update desgin docs (#269)

Signed-off-by: hsliu <liuhongsheng4@huawei.com>

docs/design/feature/ray_based_execution.md

@@ -0,0 +1,61 @@
+# VLLM-Omni Distributed Utilities
+
+This directory (vllm_omni/distributed/ray_utils) contains utilities for distributed execution in vllm-omni, supporting both **Ray** and **Multiprocessing** backends.
+
+## 1. Ray Utils
+
+The `ray_utils` module provides helper functions for managing Ray clusters and actors, which is essential for:
+*   **Multi-node deployment**: Running pipeline stages across different physical machines.
+*   **Resource management**: Efficient GPU/CPU allocation.
+
+### 1.1 Basic Usage
+
+To use the Ray backend, specify `worker_backend="ray"` when initializing the engine.
+
+**Command Line Example:**
+```bash
+vllm serve Qwen/Qwen2.5-Omni-7B \
+  --omni \
+  --port 8091 \
+  --worker-backend ray \
+  --ray-address auto
+```
+
+### 1.2 Cluster Setup
+
+**Step 1: Start Head Node**
+Run this on your primary machine:
+```bash
+ray start --head --port=6399
+```
+
+**Step 2: Connect Worker Nodes**
+Run this on each worker machine:
+```bash
+ray start --address=<HEAD_NODE_IP>:6399
+```
+
+> **Tip**: For a complete cluster setup script, refer to the vLLM example:
+> [run_cluster.sh](https://github.com/vllm-project/vllm/blob/main/examples/online_serving/run_cluster.sh)
+
+### 1.3 Distributed Connector Support
+
+When running on Ray, the system automatically adapts its communication strategy:
+
+*   **Cross-Node**: Recommended to use `MooncakeConnector` (requires separate configuration).
+*   **Same-Node**: Can still use `SharedMemoryConnector` for efficiency, or Ray's native object store (plasma).
+*   **SHM threshold default differs**: when `worker_backend="ray"`, the SharedMemoryConnector default threshold is set to `sys.maxsize`, which forces payloads to go inline (no SHM). Override `shm_threshold_bytes` in the connector config if you want SHM for Ray runs.
+
+### 1.4 Internal Helpers
+
+*   **`initialize_ray_cluster`**: Connects to an existing Ray cluster or starts a local one.
+
+## 2. Troubleshooting
+
+*   **Connection Issues**: Ensure the Ray head node is accessible and ports (default 6399 in this example) are open.
+*   **Version Mismatch**: Ensure all nodes run the same version of Ray and Python.
+
+### Installation
+```bash
+pip install "ray[default]"
+```