Building Standard TensorFlow Model Server

This tutorial shows you how to use TensorFlow Serving components to build the standard TensorFlow model server that dynamically discovers and serves new versions of a trained TensorFlow model. If you just want to use the standard server to serve your models, see TensorFlow Serving basic tutorial.

This tutorial uses the simple Softmax Regression model introduced in the TensorFlow tutorial for handwritten image (MNIST data) classification. If you don’t know what TensorFlow or MNIST is, see the MNIST For ML Beginners tutorial.

The code for this tutorial consists of two parts:

  • A Python file that trains and exports multiple versions of the model.

  • A C++ file which is the standard TensorFlow model server that discovers new exported models and runs a gRPC service for serving them.

This tutorial steps through the following tasks:

  1. Train and export a TensorFlow model.
  2. Manage model versioning with TensorFlow Serving ServerCore.
  3. Configure batching using SessionBundleSourceAdapterConfig.
  4. Serve request with TensorFlow Serving ServerCore.
  5. Run and test the service.

Before getting started, please complete the prerequisites.

Train And Export TensorFlow Model

Clear the export directory if it already exists:

$>rm -rf /tmp/mnist_model

Train (with 100 iterations) and export the first version of model:

$>bazel build //tensorflow_serving/example:mnist_saved_model
$>bazel-bin/tensorflow_serving/example/mnist_saved_model --training_iteration=100 --model_version=1 /tmp/mnist_model

Train (with 2000 iterations) and export the second version of model:

$>bazel-bin/tensorflow_serving/example/mnist_saved_model --training_iteration=2000 --model_version=2 /tmp/mnist_model

As you can see in, the training and exporting is done the same way it is in the TensorFlow Serving basic tutorial. For demonstration purposes, you’re intentionally dialing down the training iterations for the first run and exporting it as v1, while training it normally for the second run and exporting it as v2 to the same parent directory – as we expect the latter to achieve better classification accuracy due to more intensive training. You should see training data for each training run in your mnist_model directory:

$>ls /tmp/mnist_model
1  2


Now imagine v1 and v2 of the model are dynamically generated at runtime, as new algorithms are being experimented with, or as the model is trained with a new data set. In a production environment, you may want to build a server that can support gradual rollout, in which v2 can be discovered, loaded, experimented, monitored, or reverted while serving v1. Alternatively, you may want to tear down v1 before bringing up v2. TensorFlow Serving supports both options – while one is good for maintaining availability during the transition, the other is good for minimizing resource usage (e.g. RAM).

TensorFlow Serving Manager does exactly that. It handles the full lifecycle of TensorFlow models including loading, serving and unloading them as well as version transitions. In this tutorial, you will build your server on top of a TensorFlow Serving ServerCore, which internally wraps an AspiredVersionsManager.

int main(int argc, char** argv) {

  ServerCore::Options options;
  options.model_server_config = model_server_config;
  options.servable_state_monitor_creator = &CreateServableStateMonitor;
  options.custom_model_config_loader = &LoadCustomModelConfig;

  ::google::protobuf::Any source_adapter_config;
      [kTensorFlowModelPlatform].mutable_source_adapter_config()) =

  std::unique_ptr<ServerCore> core;
  TF_CHECK_OK(ServerCore::Create(options, &core));
  RunServer(port, std::move(core));

  return 0;

ServerCore::Create() takes a ServerCore::Options parameter. Here are a few commonly used options:

  • ModelServerConfig that specifies models to be loaded. Models are declared either through model_config_list, which declares a static list of models, or through dynamic_model_config, which declares a dynamic list of models that may get updated at runtime.
  • PlatformConfigMap that maps from the name of the platform (such as tensorflow) to the PlatformConfig, which is used to create the SourceAdapter. SourceAdapter adapts StoragePath (the path where a model version is discovered) to model Loader (loads the model version from storage path and provides state transition interfaces to the Manager). If PlatformConfig contains SavedModelBundleSourceAdapterConfig, a SavedModelBundleSourceAdapter will be created, which we will explain later.

SavedModelBundle is a key component of TensorFlow Serving. It represents a TensorFlow model loaded from a given path and provides the same Session::Run interface as TensorFlow to run inference. SavedModelBundleSourceAdapter adapts storage path to Loader<SavedModelBundle> so that model lifetime can be managed by Manager. Please note that SavedModelBundle is the successor of deprecated SessionBundle. Users are encouraged to use SavedModelBundle as the support for SessionBundle will soon be removed.

With all these, ServerCore internally does the following:

  • Instantiates a FileSystemStoragePathSource that monitors model export paths declared in model_config_list.
  • Instantiates a SourceAdapter using the PlatformConfigMap with the model platform declared in model_config_list and connects the FileSystemStoragePathSource to it. This way, whenever a new model version is discovered under the export path, the SavedModelBundleSourceAdapter adapts it to a Loader<SavedModelBundle>.
  • Instantiates a specific implementation of Manager called AspiredVersionsManager that manages all such Loader instances created by the SavedModelBundleSourceAdapter. ServerCore exports the Manager interface by delegating the calls to AspiredVersionsManager.

