Triton Inference Server Backend

A Triton backend is the implementation that executes a model. A backend can be a wrapper around a deep-learning framework, like PyTorch, TensorFlow, TensorRT or ONNX Runtime. Or a backend can be custom C/C++ logic performing any operation (for example, image pre-processing).

This repo contains documentation on Triton backends and also source, scripts and utilities for creating Triton backends. You do not need to use anything provided in this repo to create a Triton backend but you will likely find its contents useful.

Frequently Asked Questions

Full documentation is included below but these shortcuts can help you get started in the right direction.

Where can I ask general questions about Triton and Triton backends?

Be sure to read all the information below as well as the general Triton documentation available in the main server repo. If you don't find your answer there you can ask questions on the main Triton issues page.

Where can I find all the backends that are available for Triton?

Anyone can develop a Triton backend, so it isn't possible for us to know about all available backends. But the Triton project does provide a set of supported backends that are tested and updated with each Triton release.

TensorRT: The TensorRT backend is used to execute TensorRT models. The server repo contains the source for the backend.

ONNX Runtime: The ONNX Runtime backend is used to execute ONNX models. The onnxruntime_backend repo contains the documentation and source for the backend.

TensorFlow: The TensorFlow backend is used to execute TensorFlow models in both GraphDef and SavedModel formats. The same backend is used to execute both TensorFlow 1 and TensorFlow 2 models. The tensorflow_backend repo contains the documentation and source for the backend.

PyTorch: The PyTorch backend is used to execute TorchScript models. The pytorch_backend repo contains the documentation and source for the backend.

OpenVINO: The OpenVINO backend is used to execute OpenVINO models. The openvino_backend repo contains the documentation and source for the backend.

Python: The Python backend allows you to write your model logic in Python. For example, you can use this backend to execute pre/post processing code written in Python, or to execute a PyTorch Python script directly (instead of first converting it to TorchScript and then using the PyTorch backend). The python_backend repo contains the documentation and source for the backend.

DALI: DALI is a collection of highly optimized building blocks and an execution engine that accelerates the pre-processing of the input data for deep learning applications. The DALI backend allows you to execute your DALI pipeline within Triton. The dali_backend repo contains the documentation and source for the backend.

FIL: The FIL (Forest Inference Library) backend is used to execute a variety of tree-based ML models, including XGBoost models, LightGBM models, Scikit-Learn random forest models, and cuML random forest models. The fil_backend repo contains the documentation and source for the backend.

Important Note! Not all the above backends are supported on every platform supported by Triton. Look at the Backend-Platform Support Matrix to learn about the same.

How can I develop my own Triton backend?

First you probably want to ask on the main Triton issues page to make sure you are not duplicating a backend that already exists. Then follow the tutorial to learn how to create your first simple Triton backend and incrementally improve it to add more features. You should also read the complete documentation on Triton backends.

Can I add (or remove) a backend to an existing Triton installation?

Yes. See Backend Shared Library for general information about how the shared library implementing a backend is managed by Triton, and Triton with Unsupported and Custom Backends for documentation on how to add your backend to the released Triton Docker image. For a standard install the globally available backends are in /opt/tritonserver/backends.

What about backends developed using the "legacy custom backend" API.

The legacy custom API is removed from Triton. If you have custom backends that you developed using this older API you must port them to the new Triton Backend API.

Backends

A Triton backend is the implementation that executes a model. A backend can be a wrapper around a deep-learning framework, like PyTorch, TensorFlow, TensorRT, ONNX Runtime or OpenVINO. A backend can also implement any functionality you want as long as it adheres to the backend API. Triton uses this API to send requests to the backend for execution and the backend uses the API to communicate with Triton.

Every model must be associated with a backend. A model's backend is specified in the model's configuration using the backend setting. For using TensorRT backend, the value of this setting should be tensorrt. Similarly, for using PyTorch, ONNX and TensorFlow backends, the backend field should be set to pytorch, onnxruntime or tensorflow respectively. For all other backends, backend must be set to the name of the backend. Some backends may also check the platform setting for categorizing the model, for example, in TensorFlow backend, platform should be set to tensorflow_savedmodel or tensorflow_graphdef according to the model format. Please refer to the specific backend repository on whether platform is used.

Backend Shared Library

Each backend must be implemented as a shared library and the name of the shared library must be libtriton_<backend-name>.so. For example, if the name of the backend is "mybackend", a model indicates that it uses the backend by setting the model configuration 'backend' setting to "mybackend", and Triton looks for libtriton_mybackend.so as the shared library that implements the backend. The tutorial shows examples of how to build your backend logic into the appropriate shared library.

