[doc]: Add gnmi feedback design for SmartSwitch

doc/smart-switch/gnmi-feedback/smart-switch-gnmi-feedback-design.md

@@ -0,0 +1,164 @@
+# Smart Switch GNMI feedback design <!-- omit in toc -->
+
+## Table of content <!-- omit in toc -->
+
+- [Revision](#revision)
+- [Overview](#overview)
+- [Requirement](#requirement)
+- [Sequence flow](#sequence-flow)
+  - [Set/Remove](#setremove)
+  - [Message](#message)
+  - [Get](#get)
+  - [Subscribe](#subscribe)
+- [Table](#table)
+  - [DPU APPL STATE DB](#dpu-appl-state-db)
+    - [Example 1 DASH\_ROUTE\_TABLE](#example-1-dash_route_table)
+    - [Example 2 DASH\_ROUTING\_APPLIANCE\_TABLE](#example-2-dash_routing_appliance_table)
+
+## Revision
+
+|  Rev  | Date  | Author | Change Description |
+| :---: | :---: | :----: | ------------------ |
+|  0.1  |       | Ze Gan | Initial version.   |
+
+## Overview
+
+The controller communicates with SmartSwitch via the GNMI interface. To enhance configuration efficiency, we provide batch GNMI operations to the GNMI client, allowing it to submit multiple operations in a single request. This document covers two communication channels: the external channel between the GNMI client and server, and the internal channel between the GNMI server and DPU SAI.
+
+## Requirement
+
+- Support for multiple operations in a single request
+- Non-child objects must always include the version ID field, and the version ID should be maintained by the controller(VNET or SDN side) side.
+- The GNMI server supports set, remove, get, and subscribe operations
+- The GNMI client can proactively subscribe to or get objects/tables via object key or table name
+
+| Child object      |
+| ----------------- |
+| DASH_ACL_RULE     |
+| DASH_ROUTE        |
+| DASH_ROUTE_RULE   |
+| DASH_VNET_MAPPING |
+
+## Sequence flow
+
+### Set/Remove
+
+```mermaid
+sequenceDiagram
+    actor CTR as Controller
+    participant GC as GNMI Client
+    box NPU SONiC
+        participant GS as GNMI Server
+        participant AD as DPU APPL DB
+        participant ASD as DPU APPL STATE DB
+    end
+    Note over AD, ASD: DPUx Redis
+    box DPU SONiC
+        participant ZMQ as ZMQ
+        participant SW as SWSS
+        participant SY as SYNCD
+    end
+
+    CTR ->> CTR: Prepare objects operations
+    alt Set command
+        CTR ->> CTR: Prepare version ID
+    end
+    CTR ->> GC: Call GNMI API
+    GC ->> GS: Update commands
+    loop Each commands:
+        GS --) AD: Insert object
+        GS ->> ZMQ: Update command
+    end
+    GS -->> GC: 
+    GC -->> CTR: 
+    ZMQ ->> SW: Consume commands
+    SW ->> SY: SAI call
+    SY -->> SW: 
+    alt Set command
+        SW ->> ASD: Update command result
+        Note over ZMQ: Include result code and version ID (If it's non-child object)
+    else Del command
+        SW ->> ASD: Remove objects
+    end
+
+```
+
+### Message
+
+### Get
+
+```mermaid
+sequenceDiagram
+    participant GC as GNMI Client
+    box NPU SONiC
+        participant GS as GNMI Server
+        participant ASD as DPU APPL STATE DB
+    end
+    Note over ASD: DPUx Redis
+
+    GC ->> GS: Query object or table result
+    GS ->> ASD: Get object via key or get objects via table name
+    ASD -->> GS: Query results
+    GS -->> GC: Query result
+
+```
+
+### Subscribe
+
+```mermaid
+sequenceDiagram
+    participant GC as GNMI Client
+    box NPU SONiC
+        participant GS as GNMI Server
+        participant ASD as DPU APPL STATE DB
+    end
+    Note over ASD: DPUx Redis
+    participant DPU as DPU
+
+    GC ->> GS: Subscribe object or table
+    GS ->> GS: Register subscribe events
+    GS ->> ASD: Get object via key or get objects via table name
+    Note over ASD: Cache result before subscribe
+    ASD -->> GS: Query results
+    GS -->> GC: Query results
+    loop Each commands
+        alt Set command
+            DPU ->> ASD: Update command result
+        else Del command
+            DPU ->> ASD: Remove objects
+        end
+    end
+    ASD -->> GS: Proactively push results
+    GS -->> GC: Proactively push results
+
+```
+
+## Table
+
+### DPU APPL STATE DB
+
+Any entry in DPU APPL STATE DB will be mapped to an entry in DPU APPL DB. Their key are totally same. But the value of entry in DPU APPL STATE DB may include its version ID and its result if it's an parent objects or only includes its result to child objects. Typically, a result of 0 indicates the object was successfully applied to the DPU, otherwise it indicates some kind of error occurred.
+
+#### Example 1 DASH_ROUTE_TABLE
+
+```text
+DASH_ROUTE_TABLE:{{group_id}}:{{prefix}}
+    "result": {{result}}
+
+key         = DASH_ROUTE_TABLE:group_id:prefix
+; field     = value
+result      = uint32 0 for success, > 0 for error code
+```
+
+#### Example 2 DASH_ROUTING_APPLIANCE_TABLE
+
+```
+DASH_ROUTE_GROUP_TABLE:{{group_id}}
+    "result": {{result}}
+    "version_id": {{string}}
+
+key         = DASH_ROUTE_GROUP_TABLE:group_id
+; field     = value
+result      = uint32 0 for success, > 0 for error code
+version_id  = string, Indicate the unique version of metadata of route group. E.G. "1", "1.1".
+```