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.

User Documentation

Python Backend
Business Logic Scripting (beta)
- Limitations
Interoperability and GPU Support
Examples
Running with Inferentia
Reporting problems, asking questions

Quick Start

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. 21.05).

Inside the container, clone the Python backend repository.

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

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

Start the Triton server.

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

In the host machine, start the client container.

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

In the client container, clone the Python backend repository.

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

Run the example client.

$ python3 python_backend/examples/add_sub/client.py

Building from Source

Requirements

cmake >= 3.17
numpy
rapidjson-dev
libarchive-dev

pip3 install numpy

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

sudo apt-get install rapidjson-dev libarchive-dev

Build Python backend. Replace <GIT_BRANCH_NAME> with the GitHub branch that you want to compile. For release branches it should be r<xx.yy> (e.g. r21.06).

$ mkdir build
$ cd build
$ cmake -DTRITON_ENABLE_GPU=ON -DTRITON_BACKEND_REPO_TAG=<GIT_BRANCH_NAME> -DTRITON_COMMON_REPO_TAG=<GIT_BRANCH_NAME> -DTRITON_CORE_REPO_TAG=<GIT_BRANCH_NAME> -DCMAKE_INSTALL_PREFIX:PATH=`pwd`/install ..
$ make install

The following required Triton repositories will be pulled and used in the build. If the CMake variables below are not specified, "main" branch of those repositories will be used. <GIT_BRANCH_NAME> should be the same as the Python backend repository branch that you are trying to compile.

triton-inference-server/backend: -DTRITON_BACKEND_REPO_TAG=<GIT_BRANCH_NAME>
triton-inference-server/common: -DTRITON_COMMON_REPO_TAG=<GIT_BRANCH_NAME>
triton-inference-server/common: -DTRITON_COMMON_REPO_TAG=<GIT_BRANCH_NAME>

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

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

Start the Triton Server

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

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

Using Custom Python Execution Environments

Python backend shipped in the NVIDIA GPU Cloud containers uses Python 3.8. If your Python model is compatible with Python 3.8 and requires only modules already included in the Triton container, then you can skip this section. If you need to use a different version of Python or if you have additional dependencies, you need to recompile the stub executable and create an execution environment as described below and include that with your model.

1. Building Custom Python Backend Stub

Python backend uses a stub process to connect your model.py file to the Triton C++ core. This stub process has an embedded Python interpreter with a fixed Python version. If you intend to use a Python interpreter with different version from the default Python backend stub, you need to compile your own Python backend stub by following the steps below:

Install the software packages below:

conda
cmake
rapidjson and libarchive (instructions for installing these packages in Ubuntu or Debian are included in Building from Source Section)

Create and activate a conda environment with your desired Python version. In this example, we will be using Python 3.6:

conda create -n python-3-6 python=3.6
conda activate python-3-6

# NumPy is required for Python models
conda install numpy

Clone the Python backend repository and compile the Python backend stub (replace <GIT_BRANCH_NAME> with the branch name that you want to use, for release branches it should be r<xx.yy>):

$ git clone https://github.com/triton-inference-server/python_backend -b
<GIT_BRANCH_NAME>
$ cd python_backend
$ mkdir build && cd build
$ cmake -DTRITON_ENABLE_GPU=ON -DCMAKE_INSTALL_PREFIX:PATH=`pwd`/install ..
$ make triton-python-backend-stub

Now, you have access to a Python backend stub with Python 3.6. You can verify that using ldd:

$ ldd triton_python_backend_stub
...
libpython3.6m.so.1.0 => /home/ubuntu/envs/miniconda3/envs/python-3-6/lib/libpython3.6m.so.1.0 (0x00007fbb69cf3000)
...

There are many other shared libraries printed in addition to the library posted above. However, it is important to see libpython3.6m.so.1.0 in the list of linked shared libraries. If you use a different Python version, you should see that version instead. You need to copy the triton_python_backend_stub to the model directory of the models that want to use the custom Python backend stub. For example, if you have model_a in your model repository, the folder structure should look like below:

models
|-- model_a
    |-- 1
    |   |-- model.py
    |-- config.pbtxt
    `-- triton_python_backend_stub

Note the location of triton_python_backend_stub in the directory structure above.

2. Packaging the Conda Environment

It is also required to create a tar file that contains your conda environment. Currently, Python backend only supports conda-pack for this purpose. conda-pack ensures that your conda environment is portable. You can create a tar file for your conda environment using conda-pack command:

$ conda-pack
Collecting packages...
Packing environment at '/home/iman/miniconda3/envs/python-3-6' to 'python-3-6.tar.gz'
[########################################] | 100% Completed |  4.5s

Important Note: Before installing the packages in your conda environment, make sure that you have exported PYTHONNOUSERSITE environment variable:

export PYTHONNOUSERSITE=True

If this variable is not exported and similar packages are installed outside your conda environment, your tar file may not contain all the dependencies required for an isolated Python environment.

