vllm_backend.py

# vllm_backend.py
import time, logging
from typing import Any, Dict, AsyncIterable

from vllm.sampling_params import SamplingParams
from backends_base import ChatBackend, ImagesBackend
from state import vllm_engine   # ✅ the single source of truth

logger = logging.getLogger(__name__)

class VLLMChatBackend(ChatBackend):
    """
    Streams completions from a local vLLM engine.
    Produces OpenAI-compatible ChatCompletionChunk dicts.
    """
    async def stream(self, request: Dict[str, Any]) -> AsyncIterable[Dict[str, Any]]:
        if vllm_engine is None:
            raise RuntimeError("vLLM engine not initialized")

        # For now: just grab the last user message
        messages = request.get("messages", [])
        prompt = messages[-1]["content"] if messages else "(empty)"

        params = SamplingParams(
            temperature=float(request.get("temperature", 0.7)),
            max_tokens=int(request.get("max_tokens", 512)),
            stream=True,
        )

        rid = f"chatcmpl-local-{int(time.time())}"
        now = int(time.time())
        model_name = request.get("model", "local-vllm")

        try:
            async for output in vllm_engine.generate(prompt, params, request_id=rid):
                text_piece = output.outputs[0].text
                yield {
                    "id": rid,
                    "object": "chat.completion.chunk",
                    "created": now,
                    "model": model_name,
                    "choices": [
                        {"index": 0, "delta": {"content": text_piece}, "finish_reason": None}
                    ],
                }
        except Exception:
            logger.exception("vLLM generation failed")
            raise

        # Final stop signal
        yield {
            "id": rid,
            "object": "chat.completion.chunk",
            "created": now,
            "model": model_name,
            "choices": [{"index": 0, "delta": {}, "finish_reason": "stop"}],
        }

class StubImagesBackend(ImagesBackend):
    """
    vLLM does not support image generation.
    For now, return a transparent PNG placeholder.
    """
    async def generate_b64(self, request: Dict[str, Any]) -> str:
        logger.warning("Image generation not supported in local vLLM backend.")
        # 1x1 transparent PNG
        return "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR4nGP4BwQACfsD/etCJH0AAAAASUVORK5CYII="