[KV Connector] Add temporary, off-by-default VLLM_DISABLE_REQUEST_ID_RANDOMIZATION workaround

[eicherseiji]

v0.14.0 and later adds a random suffix to frontend-provided request IDs - #27987

This change broke several in-tree KV connectors that do not have CI coverage. See:

• [Bugfix][KV Transfer] Use kv_transfer_params for P2pNcclConnector coordination #33947 - P2P NCCL connector
• [ROCm][Bugfix] Resolve request_id mismatch and prevent crashes in Disaggregated Serving for moriio kv-connector #32630 and [DO NOT MERGE][ROCM][PD] Fix MoRIIO connector with transfer_id for P/D coordination #32937 - MoRIIO connector
• [P/D] rework mooncake connector and introduce its bootstrap server #31034 - Mooncake (fixed?)

As a temporary measure, add a VLLM_DISABLE_REQUEST_ID_RANDOMIZATION=1 env var that will restore the pre-v0.14.0 behavior so that users of these connectors can work around the breakage.

This env var will be removed again in a forthcoming release.

Manual testing

pip install quart aiohttp pyzmq msgpack

# Terminal 1: Proxy
python examples/online_serving/disaggregated_serving_p2p_nccl_xpyd/disagg_proxy_p2p_nccl_xpyd.py

# Terminal 2: Prefill (GPU 0), add VLLM_DISABLE_REQUEST_ID_RANDOMIZATION=1 for fix
CUDA_VISIBLE_DEVICES=0 vllm serve facebook/opt-125m \
    --enforce-eager --host 0.0.0.0 --port 20003 \
    --dtype float16 --max-model-len 2048 --gpu-memory-utilization 0.5 \
    --kv-transfer-config \
    '{"kv_connector":"P2pNcclConnector","kv_role":"kv_producer","kv_port":"21001","kv_connector_extra_config":{"proxy_ip":"0.0.0.0","proxy_port":"30001","http_port":"20003","send_type":"PUT_ASYNC"}}'

# Terminal 3: Decode (GPU 1), add VLLM_DISABLE_REQUEST_ID_RANDOMIZATION for fix
CUDA_VISIBLE_DEVICES=1 vllm serve facebook/opt-125m \
    --enforce-eager --host 0.0.0.0 --port 20005 \
    --dtype float16 --max-model-len 2048 --gpu-memory-utilization 0.5 \
    --kv-transfer-config \
    '{"kv_connector":"P2pNcclConnector","kv_role":"kv_consumer","kv_buffer_size":"5e9","kv_port":"22001","kv_connector_extra_config":{"proxy_ip":"0.0.0.0","proxy_port":"30001","http_port":"20005","send_type":"PUT_ASYNC"}}'

# Terminal 4: Test
curl -s http://localhost:11001/v1/completions \
  -H "Content-Type: application/json" \
  -d '{"model":"facebook/opt-125m","prompt":"The capital of France is","max_tokens":20}' \
  | python -m json.tool

Without fix

With VLLM_DISABLE_REQUEST_ID_RANDOMIZATION=1, the warning is visible and fires only once

[gemini-code-assist]

Code Review

This pull request introduces a new, temporary, opt-in environment variable VLLM_DISABLE_REQUEST_ID_RANDOMIZATION. This flag allows users to disable the automatic randomization of request ID suffixes, which is a hotfix for compatibility with connectors like P2pNcclConnector that require consistent request IDs across distributed instances.

The changes are well-implemented and minimal:

1. A new environment variable VLLM_DISABLE_REQUEST_ID_RANDOMIZATION is added in vllm/envs.py, following existing patterns for boolean flags in the project.
2. The request ID randomization logic in vllm/v1/engine/input_processor.py is now conditional on this new flag.

The implementation is clean, straightforward, and ensures backward compatibility since the new behavior is opt-in. I have reviewed the changes and found no issues of high or critical severity.

[markmc]

While in general we're trying to avoid adding new env vars (#25700) I think this is a pragmatic solution to provide a temporary workaround for broken KV connectors

[njhill]

We should add a warning that enabling this may cause failures and/or subtle correctness errors if there are any duplicate externally-provided request ids.

[eicherseiji]

Thanks @njhill, done