For a model, M that specifies backend B, Triton searches for the backend shared library in the following places, in this order:

• <model_repository>/M/<version_directory>/libtriton_B.so

• <model_repository>/M/libtriton_B.so

• <global_backend_directory>/B/libtriton_B.so

Where <global_backend_directory> is by default /opt/tritonserver/backends. The --backend-directory flag can be used to override the default.

Typically you will install your backend into the global backend directory. For example, if using Triton Docker images you can follow the instructions in Triton with Unsupported and Custom Backends. Continuing the example of a backend names "mybackend", you would install into the Triton image as:

/opt/
  tritonserver/
    backends/
      mybackend/
        libtriton_mybackend.so
        ... # other files needed by mybackend

Triton Backend API

A Triton backend must implement the C interface defined in tritonbackend.h. The following abstractions are used by the API.

TRITONBACKEND_Backend

A TRITONBACKEND_Backend object represents the backend itself. The same backend object is shared across all models that use the backend. The associated API, like TRITONBACKEND_BackendName, is used to get information about the backend and to associate a user-defined state with the backend.

A backend can optionally implement TRITONBACKEND_Initialize and TRITONBACKEND_Finalize to get notification of when the backend object is created and destroyed (for more information see backend lifecycles).

TRITONBACKEND_Model

A TRITONBACKEND_Model object represents a model. Each model loaded by Triton is associated with a TRITONBACKEND_Model. Each model can use the TRITONBACKEND_ModelBackend API to get the backend object representing the backend that is used by the model.

The same model object is shared across all instances of that model. The associated API, like TRITONBACKEND_ModelName, is used to get information about the model and to associate a user-defined state with the model.

Most backends will implement TRITONBACKEND_ModelInitialize and TRITONBACKEND_ModelFinalize to initialize the backend for a given model and to manage the user-defined state associated with the model (for more information see backend lifecycles).

The backend must take into account threading concerns when implementing TRITONBACKEND_ModelInitialize and TRITONBACKEND_ModelFinalize. Triton will not perform multiple simultaneous calls to these functions for a given model; however, if a backend is used by multiple models Triton may simultaneously call the functions with a different thread for each model. As a result, the backend must be able to handle multiple simultaneous calls to the functions. Best practice for backend implementations is to use only function-local and model-specific user-defined state in these functions, as is shown in the tutorial.

TRITONBACKEND_ModelInstance

A TRITONBACKEND_ModelInstance object represents a model instance. Triton creates one or more instances of the model based on the instance_group settings specified in the model configuration. Each of these instances is associated with a TRITONBACKEND_ModelInstance object.

The only function that the backend must implement is TRITONBACKEND_ModelInstanceExecute. The TRITONBACKEND_ModelInstanceExecute function is called by Triton to perform inference/computation on a batch of inference requests. Most backends will also implement TRITONBACKEND_ModelInstanceInitialize and TRITONBACKEND_ModelInstanceFinalize to initialize the backend for a given model instance and to manage the user-defined state associated with the model (for more information see backend lifecycles).

The backend must take into account threading concerns when implementing TRITONBACKEND_ModelInstanceInitialize, TRITONBACKEND_ModelInstanceFinalize and TRITONBACKEND_ModelInstanceExecute. Triton will not perform multiple simultaneous calls to these functions for a given model instance; however, if a backend is used by a model with multiple instances or by multiple models Triton may simultaneously call the functions with a different thread for each model instance. As a result, the backend must be able to handle multiple simultaneous calls to the functions. Best practice for backend implementations is to use only function-local and model-specific user-defined state in these functions, as is shown in the tutorial.

TRITONBACKEND_Request

A TRITONBACKEND_Request object represents an inference request made to the model. The backend takes ownership of the request object(s) in TRITONBACKEND_ModelInstanceExecute and must release each request by calling TRITONBACKEND_RequestRelease. However, the ownership of request object is returned back to Triton in case TRITONBACKEND_ModelInstanceExecute returns an error. See Inference Requests and Responses for more information about request lifecycle.

The Triton Backend API allows the backend to get information about the request as well as the input and request output tensors of the request. Each request input is represented by a TRITONBACKEND_Input object.

