gNMI Depth Extension documentation (#205)

* added gnmi depth ext documentation

* remove json syntax

* remove sections that don't bear signigicant meaning for the doc

* added missing backquotes

Co-authored-by: Rob Shakir <robjs@google.com>

* addressed review comments

---------

Co-authored-by: Rob Shakir <robjs@google.com>

rpc/gnmi/gnmi-depth.md

@@ -0,0 +1,308 @@
+# gNMI Depth Extension
+
+**Contributors:** Roman Dodin, Matthew MacDonald
+
+**Date:** March 9, 2024
+
+**Version:** 0.1.0
+
+## 1. Purpose
+
+The implicit recursive data fetching used by the gNMI servers makes the
+implementations simple. Whilst the implementation simplicity is important, the
+lack of server-side filtering for the data requested in gNMI RPCs may be
+considered a limitation for some gNMI users and systems.
+
+The gNMI Depth Extension allows a client to control the depth of the recursion
+when the server evaluates a group of paths in the `Subscribe` or `Get` RPC.
+
+Orchestration, Network Management and Monitoring Systems can benefit from this
+extension as it:
+
+1. reduces the load on the server when data is to be fetched from the Network
+OS during the recursive data extraction
+2. reduces the bytes on the wire payload, by sending fewer data
+
+gNMI Depth Extension proto specification is defined in
+[gnmi_ext.proto](<https://github.com/openconfig/gnmi/blob/master/proto/gnmi_ext/g>
+nmi_ext.proto).
+
+## 2. Demo model
+
+To explain the concept of the depth-based filtering, consider the following
+model that is used in the implementation examples throughout this document:
+
+```yang
+    container basket {
+      leaf name {
+        type string;
+      }
+
+      leaf-list contents {
+        type string;
+      }
+
+      list fruits {
+        key "name";
+
+        leaf name {
+          type string;
+        }
+
+        leaf-list colors {
+          type string;
+        }
+
+        leaf size {
+          type string;
+        }
+
+        container origin {
+          leaf country {
+            type string;
+          }
+          
+          leaf city {
+            type string;
+          }
+        }
+      }
+
+      container description {
+        leaf fabric {
+          type string;
+        }
+      
+      }
+      container broken {
+        presence "This container is broken";
+        leaf reason {
+          type string;
+        }
+      }
+    }
+```
+
+It's tree representation:
+
+```
+module: app
+  +--rw basket
+     +--rw name?          string
+     +--rw contents*      string
+     +--rw fruits* [name]
+     |  +--rw name      string
+     |  +--rw colors*   string
+     |  +--rw size?     string
+     |  +--rw origin
+     |     +--rw country?   string
+     |     +--rw city?      string
+     +--rw description
+     |  +--rw fabric?   string
+     +--rw broken!
+        +--rw reason?   string
+```
+
+We populate this data schema with the following values:
+
+```
+    basket {
+        contents [
+            fruits
+            vegetables
+        ]
+        fruits apples {
+            size XL
+            colors [
+                red
+                yellow
+            ]
+            origin {
+                country NL
+                city Amsterdam
+            }
+        }
+        fruits orange {
+            size M
+        }
+        description {
+            fabric cotton
+        }
+        broken {
+            reason "too heavy"
+        }
+    }
+```
+
+## 3. Concepts
+
+The Depth extension allows clients to specify the depth of the subtree to be
+returned in the response. The depth is specified as the number of levels below
+the specified path.
+
+The extension itself has a single field that controls the depth level:
+
+```proto
+message Depth {
+  uint32 level = 1;
+}
+```
+
+### 3.1 Depth level values
+
+#### 3.1.1 Value 0
+
+Depth value of 0 means no depth limit and behaves the same as if the extension
+was not specified at all.
+
+#### 3.1.2 Value 1
+
+Value of 1 means only the specified path and its direct children will be
+returned. See Children section for more info.
+
+#### 3.1.2 Value of N
+
+Value of N where N>1 means all elements of the specified path up to N level and
+direct children of N-th level.
+
+### 3.2 Children nodes
+
+The Depth extension operates the value of "direct children of a schema node".
+When YANG data modelling language is used, the following elements are
+considered as children nodes of a schema node:
+
+1. leafs
+2. leaf-lists
+
+In a generic data model language, the children nodes are the elements of scalar
+type, and arrays of these elements, that are direct descendants of the schema
+node.
+
+Only these elements are to be returned if depth extension with non-0 value is
+specified for a specified depth level.
+
+### 3.3 RPC support
+
+The Depth extension applies to Get and Subscribe requests only. When used with
+Capability and Set RPC the server should return an error.
+
+## 4 Examples
+
+Using the data model from Section 2 we will run through a set of examples using
+the patched version of [openconfig/gnmic](https://gnmic.openconfig.net/) client
+with the added Depth extension support.
+
+### 4.1 depth 1, path `/basket`
+
+The most common way to use the depth extension (as we see it) is to use it with
+level=1. This gets you the immediate child nodes of the schema node targeted by
+a path.
+
+Consider the following gnmic command targeting `/basket` path:
+
+```bash
+gnmic -e json_ietf get --path /basket --depth 1
+```
+
+```json
+[
+  {
+    "contents": [
+      "fruits",
+      "vegetables"
+    ]
+  }
+]
+```
+
+As per the design, only the leaf and leaf-list nodes are returned. Since our
+`/basket` container has only `leaf-list` elements (no leafs) a single element
+`contents` is returned.
+
+You can see how this makes it possible to reduce the amount of data extracted
+by the server and sent over the wire. Many applications might require fetching
+only leaf values of a certain container to make some informed decision without
+requiring any of the nested data.
+
+### 4.2 depth 1, path `/basket/fruits`
+
+When the path targets the list schema node, all elements of this list is
+returned with their children nodes
+
+```bash
+gnmic -e json_ietf get --path /basket/fruits --depth 1
+```
+
+```json
+[
+  {
+    "fruits": [
+      {
+        "colors": [
+          "red",
+          "yellow"
+        ],
+        "name": "apples",
+        "size": "XL"
+      },
+      {
+        "name": "orange",
+        "size": "M"
+      }
+    ]
+  }
+]
+```
+
+Again, please keep in mind that only leafs and leaf-lists are returned for
+every list element.
+
+### 4.3 depth 2, path `/basket`
+
+When the depth level is set to values >1, all elements from the path to the
+provided level value are returned in full with the last level including only
+leafs and leaf-lists.
+
+```bash
+gnmic -e json_ietf get --path /basket --depth 2
+```
+
+```json
+[
+  {
+    "broken": {
+      "reason": "too heavy"
+    },
+    "contents": [
+      "fruits",
+      "vegetables"
+    ],
+    "description": {
+      "fabric": "cotton"
+    },
+    "fruits": [
+      {
+        "colors": [
+          "red",
+          "yellow"
+        ],
+        "name": "apples",
+        "size": "XL"
+      },
+      {
+        "name": "orange",
+        "size": "M"
+      }
+    ]
+  }
+]
+```
+
+Here is what happens:
+
+<img width="640" alt="image"
+src="https://github.com/openconfig/gnmi/assets/5679861/331a20a5-ff19-4932-873a-7aff9b36dd62">
+
+The 1st level elements are returned, since depth level is 2.
+On the 2nd level we return only leafs and leaf-lists, hence the
+`.fruits.origin` is not present.