Save and load trained models

Learn how to save and load trained models in your application.

Throughout the model building process, a model lives in memory and is accessible throughout the application's lifecycle. However, once the application stops running, if the model is not saved somewhere locally or remotely, it's no longer accessible. Typically models are used at some point after training in other applications either for inference or re-training. Therefore, it's important to store the model. Save and load models using the steps described in subsequent sections of this document when using data preparation and model training pipelines like the one detailed below. Although this sample uses a linear regression model, the same process applies to other ML.NET algorithms.

HousingData[] housingData = new HousingData[]
{
    new HousingData
    {
        Size = 600f,
        HistoricalPrices = new float[] { 100000f, 125000f, 122000f },
        CurrentPrice = 170000f
    },
    new HousingData
    {
        Size = 1000f,
        HistoricalPrices = new float[] { 200000f, 250000f, 230000f },
        CurrentPrice = 225000f
    },
    new HousingData
    {
        Size = 1000f,
        HistoricalPrices = new float[] { 126000f, 130000f, 200000f },
        CurrentPrice = 195000f
    }
};

// Create MLContext
MLContext mlContext = new MLContext();

// Load Data
IDataView data = mlContext.Data.LoadFromEnumerable<HousingData>(housingData);

// Define data preparation estimator
EstimatorChain<RegressionPredictionTransformer<LinearRegressionModelParameters>> pipelineEstimator =
    mlContext.Transforms.Concatenate("Features", new string[] { "Size", "HistoricalPrices" })
        .Append(mlContext.Transforms.NormalizeMinMax("Features"))
        .Append(mlContext.Regression.Trainers.Sdca());

// Train model
ITransformer trainedModel = pipelineEstimator.Fit(data);

// Save model
mlContext.Model.Save(trainedModel, data.Schema, "model.zip");

Because most models and data preparation pipelines inherit from the same set of classes, the save and load method signatures for these components is the same. Depending on your use case, you can either combine the data preparation pipeline and model into a single EstimatorChain which would output a single ITransformer or separate them thus creating a separate ITransformer for each.

Save a model locally

When saving a model you need two things:

1. The ITransformer of the model.
2. The DataViewSchema of the ITransformer's expected input.

After training the model, use the Save method to save the trained model to a file called model.zip using the DataViewSchema of the input data.

// Save Trained Model
mlContext.Model.Save(trainedModel, data.Schema, "model.zip");

Save an ONNX model locally

To save an ONNX version of your model locally you will need the Microsoft.ML.OnnxConverter NuGet package installed.

With the OnnxConverter package installed, we can use it to save our model into the ONNX format. This requires a Stream object which we can provide as a FileStream using the File.Create method. The File.Create method takes in a string as a parameter which will be the path of the ONNX model.

using FileStream stream = File.Create("./onnx_model.onnx");

With the stream created, we can call the ConvertToOnnx method and give it the trained model, the data used to train the model, and the stream. However, not all trainers and transformers are exportable to ONNX. For a complete list, visit the Transforms and How to Choose an ML.NET Algorithm guides.

mlContext.Model.ConvertToOnnx(trainedModel, data, stream);

Load a model stored locally

Models stored locally can be used in other processes or applications like ASP.NET Core and Serverless Web Applications. See Use ML.NET in Web API and Deploy ML.NET Serverless Web App how-to articles to learn more.

In a separate application or process, use the Load method along with the file path to get the trained model into your application.

//Define DataViewSchema for data preparation pipeline and trained model
DataViewSchema modelSchema;

// Load trained model
ITransformer trainedModel = mlContext.Model.Load("model.zip", out modelSchema);

Load an ONNX model locally

To load in an ONNX model for predictions, you will need the Microsoft.ML.OnnxTransformer NuGet package.

With the OnnxTransformer package installed, you can load an existing ONNX model by using the ApplyOnnxModel method. The required parameter is a string which is the path of the local ONNX model.

OnnxScoringEstimator estimator = mlContext.Transforms.ApplyOnnxModel("./onnx_model.onnx");

The ApplyOnnxModel method returns an OnnxScoringEstimator object. First, we need to load in the new data.

HousingData[] newHousingData = new HousingData[]
{
    new()
    {
        Size = 1000f,
        HistoricalPrices = new[] { 300_000f, 350_000f, 450_000f },
        CurrentPrice = 550_00f
    }
};

With the new data we can load that into an IDataView using the LoadFromEnumerable method.

IDataView newHousingDataView = mlContext.Data.LoadFromEnumerable(newHousingData);

Now, we can use the new IDataView to fit on the new data.

estimator.Fit(newHousingDataView);

After using the Fit method on an estimator from ApplyOnnxModel, it can then be saved as a new model using the Save method mentioned save a model locally section.

Load a model stored remotely

To load data preparation pipelines and models stored in a remote location into your application, use a Stream instead of a file path in the Load method.

// Create MLContext
MLContext mlContext = new MLContext();

// Define DataViewSchema and ITransformers
DataViewSchema modelSchema;
ITransformer trainedModel;

// Load data prep pipeline and trained model
using (HttpClient client = new HttpClient())
{
    Stream modelFile = await client.GetStreamAsync("<YOUR-REMOTE-FILE-LOCATION>");

    trainedModel = mlContext.Model.Load(modelFile, out modelSchema);
}

Working with separate data preparation and model pipelines

When working with separate data preparation pipelines and models, the same process as single pipelines applies; except now both pipelines need to be saved and loaded simultaneously.

Given separate data preparation and model training pipelines:

// Define data preparation estimator
IEstimator<ITransformer> dataPrepEstimator =
    mlContext.Transforms.Concatenate("Features", new string[] { "Size", "HistoricalPrices" })
        .Append(mlContext.Transforms.NormalizeMinMax("Features"));

// Create data preparation transformer
ITransformer dataPrepTransformer = dataPrepEstimator.Fit(data);

// Define StochasticDualCoordinateAscent regression algorithm estimator
var sdcaEstimator = mlContext.Regression.Trainers.Sdca();

// Pre-process data using data prep operations
IDataView transformedData = dataPrepTransformer.Transform(data);

// Train regression model
RegressionPredictionTransformer<LinearRegressionModelParameters> trainedModel = sdcaEstimator.Fit(transformedData);

Save data preparation pipeline and trained model

To save both the data preparation pipeline and trained model, use the following commands:

// Save Data Prep transformer
mlContext.Model.Save(dataPrepTransformer, data.Schema, "data_preparation_pipeline.zip");

// Save Trained Model
mlContext.Model.Save(trainedModel, transformedData.Schema, "model.zip");

Load data preparation pipeline and trained model

In a separate process or application, load the data preparation pipeline and trained model simultaneously as follows:

// Create MLContext
MLContext mlContext = new MLContext();

// Define data preparation and trained model schemas
DataViewSchema dataPrepPipelineSchema, modelSchema;

// Load data preparation pipeline and trained model
ITransformer dataPrepPipeline = mlContext.Model.Load("data_preparation_pipeline.zip",out dataPrepPipelineSchema);
ITransformer trainedModel = mlContext.Model.Load("model.zip", out modelSchema);