Introduction 

CDAP pipeline is composed of various plugins that can be configured by users as CDAP pipelines are being developed. While building CDAP pipelines, pipeline developer can provide invalid plugin configurations. For example, the BigQuery sink plugin can have schema which does not match with an underlying BigQuery table. CDAP pipeline developer can use new validation endpoint to validate the stages before deploying the pipeline. In order to fail fast, validation endpoint should return all the validation errors from a given stage when this endpoint is called. 

Data pipeline app exposes various exceptions to plugins so that appropriate exception is thrown while validating the plugins. In future releases, new exception types can be introduced. When plugins with new exception types are pushed to hub, data pipeline artifacts need to be upgraded for every new type of exception that is introduced. This is because the validation exceptions are defined in the data pipeline app itself. A better approach would be to modify data pipeline app so that app artifacts do not need to be replaced for every new type of exception.

Goals

• To fail fast, introduce a new api to collect multiple error messages from plugins at configure time

• Decouple various validation exception types from data pipeline app

• Instrument plugins to use this api to return multiple error messages for validation endpoint

User Stories 

• As a CDAP pipeline developer, when I validate a stage, I expect that all the invalid config properties and schema fields are highlighted on CDAP UI with appropriate error message.
  
• As a plugin developer, I should be able to specify all the validation errors while configuring the plugin for better user experience.
• As a plugin developer, I should be able to use new validation exception types without replacing data pipeline app artifacts. 

API Changes for Plugin Validation

Approach 1

Collect multiple errors from plugins 

To collect multiple stage validation errors from the stage, StageConfigurer, MultiInputStageConfigurer and MultiOutputStageConfigurer can be modified as below. If there are any validation errors added to stage configurer, the pipeline deployment will fail and all the errors will be returned as a response to stage validation REST endpoint.

public interface StageConfigurer {

  ...

  /**
   * add validation errors for this stage to the configurer if pipeline stage is invalid. 
   *
   * @param error {@link ValidationError} when a pipeline stage is invalid for any reason.
   */
  void addValidationError(ValidationError error);
}

Decoupling plugin error types from data pipeline app

A new ValidationError class is introduced to collect multiple validation errors in stage configurer. This class will be exposed by data pipeline app to the plugins. Each new type of error can be added to hydrator-common which can be added as compile time dependency to the plugins. This approach allows us to easily add more types of validation errors for plugins while removing a need to update data pipeline artifact for each new error type. Below are some of the exception types that will be added to hydrator-common module.

/**
 * Represents some sort of 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();
  }

  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);
  }
}

Following errors will be added to hydrator-common. Note that this list will keep evolving as new types of errors are added to the module.

/**
 * Represents some sort of error that occurred during stage validation.
 */
@Beta
public class InvalidStageError extends ValidationError {
  private final String stage;

  protected InvalidStageError(String message, String stage) {
    super(message);
    this.stage = stage;
  }

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

/**
 * Represents invalid stage property error that occurred during stage validation.
 */
public class InvalidStagePropertyException extends InvalidStageException {
  private static final String TYPE = "INVALID_FIELD";

  public InvalidStagePropertyException(String message, String stage, String property) {
    super(message, stage);
    props.put("property", property);
  }

  public InvalidStagePropertyException(String message, Throwable cause, String stage, String property) {
    super(message, cause, stage);
    props.put("property", property);
  }

  public String getType() {
    return TYPE;
  }
}

Sources and sinks can have schema mismatch with underlying storage. A new type of exception can be introduced so that invalid schema fields can be highlighted when schema mismatch occurs:

/**
 * Represents schema mismatch that occurred during stage validation.
 */
@Beta
public class InvalidSchemaFieldException extends InvalidStageException {
  private static final String TYPE = "INVALID_SCHEMA";

  public InvalidSchemaFieldException(String message, String stage, String field) {
    super(message, stage);
    props.put("field", field);
  }

  public InvalidSchemaFieldException(String message, Throwable cause, String stage, String field) {
    super(message, cause, stage);
    props.put("field", field);
  }

  public String getType() {
    return TYPE;
  }
}

Usage of the API in Plugins

@Override
public void configurePipeline(PipelineConfigurer pipelineConfigurer) {
  pipelineConfigurer.createDataset(conf.destinationFileset, FileSet.class);
  StageConfigurer stageConfigurer = pipelineConfigurer.getStageConfigurer();
  try {
    Pattern.compile(conf.filterRegex);
  } catch (Exception e) {  
    stageConfigurer.addValidationError(new InvalidStagePropertyException(e.getMessage(), "filterRegex"));
  }
  if (conf.sourceFileset.equals(conf.destinationFileset)) {
    stageConfigurer.addValidationError(new ValidationException("source and destination filesets must be different"));
  }
}

Impact on UI

UI should be able to handle new error types that are introduced. For example, for invalid stage properties, UI should highlight all the invalid properties for a given stage. For schema mismatch, UI should be able to highlight schema fields that are mismatching. 

Below are the responses to the validation endpoint for each type of exception:

{
  "type" : "INVALID_FIELD", 
  "stage" : "src",
  "message" : "Invalid config for property 'port'", 
  "property" : "port"
}

{
  "type" : "INVALID_FIELD", 
  "stage" : "sink",
  "message" : "Invalid schema for the field 'name'", 
  "field" : "name"
}

{
  "errors": [
    {
      "stage": "src",
      "type": "PLUGIN_NOT_FOUND",
      "message": "Plugin named 'Mock' of type 'batchsource' not found.",
      "pluginType": "batchsource",
      "pluginName": "Mock",
      "requestedArtifact": {
        "scope": "USER",
        "name": "app-mocks-ghost",
        "version": "1.0.0"
      },
      "suggestedArtifact": {
        "scope": "USER",
        "name": "app-mocks",
        "version": "1.0.0"
      },
    }
  ]
}

Future Work

• error codes

Test Scenarios

Releases

Release 6.1.0

Related Work

• Work #1
• Work #2
• Work #3

Future work