TRITONBACKEND_Response

A TRITONBACKEND_Response object represents a response sent by the backend for a specific request. The backend uses the response API to set the name, shape, datatype and tensor values for each output tensor included in the response. The response can indicate either a failed or a successful request. See Inference Requests and Responses for more information about request-response lifecycle.

TRITONBACKEND_BackendAttribute

A TRITONBACKEND_BackendAttribute allows a backend to set certain attributes which are queried by Triton to inform certain feature support, preferred configurations, and other types of backend-specific behavior.

When initializing a backend, Triton will query the TRITONBACKEND_GetBackendAttribute function if implemented by the backend. This function is optional to implement, but is generally used to call the related TRITONBACKEND_BackendAttribute APIs for setting backend-specific attributes.

Some of the relevant BackendAttribute setter APIs are listed below:

• TRITONBACKEND_BackendSetExecutionPolicy
• TRITONBACKEND_BackendAttributeAddPreferredInstanceGroup
  • Defines a priority list of instance groups to prefer for this backend if a model config doesn't explicitly define any instance groups.
• TRITONBACKEND_BackendAttributeSetParallelModelInstanceLoading
  • Defines whether the backend can safely handle concurrent calls to TRITONBACKEND_ModelInstanceInitialize or not.
  • Loading model instances in parallel can improve server startup times for large instance counts.
  • By default, this attribute is set to false, meaning that parallel instance loading is disabled for all backends unless explicitly enabled.
  • The following official backends currently support loading model instances in parallel:
    • Python
    • ONNXRuntime

The full list of TRITONBACKEND_BackendAttribute related APIs are defined in tritonbackend.h.

Backend Lifecycles

A backend must carefully manage the lifecycle of the backend itself, the models and model instances that use the backend and the inference requests that execute on the model instances using the backend.

Backend and Model

Backend, model and model instance initialization is triggered when Triton loads a model.

• If the model requires a backend that is not already in use by an already loaded model, then:

  • Triton loads the shared library that implements the backend required by the model.

  • Triton creates the TRITONBACKEND_Backend object that represents the backend.

  • Triton calls TRITONBACKEND_Initialize if it is implemented in the backend shared library. TRITONBACKEND_Initialize should not return until the backend is completely initialized. If TRITONBACKEND_Initialize returns an error, Triton will report that the model failed to load.

• Triton creates the TRITONBACKEND_Model object that represents the model. Triton calls TRITONBACKEND_ModelInitialize if it is implemented in the backend shared library. TRITONBACKEND_ModelInitialize should not return until the backend is completely initialized for the model. If TRITONBACKEND_ModelInitialize returns an error, Triton will show that the model failed to load.

• For each model instance specified for the model in the model configuration:

  • Triton creates the TRITONBACKEND_ModelInstance object that represents the model instance.

  • Triton calls TRITONBACKEND_ModelInstanceInitialize if it is implemented in the backend shared library. TRITONBACKEND_ModelInstanceInitialize should not return until the backend is completely initialized for the instance. If TRITONBACKEND_ModelInstanceInitialize returns an error, Triton will show that the model failed to load.

Backend, model and model instance finalization is triggered when Triton unloads a model.

• For each model instance:

  • Triton calls TRITONBACKEND_ModelInstanceFinalize if it is implemented in the backend shared library. TRITONBACKEND_ModelInstanceFinalize should not return until the backend is completely finalized, including stopping any threads create for the model instance and freeing any user-defined state created for the model instance.

  • Triton destroys the TRITONBACKEND_ModelInstance object that represents the model instance.

• Triton calls TRITONBACKEND_ModelFinalize if it is implemented in the backend shared library. TRITONBACKEND_ModelFinalize should not return until the backend is completely finalized, including stopping any threads create for the model and freeing any user-defined state created for the model.

• Triton destroys the TRITONBACKEND_Model object that represents the model.

• Even if no other loaded model requires the backend, Triton does not finalize and unload the backend until the tritonserver process is exiting. When the tritonserver process exits:

  • Triton calls TRITONBACKEND_Finalize if it is implemented in the backend shared library. TRITONBACKEND_ModelFinalize should not return until the backend is completely finalized, including stopping any threads create for the backend and freeing any user-defined state created for the backend.

  • Triton destroys the TRITONBACKEND_Backend object that represents the backend.

