For larger scale deployments especially, it can make sense to handle the orchestration and load balancing of data parallel ranks externally.
In this case, it's more convenient to treat each DP rank like a separate vLLM deployment, with its own endpoint, and have an external router balance HTTP requests between them, making use of appropriate real-time telemetry from each server for routing decisions.
## Getting Start
The functionality of [external DP](https://docs.vllm.ai/en/latest/serving/data_parallel_deployment/?h=external#external-load-balancing) is already natively supported by vLLM. In vllm-ascend we provide two enhanced functionalities:
1. A launch script which helps to launch multi vllm instances in one command.
2. A request-length-aware load balance proxy for external dp.
First you need to have at least two vLLM servers running in data parallel. These can be mock servers or actual vLLM servers. Note that this proxy also works with only one vLLM server running, but will fall back to direct request forwarding which is meaningless.
You can start external vLLM dp servers one-by-one manually or using the launch script in `examples/external_online_dp`. For scenarios of large dp size across multi nodes, we recommend using our launch script for convenience.
Firstly, you need to modify the `examples/external_online_dp/run_dp_template.sh` according to your vLLM configuration. Then you can use `examples/external_online_dp/launch_online_dp.py` to launch multiple vLLM instances in one command each node. It will internally call `examples/external_online_dp/run_dp_template.sh` for each DP rank with proper DP-related parameters.
An example of running external DP in one single node:
After all vLLM DP instances are launched, you can now launch the load-balance proxy server which serves as entrypoint for coming requests and load balance them between vLLM DP instances.
