Python Backend

The Triton backend for Python. The goal of Python backend is to let you serve models written in Python by Triton Inference Server without having to write any C++ code.

Quick Start

1. Run the Triton Inference Server container.

$ docker run --shm-size=1g --ulimit memlock=-1 -p 8000:8000 -p 8001:8001 -p 8002:8002 --ulimit stack=67108864 -ti nvcr.io/nvidia/tritonserver:<xx.yy>-py3

Replace <xx.yy> with the Triton version (e.g. 20.11).

2. Inside the container, clone the Python backend repository.

$ git clone https://github.com/triton-inference-server/python_backend -b r<xx.yy>

3. Install example model.

$ cd python_backend
$ mkdir -p models/add_sub/1/
$ cp examples/add_sub/model.py models/add_sub/1/model.py
$ cp examples/add_sub/config.pbtxt models/add_sub/config.pbtxt

4. Start the Triton server.

$ tritonserver --model-repository `pwd`/models

5. In the host machine, start the client container.

 docker run -ti --net host nvcr.io/nvidia/tritonserver:<xx.yy>-py3-sdk /bin/bash

6. In the client container, clone the Python backend repository.

$ git clone https://github.com/triton-inference-server/python_backend -b r<xx.yy>

7. Run the example client.

$ python3 python_backend/examples/add_sub/client.py

Building from Source

1. Requirements

• cmake >= 3.17
• numpy
• rapidjson-dev

pip3 install numpy

On Ubuntu or Debian you can use the command below to install rapidjson:

sudo apt-get install rapidjson-dev

2. Build Python backend

$ mkdir build
$ cd build
$ cmake -DCMAKE_INSTALL_PREFIX:PATH=`pwd`/install ..
$ make install

The following required Triton repositories will be pulled and used in the build. By default the "main" branch/tag will be used for each repo but the listed CMake argument can be used to override.

• triton-inference-server/backend: -DTRITON_BACKEND_REPO_TAG=[tag]
• triton-inference-server/common: -DTRITON_COMMON_REPO_TAG=[tag]

Set DCMAKE_INSTALL_PREFIX to the location where the Triton Server is installed. In the released containers, this location is /opt/tritonserver.

3. Copy example model and configuration

$ mkdir -p models/add_sub/1/
$ cp examples/add_sub/model.py models/add_sub/1/model.py
$ cp examples/add_sub/config.pbtxt models/add_sub/config.pbtxt

4. Start the Triton Server

$ /opt/tritonserver/bin/tritonserver --model-repository=`pwd`/models

5. Use the client app to perform inference

$ python3 examples/add_sub/client.py

Usage

In order to use the Python backend, you need to create a Python file that has a structure similar to below:

import triton_python_backend_utils as pb_utils


class TritonPythonModel:
    """Your Python model must use the same class name. Every Python model
    that is created must have "TritonPythonModel" as the class name.
    """

    def initialize(self, args):
        """`initialize` is called only once when the model is being loaded.
        Implementing `initialize` function is optional. This function allows
        the model to initialize any state associated with this model.

        Parameters
        ----------
        args : dict
          Both keys and values are strings. The dictionary keys and values are:
          * model_config: A JSON string containing the model configuration
          * model_instance_kind: A string containing model instance kind
          * model_instance_device_id: A string containing model instance device ID
          * model_repository: Model repository path
          * model_version: Model version
          * model_name: Model name
        """
        print('Initialized...')

    def execute(self, requests):
        """`execute` must be implemented in every Python model. `execute`
        function receives a list of pb_utils.InferenceRequest as the only
        argument. This function is called when an inference is requested
        for this model.

        Parameters
        ----------
        requests : list
          A list of pb_utils.InferenceRequest

        Returns
        -------
        list
          A list of pb_utils.InferenceResponse. The length of this list must
          be the same as `requests`
        """

        responses = []

        # Every Python backend must iterate through list of requests and create
        # an instance of pb_utils.InferenceResponse class for each of them. You
        # should avoid storing any of the input Tensors in the class attributes
        # as they will be overridden in subsequent inference requests. You can
        # make a copy of the underlying NumPy array and store it if it is
        # required.
        for request in requests:
            # Perform inference on the request and append it to responses list...

        # You must return a list of pb_utils.InferenceResponse. Length
        # of this list must match the length of `requests` list.
        return responses

    def finalize(self):
        """`finalize` is called only once when the model is being unloaded.
        Implementing `finalize` function is optional. This function allows
        the model to perform any necessary clean ups before exit.
        """
        print('Cleaning up...')

