[Rust Frontend] Extend the existing gRPC service protocol

[connorcarpenter15]

Purpose

Extend the existing vLLM gRPC listener with protocol definitions for control-plane operations required by later PRs. Generation remains on vllm.Generate; the new operations are grouped under a separate vllm.Engine service on the same listener.

Every vllm.Engine RPC returns UNIMPLEMENTED until its implementation PR lands. This PR also registers the canonical grpc.health.v1.Health service.

What changes

• Extend rust/proto/vllm_grpc.proto with a vllm.Engine service for:
  • deployment and model discovery
  • abort and drain
  • dynamic LoRA lifecycle
  • KV event-source discovery
• Add deployment metadata for:
  • instance and API versions
  • supported models
  • tensor, pipeline, data, and decode-context parallelism
  • KV-cache and scheduler capacity
• Add model metadata for:
  • served names and aliases
  • context and output limits
  • tokenizer modes and LoRA capacity
  • supported input and parsing capabilities
• Extend GenerateRequest with multimodal media inputs and loaded LoRA selection by name.
• Keep the existing unary Generate and streaming GenerateStream methods unchanged.
• Generate bindings through the existing protobuf build path.
• Add an unimplemented EngineServiceImpl and register it on the existing listener.
• Register tonic-health on the same listener and report SERVING for vllm.Generate, vllm.Engine, and the overall server.
• Test canonical health responses and verify every control RPC returns gRPC UNIMPLEMENTED.

Review boundaries

• Reuses the existing gRPC listener and --grpc-port.
• Reuses existing host, TLS, keepalive, and shutdown behavior.
• Adds no CLI arguments, listener, port, or server process.
• Separates data and control APIs at the service level only.
• Does not implement discovery, lifecycle, KV event-source, or LoRA behavior.
• Does not expose connector handshake metadata or forced data-parallel routing in request payloads.
• Existing unary and streaming generation behavior is unchanged.

Test Plan

• Verify vllm.Generate and vllm.Engine are reported as SERVING through the canonical gRPC health API.
• Verify every declared vllm.Engine RPC reaches the existing listener and returns UNIMPLEMENTED.
• Run an existing unary generation test to confirm the data service is unchanged.
• Check Rust compilation, formatting, and whitespace.

Test Result

• cargo check -p vllm-server — passed
• cargo test -p vllm-server grpc::tests::canonical_health_and_unimplemented_extensions_share_listener -- --exact --nocapture — passed
• cargo test -p vllm-server grpc::tests::unary_generate_returns_collected_text -- --exact --nocapture — passed
• cargo fmt --all -- --check — passed
• git diff --check upstream/main...HEAD — passed
• Final diff: 7 files, 369 additions, 31 deletions; no new listener, port, process, or CLI argument

AI assistance disclosure: This PR was authored with AI assistance.

---

Essential elements checklist

• Purpose and scope are described.
• Test plan and results are included.
• Data and control services share the existing listener.
• Existing gRPC generation behavior and compatibility boundaries are stated.
• Documentation impact was considered; this PR declares unimplemented protocol extensions and canonical health registration.

BEFORE SUBMITTING, PLEASE READ https://docs.vllm.ai/en/latest/contributing

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[claude]

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[njhill]

Thanks @connorcarpenter15, much/most of this looks good to me

[njhill]

Just thinking aloud, I wonder whether DeploymentConfig / DeploymentInfo would be more appropriate.

[connorcarpenter15]

What about ServerInfo? Fine with DeploymentInfo though if you think that's better.

[njhill]

What is the purpose of drain here? Shouldn't the router be able to handle this by itself?

[connorcarpenter15]

The router can stop sending new requests, but I kept this to provide a server-side admission barrier and wait for in-flight requests to finish before shutdown. This only affects this gRPC service.

[njhill]

But the router also has full knowledge of in-flight requests and can wait for/abort them as needed. Just trying to understand what this API actually buys you...

[connorcarpenter15]

Yes. This was originally meant to clearly support graceful shutdowns, but I agree that it doesn't actually provide much functional value. We can remove it now and add it back later if we discover a scenario where provides real value.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @connorcarpenter15.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork