
+                  key "code";
+
+                  leaf code {
+                      type uint16;
+                  }
+
+                  leaf name {
+                      type string;
+                  }
+
+                  leaf numberOfBooks {
+                      type uint16;
+                  }
+
+                  container books {
+
+                      list book {
+                          key title;
+
+                          leaf title {
+                              type string;
+                          }
+                          leaf price {
+                              type uint16;
+                          }
+                          leaf-list label {
+                              type string;
+                          }
+                          leaf-list edition {
+                              type string;
+                          }
+                      }
+                  }
+              }
+          }
+      }
+  }
+
+**Note.** 'categories' is a Yang List and 'code' is its key leaf. All other data nodes are Yang Containers. 'label' and 'edition' are both leaf-lists.
+
+Sample Data in Json
+===================
+
+.. code-block:: json
+
+    {
+      "shops": {
+        "bookstore": {
+          "bookstore-name": "Chapters",
+          "name": "Chapters",
+          "categories": [
+            {
+              "code": 1,
+              "name": "SciFi",
+              "numberOfBooks": 2,
+              "books": {
+                "book": [
+                  {
+                    "title": "2001: A Space Odyssey",
+                    "price": 5,
+                    "label": ["sale", "classic"],
+                    "edition": ["1968", "2018"]
+                  },
+                  {
+                    "title": "Dune",
+                    "price": 5,
+                    "label": ["classic"],
+                    "edition": ["1965"]
+                  }
+                ]
+              }
+            },
+            {
+              "code": 2,
+              "name": "Kids",
+              "numberOfBooks": 1,
+              "books": {
+                "book": [
+                  {
+                    "title": "Matilda"
+                  }
+                ]
+              }
+            }
+          ]
+        }
+      }
+    }
+
+Nodes
+=====
+
+- Xpaths uses 'nodes' to navigate through the data.
+    - Nodes typically used in CPS:
+        - container
+        - list
+        - leaf (CPS does not support addressing leaf nodes using XPath syntax)
+- JSON data and XML data are node trees
+    - the 'root node' contains all the configuration data nodes that exist at the top-level in all modules, serving as their parent node.
+
+Relationships
+=============
+
+- Every node has a parent, not every node has a children
+- Ancestor: a node's parent, parent's parent , parent's parent's parents …
+    - In the given sample model and data, the top data node 'shops' is the ancestor of every other node
+- Descendants: a node's children, children's children,  children's children's children
+    - In the given sample model and data, the list node 'books' is a descendant of the container node 'books'
+
+Supported XPath syntax in CPS
+=============================
+
+Selecting nodes
+---------------
+
+  - ``/``: Selects from the root node
+    - **Note:** CPS uses absolute location path which means that XPath expression expected in CPS must start with / followed by the nodes leading to the node being selected.
+  - ``@``: Selects attributes
+    - **Note:** Providing the attribute value can be enclosed in quotations i.e. ``[@code=1]`` will provide same result as ``[@code='1']``
+
+.. list-table::
+   :header-rows: 1
+
+   * - XPath
+     - Description
+     - Selected node(s) without descendants
+   * - - /
+       - /shops
+     - Selects the root node
+     - .. code-block:: json
+
+            [
+              {
+                "book-store:shops": {}
+              }
+            ]
+   * - /shops/bookstore
+     - Selects the bookstore container node
+     - .. code-block:: json
+
+            [
+              {
+                "book-store:bookstore": {
+                  "name": "Chapters",
+                  "bookstore-name": "Chapters"
+                }
+              }
+            ]
+   * - /shops/bookstore/categories[@code='1']
+     - Selects 'categories' list node in 'bookstore' container that contains the key attribute 'code=1'
+     - .. code-block:: json
+
+            [
+              {
+                "book-store:categories": {
+                  "code": 1,
+                  "name": "SciFi",
+                  "numberOfBooks": 2
+                }
+              }
+            ]
+   * - /shops/bookstore/categories[@code='1']/books
+     - Selects the 'books' container node in 'categories' list node in 'bookstore' container that contains the key attribute 'code=1'
+     - .. code-block:: json
+
+            [
+              {
+                "book-store:books": {}
+              }
+            ]
+