After creating the tar file from the conda environment, you need to tell Python backend to use that environment for your model. You can do this by adding the lines below to the config.pbtxt file:

name: "model_a"
backend: "python"

...

parameters: {
  key: "EXECUTION_ENV_PATH",
  value: {string_value: "/home/iman/miniconda3/envs/python-3-6/python3.6.tar.gz"}
}

It is also possible to provide the execution environment path relative to the model folder in model repository:

name: "model_a"
backend: "python"

...

parameters: {
  key: "EXECUTION_ENV_PATH",
  value: {string_value: "$$TRITON_MODEL_DIRECTORY/python3.6.tar.gz"}
}

In this case, python3.tar.gz should be placed in the model folder and the model repository should look like below:

models
|-- model_a
|   |-- 1
|   |   `-- model.py
|   |-- config.pbtxt
|   |-- python3.6.tar.gz
|   `-- triton_python_backend_stub

In the example above, $$TRITON_MODEL_DIRECTORY is resolved to $pwd/models/model_a.

This is useful if you want to use S3, GCS, or Azure and you do not have access to the absolute path of the execution env that is stored in the cloud object storage service.

Important Notes

The version of the Python interpreter in the execution environment must match the version of triton_python_backend_stub.
If you don't want to use a different Python interpreter, you can skip Building Custom Python Backend Stub Step. In this case you only need to pack your environment using conda-pack and provide the path to tar file in the model config. However, the previous note still applies here and the version of the Python interpreter inside the conda environment must match the Python version of stub used by Python backend. The default version of the stub is Python 3.8.
You can share a single execution environment across multiple models. You need to provide the path to the tar file in the EXECUTION_ENV_PATH in the config.pbtxt of all the models that want to use the execution 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.

Business Logic Scripting (beta)

Triton's ensemble feature supports many use cases where multiple models are composed into a pipeline (or more generally a DAG, directed acyclic graph). However, there are many other use cases that are not supported because as part of the model pipeline they require loops, conditionals (if-then-else), data-dependent control-flow and other custom logic to be intermixed with model execution. We call this combination of custom logic and model executions Business Logic Scripting (BLS).

Starting from 21.08, you can implement BLS in your Python model. A new set of utility functions allows you to execute inference requests on other models being served by Triton as a part of executing your Python model. Example below shows how to use this feature:

import triton_python_backend_utils as pb_utils


class TritonPythonModel:
  ...
    def execute(self, requests):
      ...
      # Create an InferenceRequest object. `model_name`,
      # `requested_output_names`, and `inputs` are the required arguments and
      # must be provided when constructing an InferenceRequest object. Make sure
      # to replace `inputs` argument with a list of `pb_utils.Tensor` objects.
      inference_request = pb_utils.InferenceRequest(
          model_name='model_name',
          requested_output_names=['REQUESTED_OUTPUT_1', 'REQUESTED_OUTPUT_2'],
          inputs=[<pb_utils.Tensor object>])

      # `pb_utils.InferenceRequest` supports request_id, correlation_id, and model
      # version in addition to the arguments described above. These arguments
      # are optional. An example containing all the arguments:
      # inference_request = pb_utils.InferenceRequest(model_name='model_name',
      #   requested_output_names=['REQUESTED_OUTPUT_1', 'REQUESTED_OUTPUT_2'],
      #   inputs=[<list of pb_utils.Tensor objects>],
      #   request_id="1", correlation_id=4, model_version=1)

      # Execute the inference_request and wait for the response
      inference_response = inference_request.exec()

      # Check if the inference response has an error
      if inference_response.has_error():
          raise pb_utils.TritonModelException(inference_response.error().message())
      else:
          # Extract the output tensors from the inference response.
          output1 = pb_utils.get_output_tensor_by_name(inference_response, 'REQUESTED_OUTPUT_1')
          output2 = pb_utils.get_output_tensor_by_name(inference_response, 'REQUESTED_OUTPUT_2')

          # Decide the next steps for model execution based on the received output
          # tensors. It is possible to use the same output tensors to for the final
          # inference response too.

In addition to the inference_request.exec function that allows you to execute blocking inference requests, inference_request.async_exec allows you to perform async inference requests. This can be useful when you do not need the result of the inference immediately. Using async_exec function, it is possible to have multiple inflight inference requests and wait for the responses only when needed. Example below shows how to use async_exec:

import triton_python_backend_utils as pb_utils
import asyncio


class TritonPythonModel:
  ...

