/**
 * A Graph acts as an Estimator. It consists of a DAG of stages, each of which is either an
 * Estimator or Transformer.
 */
@PublicEvolving
public final class Graph implements Estimator<Graph, GraphModel> {
    public Graph(...) {...}

    @Override
    public GraphModel fit(Table... inputs) {...}

    @Override
    public TableSchema[] transformSchemas(TableSchema... schemas) {
        return schemas;
    }

    /** Skipped a few methods, including the implementations of some Estimator APIs. */
}

/** A GraphModel acts as a Transformer. It consists of a DAG of Transformers. */
@PublicEvolving
public final class GraphModel implements Transformer<GraphModel> {
    /** Skipped a few methods, including the implementations of the Transformer APIs. */
}

/** A GraphBuilder helps connect Stage instances into a Graph or GraphModel. */
@PublicEvolving
public final class GraphBuilder {
    /**
     * Specifies the upper bound (could be loose) of the number of output tables that can be
     * returned by the Transformer::getStateStreams and Transformer::transform methods, for any
     * stage involved in this Graph.
     *
     * <p>The default upper bound is 20.
     */
    public GraphBuilder setMaxOutputLength(int maxOutputLength) {...}

    /**
     * Creates a TableId associated with this GraphBuilder. It can be used to specify the passing of
     * tables between stages, as well as the input/output tables of the Graph/GraphModel generated
     * by this builder.
     */
    public TableId createTableId() {...}

    /**
     * The Graph::fit and GraphModel::transform should invoke the fit/transform of the corresponding
     * stage with the corresponding inputs.
     *
     * <p>Returns a list of TableIds, which represents outputs of the Transformer::transform
     * invocation.
     */
    public TableId[] getOutputs(Stage<?> stage, TableId... inputs) {...}

    /**
     * The GraphModel::setStateStreams should invoke the setStateStreams of the corresponding stage
     * with the corresponding inputs.
     */
    void setStateStreams(Stage<?> stage, TableId... inputs) {...}

    /**
     * The GraphModel::getStateStreams should invoke the getStateStreams of the corresponding stage.
     *
     * <p>Returns a list of TableIds, which represents outputs of the getStateStreams invocation.
     */
    TableId[] getStateStreams(Stage<?> stage) {...}

    /**
     * Returns a Graph instance which the following API specification: - Graph::fit should take
     * inputs and returns a GraphModel with the following specification. - GraphModel::transform
     * should take inputs and returns outputs. - GraphModel::setStateStreams should take
     * inputStates. - GraphModel::getStateStreams should return outputStates.
     *
     * <p>The fit/transform/setStateStreams/getStateStreams should invoke the APIs of the internal
     * stages in the order specified by the DAG of stages.
     */
    Graph build(TableId[] inputs, TableId[] outputs, TableId[] inputStates, TableId[] outputStates) {...}

    /**
     * Returns a GraphModel instance which the following API specification: - GraphModel::transform
     * should take inputs and returns outputs. - GraphModel::setStateStreams should take
     * inputStates. - GraphModel::getStateStreams should return outputStates.
     *
     * <p>The transform/setStateStreams/getStateStreams should invoke the APIs of the internal
     * stages in the order specified by the DAG of stages.
     *
     * <p>This method throws exception if any stage of this graph is an Estimator.
     */
    GraphModel buildModel(TableId[] inputs, TableId[] outputs, TableId[] inputStates, TableId[] outputStates) {...}

    // The TableId is necessary to pass the inputs/outputs of various API calls across the
    // Graph/GraphModel stags.
    static class TableId {}

}

GraphBuilder builder = new GraphBuilder();

// Creates nodes
Stage<?> stage1 = new TransformerA();
Stage<?> stage2 = new TransformerA();
Stage<?> stage3 = new EstimatorB();
// Creates inputs and inputStates
TableId input1 = builder.createTableId();
TableId input2 = builder.createTableId();
// Feeds inputs to nodes and gets outputs.
TableId output1 = builder.getOutputs(stage1, input1)[0];
TableId output2 = builder.getOutputs(stage2, input2)[0];
TableId output3 = builder.getOutputs(stage3, output1, output2)[0];

// Specifies the ordered lists of inputs, outputs, input states and output states that will
// be used as the inputs/outputs of the corresponding Graph and GraphModel APIs.
TableId[] inputs = new TableId[] {input1, input2};
TableId[] outputs = new TableId[] {output3};
// Generates the Graph instance.
Graph graph = builder.build(inputs, outputs, new TableId[]{}, new TableId[]{});

// Use the Graph instance as an Estimator.
GraphModel model = graph.fit(...);
Table[] results = model.transform(...);

void runInferenceOnWebServer(...) {

  // Reads the transformer_json from the same remote file written by the above code snippet.
  String transformer_json = ...;
  // Creates the state stream from Kafka topicA which is written by the above code snippet. 
  Table state_stream = ...;
  // Creates the input stream that needs inference.
  Table input_stream = ...;

  Transformer transformer = new Transformer(...);
  transformer.loadJson(transformer_json);
  transformer.setStateStreams(new Table[]{state_stream});
  Table output_stream = transformer.transform(input_stream);

  // Do something with the output_stream.

  // Executes the operators generated by the Transformer::transform(...), which reads from state_stream to update its parameters. It also does inference on input_stream and produces results to the output_stream.
  env.execute()
}