Truss processes client requests by extracting and validating payloads. For advanced use cases, you can access the raw request object to:
  • Customize payload deserialization (e.g., binary protocol buffers).
  • Handle disconnections & cancel long-running predictions.
You can mix request objects with standard inputs or use requests exclusively for performance optimization.

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Using Request Objects in Truss

You can define request objects in preprocess, predict, and postprocess: 
import fastapi

class Model:
    def preprocess(self, request: fastapi.Request):
        ...

    def predict(self, inputs, request: fastapi.Request):
        ...

    def postprocess(self, inputs, request: fastapi.Request):
        ...

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Rules for Using Requests

  • The request must be type-annotated as fastapi.Request.
  • If only requests are used, Truss skips payload extraction for better performance.
  • If both request objects and standard inputs are used:
    • Request must be the second argument.
    • Preprocessing transforms inputs, but the request object remains unchanged.
    • postprocess cannot use only the request—it must receive the model’s output.
    • If predict only uses the request, preprocess cannot be used.
import fastapi, asyncio, logging

class Model:
    async def predict(self, inputs, request: fastapi.Request):
        await asyncio.sleep(1)
        if await request.is_disconnected():
            logging.warning("Cancelled before generation.")
            return  # Cancel request on the model engine here.

        for i in range(5):
            await asyncio.sleep(1.0)
            logging.warning(i)
            yield str(i)  # Streaming response
            if await request.is_disconnected():
                logging.warning("Cancelled during generation.")
                return  # Cancel request on the model engine here.
You must implement request cancellation at the model level, which varies by framework.

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Cancelling Requests in Specific Frameworks

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TRT-LLM (Polling-Based Cancellation)

For TensorRT-LLM, use response_iterator.cancel() to terminate streaming requests: 
async for request_output in response_iterator:
    if await is_cancelled_fn():
        logging.info("Request cancelled. Cancelling Triton request.")
        response_iterator.cancel()
        return
See full example in TensorRT-LLM Docs.

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vLLM (Abort API)

For vLLM, use engine.abort() to stop processing: 
async for request_output in results_generator:
    if await request.is_disconnected():
        await engine.abort(request_id)
        return
See full example in vLLM Docs.

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Unsupported Request Features

  • Streaming file uploads – Use URLs instead of embedding large data in the request.
  • Client-side headers – Most headers are stripped; include necessary metadata in the payload.