    # You must add the Python 'async' keyword to the beginning of `execute`
    # function if you want to use `async_exec` function.
    async def execute(self, requests):
      ...
      # Create an InferenceRequest object. `model_name`,
      # `requested_output_names`, and `inputs` are the required arguments and
      # must be provided when constructing an InferenceRequest object. Make sure
      # to replace `inputs` argument with a list of `pb_utils.Tensor` objects.
      inference_request = pb_utils.InferenceRequest(
          model_name='model_name',
          requested_output_names=['REQUESTED_OUTPUT_1', 'REQUESTED_OUTPUT_2'],
          inputs=[<pb_utils.Tensor object>])

      infer_response_awaits = []
      for i in range(4):
        # async_exec function returns an
        # [Awaitable](https://docs.python.org/3/library/asyncio-task.html#awaitables)
        # object.
        inference_response_awaits.append(inference_request.async_exec())

      # Wait for all of the inference requests to complete.
      infer_responses = await asyncio.gather(*infer_response_awaits)

      for infer_response in infer_responses:
        # Check if the inference response has an error
        if inference_response.has_error():
            raise pb_utils.TritonModelException(inference_response.error().message())
        else:
            # Extract the output tensors from the inference response.
            output1 = pb_utils.get_output_tensor_by_name(inference_response, 'REQUESTED_OUTPUT_1')
            output2 = pb_utils.get_output_tensor_by_name(inference_response, 'REQUESTED_OUTPUT_2')

            # Decide the next steps for model execution based on the received output
            # tensors.

A complete example for sync and async BLS in Python backend is included in the Examples section.

Limitations

The number of inference requests that can be executed as a part of your model execution is limited to the amount of shared memory available to the Triton server. If you are using Docker to start the TritonServer, you can control the shared memory usage using the --shm-size flag.
You need to make sure that the inference requests performed as a part of your model do not create a circular dependency. For example, if model A performs an inference request on itself and there are no more model instances ready to execute the inference request, the model will block on the inference execution forever.

Interoperability and GPU Support

Starting from 21.09 release, Python backend supports DLPack for zero-copy transfer of Python backend tensors to other frameworks. The methods below are added to the pb_utils.Tensor object to facilitate the same:

`pb_utils.Tensor.to_dlpack() -> PyCapsule`

This method can be called on existing instantiated tensors to convert a Tensor to DLPack. The code snippet below shows how this works with PyTorch:

from torch.utils.dlpack import from_dlpack
import triton_python_backend_utils as pb_utils

class TritonPythonModel:

  def execute(self, requests):
    ...
    input0 = pb_utils.get_input_tensor_by_name(request, "INPUT0")

    # We have converted a Python backend tensor to a PyTorch tensor without
    # making any copies.
    pytorch_tensor = from_dlpack(input0.to_dlpack())

`pb_utils.Tensor.from_dlpack() -> Tensor`

This static method can be used for creating a Tensor object from the DLPack encoding of the tensor. For example:

from torch.utils.dlpack import to_dlpack
import torch
import triton_python_backend_utils as pb_utils

class TritonPythonModel:

  def execute(self, requests):
    ...
    pytorch_tensor = torch.tensor([1, 2, 3], device='cuda')

    # Create a Python backend tensor from the DLPack encoding of a PyTorch
    # tensor.
    input0 = pb_utils.Tensor.from_dlpack("INPUT0", to_dlpack(pytorch_tensor))

This method only supports contiguous Tensors that are in C-order. If the tensor is not C-order contiguous an exception will be raised.

`pb_utils.Tensor.is_cpu() -> bool`

This function can be used to check whether a tensor is placed in CPU or not.

Input Tensor Device Placement

By default, the Python backend moves all input tensors to CPU before providing them to the Python model. Starting from 21.09, you can change this default behavior. By setting FORCE_CPU_ONLY_INPUT_TENSORS to "no", Triton will not move input tensors to CPU for the Python model. Instead, Triton will provide the input tensors to the Python model in either CPU or GPU memory, depending on how those tensors were last used. You cannot predict which memory will be used for each input tensor so your Python model must be able to handle tensors in both CPU and GPU memory. To enable this setting, you need to add this setting to the parameters section of model configuration:

parameters: { key: "FORCE_CPU_ONLY_INPUT_TENSORS" value: {string_value:"no"}}

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. Alternatively, you can create a Python Execution Environment. You can find the files for this example in examples/pytorch.

Business Logic Scripting

The BLS example needs the dependencies required for both of the above examples. You can find the complete example instructions in examples/bls.

Preprocessing

The Preprocessing example shows how to use Python Backend to do model preprocessing. You can find the complete example instructions in examples/preprocessing.

Running with Inferentia

Please see the README.md located in the python_backend/inferentia sub folder.

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.

okdimok/python_backend