public interface StageConfigurer {

  ...

/**
 * get the stage name.
 * @return stage name
 */
String getStageName();

/**
 * add validation errors.
 * @param e errors
 */
void addValidationError(ValidationError e);

/**
 * Represents error that occurred during validation.
 */
@Beta
public class ValidationError {
  private final String message;
  private final String type;

  /**
   * Creates an error with provided error message.
   * @param message error message
   */
  public ValidationError(String message) {
    this.message = message;
    this.type = getType();
  }

  /**
   * Returns the type of the error.
   */
  public String getType() {
    return "ERROR";
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) {
      return true;
    }
    if (o == null || getClass() != o.getClass()) {
      return false;
    }
    ValidationError error = (ValidationError) o;
    return message.equals(error.message) &&
      type.equals(error.type);
  }

  @Override
  public int hashCode() {
    return Objects.hash(message, type);
  }
}

public class InvalidStageError extends ValidationError {
  private final String stage;

  /**
   * Creates error that represents a stage validation error.
   *
   * @param message error message
   * @param stage name of the stage
   */
  public InvalidStageError(String message, String stage) {
    super(message);
    this.stage = stage;
  }

  @Override
  public String getType() {
    return "STAGE_ERROR";
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) {
      return true;
    }
    if (o == null || getClass() != o.getClass()) {
      return false;
    }
    if (!super.equals(o)) {
      return false;
    }
    InvalidStageError that = (InvalidStageError) o;
    return Objects.equals(stage, that.stage);
  }

  @Override
  public int hashCode() {
    return Objects.hash(super.hashCode(), stage);
  }
}

/**
 * Represents invalid stage property error that occurred during stage validation.
 */
@Beta
public class InvalidStagePropertyError extends InvalidStageError {
  private final String property;

  /**
   * Creates error that represents invalid stage property.
   *
   * @param message error message
   * @param stage name of the stage
   * @param property invalid property
   */
  public InvalidStagePropertyError(String message, String stage, String property) {
    super(message, stage);
    this.property = property;
  }

  @Override
  public String getType() {
    return "INVALID_FIELD";
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) {
      return true;
    }
    if (o == null || getClass() != o.getClass()) {
      return false;
    }
    if (!super.equals(o)) {
      return false;
    }
    InvalidStagePropertyError that = (InvalidStagePropertyError) o;
    return property.equals(that.property);
  }

  @Override
  public int hashCode() {
    return Objects.hash(super.hashCode(), property);
  }
}

/**
 * Represents schema mismatch that occurred during stage validation.
 */
@Beta
public class InvalidSchemaFieldError extends InvalidStageError {
  private final String field;
  private final boolean isInputField;

  /**
   * Creates error that represents schema mismatch
   *
   * @param message error message
   * @param stage name of the stage
   * @param field invalid field
   * @param isInputField true indicates that the field is from input schema.
   *                     false indicates that the field is from output schema
   */
  public InvalidSchemaFieldError(String message, String stage, String field, boolean isInputField) {
    super(message, stage);
    this.field = field;
    this.isInputField = isInputField;
  }

  @Override
  public String getType() {
    return "INVALID_SCHEMA";
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) {
      return true;
    }
    if (o == null || getClass() != o.getClass()) {
      return false;
    }
    if (!super.equals(o)) {
      return false;
    }
    InvalidSchemaFieldError that = (InvalidSchemaFieldError) o;
    return isInputField == that.isInputField && field.equals(that.field);
  }

  @Override
  public int hashCode() {
    return Objects.hash(super.hashCode(), field, isInputField);
  }
}

@Override
public void configurePipeline(PipelineConfigurer pipelineConfigurer) {
  pipelineConfigurer.createDataset(conf.destinationFileset, FileSet.class);
  StageConfigurer stageConfigurer = pipelineConfigurer.getStageConfigurer();
  // get the name of the stage 
  String stageName = stageConfigurer.getStageName();
  try {
    Pattern.compile(conf.filterRegex);
  } catch (Exception e) {  
    // add validation error to stage configurer
    stageConfigurer.addValidationError(new InvalidStagePropertyError(e.getMessage(), stageName, "filterRegex"));
  }
  if (conf.sourceFileset.equals(conf.destinationFileset)) {
    // add validation error to stage configurer
    stageConfigurer.addValidationError(new InvalidStageError("source and destination filesets must be different", stageName));
  }
}