Inference Requests and Responses

Triton calls TRITONBACKEND_ModelInstanceExecute to execute inference requests on a model instance. Each call to TRITONBACKEND_ModelInstanceExecute communicates a batch of requests to execute and the instance of the model that should be used to execute those requests. The backend should not allow the caller thread to return from TRITONBACKEND_ModelInstanceExecute until that instance is ready to handle another set of requests. Typically this means that the TRITONBACKEND_ModelInstanceExecute function will create responses and release the requests before returning. However, in case TRITONBACKEND_ModelInstanceExecute returns an error, the ownership of requests is transferred back to Triton which will then be responsible for releasing them. Therefore, in the case where TRITONBACKEND_ModelInstanceExecute returns an error, the backend must not retain references to the requests or access them in any way. For more detailed description of request/response lifetimes, study the documentation of TRITONBACKEND_ModelInstanceExecute in tritonbackend.h.

Single Response

Most backends will create a single response for each request. For that kind of backend, executing a single inference request requires the following steps:

• Create a response for the request using TRITONBACKEND_ResponseNew.

• For each request input tensor use TRITONBACKEND_InputProperties to get shape and datatype of the input as well as the buffer(s) containing the tensor contents.

• For each output tensor which the request expects to be returned, use TRITONBACKEND_ResponseOutput to create the output tensor of the required datatype and shape. Use TRITONBACKEND_OutputBuffer to get a pointer to the buffer where the tensor's contents should be written.

• Use the inputs to perform the inference computation that produces the requested output tensor contents into the appropriate output buffers.

• Optionally set parameters in the response.

• Send the response using TRITONBACKEND_ResponseSend.

• Release the request using TRITONBACKEND_RequestRelease.

For a batch of requests the backend should attempt to combine the execution of the individual requests as much as possible to increase performance.

Decoupled Responses

It is also possible for a backend to send multiple responses for a request or not send any responses for a request. A backend may also send responses out-of-order relative to the order that the request batches are executed. Such backends are called decoupled backends. The decoupled backends use one ResponseFactory object per request to keep creating and sending any number of responses for the request. For this kind of backend, executing a single inference request typically requires the following steps:

• For each request input tensor use TRITONBACKEND_InputProperties to get shape and datatype of the input as well as the buffer(s) containing the tensor contents.

• Create a ResponseFactory object for the request using TRITONBACKEND_ResponseFactoryNew.

  1. Create a response from the ResponseFactory object using TRITONBACKEND_ResponseNewFromFactory. As long as you have ResponseFactory object you can continue creating responses.

  2. For each output tensor which the request expects to be returned, use TRITONBACKEND_ResponseOutput to create the output tensor of the required datatype and shape. Use TRITONBACKEND_OutputBuffer to get a pointer to the buffer where the tensor's contents should be written.

  3. Use the inputs to perform the inference computation that produces the requested output tensor contents into the appropriate output buffers.

  4. Optionally set parameters in the response.

  5. Send the response using TRITONBACKEND_ResponseSend. If this is the last request then use TRITONSERVER_ResponseCompleteFlag with TRITONBACKEND_ResponseSend. Otherwise continue with Step 1 for sending next request

• Release the request using TRITONBACKEND_RequestRelease.

Special Cases

The decoupled API is powerful and supports various special cases:

• If the backend should not send any more responses for the request, TRITONBACKEND_ResponseFactorySendFlags can be used to send TRITONSERVER_RESPONSE_COMPLETE_FINAL using the ResponseFactory.

• The model can also send responses out-of-order in which it received requests.

• The backend can copy out the contents of the input buffer(s) if request is to be released before the contents are completely consumed to generate responses. After copy, the request can be released anytime before exiting TRITONBACKEND_ModelInstanceExecute. The copies and ResponseFactory object can be passed to a separate thread in backend. This means main caller thread can exit from TRITONBACKEND_ModelInstanceExecute and the backend can still continue generating responses as long as it holds ResponseFactory object.

The repeat example demonstrates full power of what can be achieved from decoupled API.

Study documentation of these TRITONBACKEND_* functions in tritonbackend.h for more details on these APIs. Read Decoupled Backends and Models for more details on how to host a decoupled model.

Build the Backend Utilities

The source in this repo builds into a single "backend utilities" library that is useful when building backends. You don't need to use these utilities but they will be helpful for most backends.

Typically you don't need to build this repo directly but instead you can include it in the build of your backend as is shown in the CMakeLists.txt files of the tutorial examples.

To build and install in a local directory use the following commands.

$ 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/common: -DTRITON_COMMON_REPO_TAG=[tag]
• triton-inference-server/core: -DTRITON_CORE_REPO_TAG=[tag]

See the CMakeLists.txt file for other build options.