Customize BentoServer#

BentoML Server runs the Service API in an ASGI web serving layer and puts Runners in a separate worker process pool managed by BentoML. The ASGI web serving layer will expose REST endpoints for inference APIs, such as POST /predict and common infrastructure APIs, such as GET /metrics for monitoring.

BentoML offers a number of ways to customize the behaviors of the web serving layer to meet the needs of the consumers.

Custom Endpoint URL#

By default, the inference APIs are generated from the @api defined within a bentoml.Service. The URL route for the inference API is determined by the function name. Take the sample service from our tutorial for example, the function name classify will be used as the REST API URL /classify:

svc = bentoml.Service("iris_classifier", runners=[iris_clf_runner])

@svc.api(input=NumpyNdarray(), output=NumpyNdarray())
def classify(input_arr):

However, user can customize this URL endpoint via the route option in the bentoml.Service#api decorator. For example, the following code will assign the endpoint with URL /v1/models/iris_classifier/predict, regardless of the API function name:

import numpy as np
import bentoml
from import NumpyNdarray

iris_clf_runner = bentoml.sklearn.get("iris_clf:latest").to_runner()

svc = bentoml.Service("iris_classifier", runners=[iris_clf_runner])

def any_func_name(input_series: np.ndarray) -> np.ndarray:
    result =
    return result

ASGI Middleware#

Since the web serving layer is built with the Python ASGI protocol, users can use the bentoml.Service#add_asgi_middleware API to mount arbitrary ASGI middleware to change anything they may need to customize in the HTTP request to response lifecycle, such as manipulating the request headers, modifying the response status code, or authorizing access to an endpoint.

Users can not only implement their own ASGI middleware class, but also use existing middleware built by the Python web development community, such as:

For example, you can add do:

from starlette.middleware.httpsredirect import HTTPSRedirectMiddleware
from starlette.middleware.trustedhost import TrustedHostMiddleware

svc = bentoml.Service('my_service', runners=[...])

svc.add_asgi_middleware(TrustedHostMiddleware, allowed_hosts=['', '*'])

Fully Customized Endpoints#

BentoML provides first-class support for mounting existing WSGI or ASGI apps onto the web serving layer, to enable common use cases such as serving existing Python web applications alongside the models, performing custom authentication and authorization, handling GET requests and web UIs, or providing streaming capabilities.

Bundle ASGI app (e.g. FastAPI)#

BentoML’s web serving layer is ASGI native, existing ASGI apps can be mounted directly to and serving side-by-side with your BentoML Service.

Here’s an example of mounting BentoML Service with an ASGI app built with FastAPI:

import numpy as np
import pandas as pd
import bentoml
from import NumpyNdarray, JSON
from pydantic import BaseModel
from fastapi import FastAPI

class IrisFeatures(BaseModel):
    sepal_len: float
    sepal_width: float
    petal_len: float
    petal_width: float

bento_model = bentoml.sklearn.get("iris_clf_with_feature_names:latest")
iris_clf_runner = bento_model.to_runner()

svc = bentoml.Service("iris_fastapi_demo", runners=[iris_clf_runner])

@svc.api(input=JSON(pydantic_model=IrisFeatures), output=NumpyNdarray())
def predict_bentoml(input_data: IrisFeatures) -> np.ndarray:
    input_df = pd.DataFrame([input_data.dict()])

fastapi_app = FastAPI()

def metadata():
    return {"name":, "version": bento_model.tag.version}

# For demo purpose, here's an identical inference endpoint implemented via FastAPI"/predict_fastapi")
def predict(features: IrisFeatures):
    input_df = pd.DataFrame([features.dict()])
    results =
    return { "prediction": results.tolist()[0] }

# BentoML Runner's async API is recommended for async endpoints"/predict_fastapi_async")
async def predict_async(features: IrisFeatures):
    input_df = pd.DataFrame([features.dict()])
    results = await iris_clf_runner.predict.async_run(input_df)
    return { "prediction": results.tolist()[0] }

In addition to FastAPI, application mounting is supported for any ASGI web applications built with any frameworks adhering to the ASGI standards.

Bundle WSGI app (e.g. Flask)#

For WSGI web apps, such as a Flask app, BentoML provides a different API mount_wsgi_app which will internally convert the provided WSGI app into an ASGI app and serve side-by-side with your BentoML Service.

Here’s an example of mounting BentoML Service with an WSGI app built with Flask:

import numpy as np
import bentoml
from import NumpyNdarray
from flask import Flask, request, jsonify

bento_model = bentoml.sklearn.get("iris_clf:latest")
iris_clf_runner = bento_model.to_runner()

svc = bentoml.Service("iris_flask_demo", runners=[iris_clf_runner])

@svc.api(input=NumpyNdarray(), output=NumpyNdarray())
def predict_bentoml(input_series: np.ndarray) -> np.ndarray:

flask_app = Flask(__name__)

def metadata():
    return {"name":, "version": bento_model.tag.version}

# For demo purpose, here's an identical inference endpoint implemented via FastAPI
@flask_app.route("/predict_flask", methods=["POST"])
def predict():
    content_type = request.headers.get('Content-Type')
    if (content_type == 'application/json'):
        input_arr = np.array(request.json, dtype=float)
        return jsonify(
        return 'Content-Type not supported!'