Every Python backend can implement three main functions:

initialize

initialize is called once the model is being loaded. Implementing initialize is optional. initialize allows you to do any necessary initializations before execution. In the initialize function, you are given an args variable. args is a Python dictionary. Both keys and values for this Python dictionary are strings. You can find the available keys in the args dictionary along with their description in the table below:

key	description
model_config	A JSON string containing the model configuration
model_instance_kind	A string containing model instance kind
model_instance_device_id	A string containing model instance device ID
model_repository	Model repository path
model_version	Model version
model_name	Model name

execute

execute function is called whenever an inference request is made. Every Python model must implement execute function. In the execute function you are given a list of InferenceRequest objects. In this function, your execute function must return a list of InferenceResponse objects that has the same length as requests.

In case one of the inputs has an error, you can use the TritonError object to set the error message for that specific request. Below is an example of setting errors for an InferenceResponse object:

import triton_python_backend_utils as pb_utils


class TritonPythonModel:
    ...

    def execute(self, requests):
        responses = []

        for request in requests:
            if an_error_occurred:
              # If there is an error, the output_tensors are ignored
              responses.append(pb_utils.InferenceResponse(
                output_tensors=[], error=pb_utils.TritonError("An Error Occurred")))

        return responses

finalize

Implementing finalize is optional. This function allows you to do any clean ups necessary before the model is unloaded from Triton server.

You can look at the add_sub example which contains a complete example of implementing all these functions for a Python model that adds and subtracts the inputs given to it. After implementing all the necessary functions, you should save this file as model.py.

Model Config File

Every Python Triton model must provide a config.pbtxt file describing the model configuration. In order to use this backend you must set the backend field of your model config.pbtxt file to python. You shouldn't set platform field of the configuration.

Your models directory should look like below:

models
└── add_sub
    ├── 1
    │   └── model.py
    └── config.pbtxt

Python Environment

Python backend by default uses python3 available inside PATH. If you are using conda or venv, they will automatically update PATH and Python backend will use the correct environment.

Error Handling

If there is an error that affects the initialize, execute, or finalize function of the Python model you can use TritonInferenceException. Example below shows how you can do error handling in finalize:

import triton_python_backend_utils as pb_utils


class TritonPythonModel:
    ...

    def finalize(self):
      if error_during_finalize:
        raise pb_utils.TritonModelException("An error occurred during finalize.")

Managing Shared Memory

Starting from 21.04 release, Python backend uses shared memory to connect user's code to Triton. Note that this change is completely transparent and does not require any change to the existing user's model code.

Python backend, by default, allocates 64 MBs for each model instance. Then, it will grow the shared memory region by 64 MBs whenever an increase is required. You can configure the default shared memory used by each model instance using the shm-default-byte-size flag. The amount of shared memory growth can be configured using the shm-growth-byte-size.

You can also configure the timeout used for connecting Triton main process to the Python backend stubs using the stub-timeout-seconds. The default value is 30 seconds.

The config values described above can be passed to Triton using --backend-config flag:

/opt/tritonserver/bin/tritonserver --model-repository=`pwd`/models --backend-config=python,<config-key>=<config-value>

Also, if you are running Triton inside a Docker container you need to properly set the --shm-size flag depending on the size of your inputs and outputs. The default value for docker run command is 64MB which is very small.

Examples

For using the Triton Python client in these examples you need to install the Triton Python Client Library. The Python client for each of the examples is in the client.py file.

AddSub in Numpy

There is no dependencies required for the AddSub numpy example. Instructions on how to use this model is explained in the quick start section. You can find the files in examples/add_sub.

AddSubNet in PyTorch

In order to use this model, you need to install PyTorch. We recommend using pip method mentioned in the PyTorch website. Make sure that PyTorch is available in the same Python environment as other dependencies. If you need to create another Python environment, please refer to the "Changing Python Runtime Path" section of this readme. You can find the files for this example in examples/pytorch.

Reporting problems, asking questions

We appreciate any feedback, questions or bug reporting regarding this project. When help with code is needed, follow the process outlined in the Stack Overflow (https://stackoverflow.com/help/mcve) document. Ensure posted examples are:

• minimal – use as little code as possible that still produces the same problem

• complete – provide all parts needed to reproduce the problem. Check if you can strip external dependency and still show the problem. The less time we spend on reproducing problems the more time we have to fix it

• verifiable – test the code you're about to provide to make sure it reproduces the problem. Remove all other problems that are not related to your request/question.