Whenever a new version is available, this AspiredVersionsManager loads the new version, and under its default behavior unloads the old one. If you want to start customizing, you are encouraged to understand the components that it creates internally, and how to configure them.

It is worth mentioning that TensorFlow Serving is designed from scratch to be very flexible and extensible. You can build various plugins to customize system behavior, while taking advantage of generic core components like ServerCore and AspiredVersionsManager. For example, you could build a data source plugin that monitors cloud storage instead of local storage, or you could build a version policy plugin that does version transition in a different way – in fact, you could even build a custom model plugin that serves non-TensorFlow models. These topics are out of scope for this tutorial. However, you can refer to the custom source and [custom servable] (custom_servable) tutorials for more information.


Another typical server feature we want in a production environment is batching. Modern hardware accelerators (GPUs, etc.) used to do machine learning inference usually achieve best computation efficiency when inference requests are run in large batches.

Batching can be turned on by providing proper SessionBundleConfig when creating the SavedModelBundleSourceAdapter. In this case we set the BatchingParameters with pretty much default values. Batching can be fine-tuned by setting custom timeout, batch_size, etc. values. For details, please refer to BatchingParameters.

SessionBundleConfig session_bundle_config;
// Batching config
if (enable_batching) {
  BatchingParameters* batching_parameters =
*saved_model_bundle_source_adapter_config.mutable_legacy_config() =

Upon reaching full batch, inference requests are merged internally into a single large request (tensor), and tensorflow::Session::Run() is invoked (which is where the actual efficiency gain on GPUs comes from).

Serve with Manager

As mentioned above, TensorFlow Serving Manager is designed to be a generic component that can handle loading, serving, unloading and version transition of models generated by arbitrary machine learning systems. Its APIs are built around the following key concepts:

  • Servable: Servable is any opaque object that can be used to serve client requests. The size and granularity of a servable is flexible, such that a single servable might include anything from a single shard of a lookup table to a single machine-learned model to a tuple of models. A servable can be of any type and interface.

  • Servable Version: Servables are versioned and TensorFlow Serving Manager can manage one or more versions of a servable. Versioning allows for more than one version of a servable to be loaded concurrently, supporting gradual rollout and experimentation.

  • Servable Stream: A servable stream is the sequence of versions of a servable, with increasing version numbers.

  • Model: A machine-learned model is represented by one or more servables. Examples of servables are:

    • TensorFlow session or wrappers around them, such as SavedModelBundle.
    • Other kinds of machine-learned models.
    • Vocabulary lookup tables.
    • Embedding lookup tables.

    A composite model could be represented as multiple independent servables, or as a single composite servable. A servable may also correspond to a fraction of a Model, for example with a large lookup table sharded across many Manager instances.

To put all these into the context of this tutorial:

  • TensorFlow models are represented by one kind of servable – SavedModelBundle. SavedModelBundle internally consists of a tensorflow:Session paired with some metadata about what graph is loaded into the session and how to run it for inference.

  • There is a file-system directory containing a stream of TensorFlow exports, each in its own subdirectory whose name is a version number. The outer directory can be thought of as the serialized representation of the servable stream for the TensorFlow model being served. Each export corresponds to a servables that can be loaded.

  • AspiredVersionsManager monitors the export stream, and manages lifecycle of all SavedModelBundle` servables dynamically.

TensorflowPredictImpl::Predict then just:

  • Requests SavedModelBundle from the manager (through ServerCore).
  • Uses the generic signatures to map logical tensor names in PredictRequest to real tensor names and bind values to tensors.
  • Runs inference.

Test and Run The Server

Copy the first version of the export to the monitored folder and start the server.

$>mkdir /tmp/monitored
$>cp -r /tmp/mnist_model/1 /tmp/monitored
$>bazel build //tensorflow_serving/model_servers:tensorflow_model_server
$>bazel-bin/tensorflow_serving/model_servers/tensorflow_model_server --enable_batching --port=9000 --model_name=mnist --model_base_path=/tmp/monitored

The server will emit log messages every one second that say “Aspiring version for servable …”, which means it has found the export, and is tracking its continued existence.

Run the test with --concurrency=10. This will send concurrent requests to the server and thus trigger your batching logic.

$>bazel build //tensorflow_serving/example:mnist_client
$>bazel-bin/tensorflow_serving/example/mnist_client --num_tests=1000 --server=localhost:9000 --concurrency=10
Inference error rate: 13.1%

Then we copy the second version of the export to the monitored folder and re-run the test:

$>cp -r /tmp/mnist_model/2 /tmp/monitored
$>bazel-bin/tensorflow_serving/example/mnist_client --num_tests=1000 --server=localhost:9000 --concurrency=10
Inference error rate: 9.5%

This confirms that your server automatically discovers the new version and uses it for serving!