Separation of Concern (CDAP vs Application)

Owned by Sagar Kapare
Last updated: Nov 08, 2016
7 min read

This document explains the separation of responsibilities between CDAP platform and Applications. 

CDAP Platform

  1. Java API: CDAP platform provides two sets of java API. External api is used by CDAP applications to interact with the preview system and internal api is used by preview REST handler.

    1. API to be used by applications:

      1. Get the instance of DebugLogger from program context. For example MapReduceContext will be updated to add new method as - 

        /**
 * MapReduce job execution context.
 */
public interface MapReduceContext ... {
   /**
    * Return the DebugLogger
    * @param loggerName the name of the logger using which the debug information will be logged
    */
   DebugLogger getLogger(String loggerName);
}

      2. Use DebugLogger to log the useful information.

         
/**
 * Interface used by the CDAP applications to log the debug data.
 */
public interface DebugLogger {
  /**
   * Logs the data at INFO level. Multiple values can be logged against the same property.
   * @param propertyName the the name of the property
   * @param propertyValue the value associated with the property
   */
  void info(String propertyName, Object propertyValue);

  /**
   * Return the name of the logger instance.
   */ 
  String getName();

  /**
   * Returns {@code true} if application is running in debug mode otherwise false is returned.
   */
  boolean isEnabled();
}
 
/**
 * DebugLoggerFactory will be injected in the Program context classes. This may not be directly used by Applications.
 */
public interface DebugLoggerFactory {
  /**
   * Get the {@link DebugLogger} used to log the debug data.
   * @param loggerName the name of the logger with which the log data to be associated
   * @return the instance of the DebugLogger
   */
  DebugLogger getLogger(String loggerName);
}

    2. API to be used by REST handler: PreviewHttpHandler will be responsible for handling the REST calls (details below). This REST handler will also interact with the preview system through API exposed by PreviewManager. Note that this is internal API.

      /**
 * Interface used to start preview and also retrieve the information associated with a preview.
 */
public interface PreviewManager {
  /**
   * Start the program in preview mode.
   * @param namespaceId the id of the namespace
   * @param request the request for the preview. This includes details about artifact, application configs, and preview configurations used by CDAP(details below)
   * @return the unique {@link PreviewId} generated for the preview run
   * @throws Exception if there were any error during starting
   */
  ApplicationId start(NamespaceId namespaceId, AppRequest<?> request) throws Exception;

  /**
   * Get the status for the specified {@link ApplicationId}.
   * @param preview the id of the preview for which status is to be returned
   * @return the status associated with the preview
   * @throws NotFoundException if the preview is not found
   */
  PreviewStatus getStatus(ApplicationId preview) throws NotFoundException;

  /**
   * Stop the preview identified by preview.
   * @param preview id of the preview
   * @throws Exception if the preview is not found or if there were any error during stop
   */
  void stop(ApplicationId previewId) throws Exception;

  /**
   * Get the list of loggers in this preview.
   * @param preview id of the preview
   * @return the {@link List} of list of loggers for a given preview
   * @throws NotFoundException if the previewId is not found
   */
  List<String> getLoggers(ApplicationId previewId) throws NotFoundException;

  /**
   * Get the data associated with the specified logger name of the preview.
   * @param preview id of the preview
   * @param loggerName the name of the logger for which data is to be returned
   * @return the {@link Map} of property name to property value associated with the given logger for a given preview
   * @throws NotFoundException if the preview is not found
   */
  Map<String, List<String>> getData(ApplicationId preview, String loggerName) throws NotFoundException;
 

  /**
   * Get metric associated with the preview.
   * @param preview the id of the preview
   * @return the {@link Collection} of metrics emitted during the preview run
   * @throws NotFoundException if the previewId is not found
   */
  Collection<MetricTimeSeries> getMetrics(ApplicationId preview) throws NotFoundException;

  /**
   * Get the logs for the preview.
   * @param preview the id of the preview for which logs to be fetched
   * @return the logs
   * @throws NotFoundException if the preview is not found
   */
  List<LogEntry> getLogs(ApplicationId preview) throws NotFoundException; 
}
 
// Instance of the PreviewStatus is returned by the getStatus call above. The details are as follows
/**
 * Represents the state of the preview.
 */
public class PreviewStatus {
  public enum Status {
    RUNNING,
    COMPLETED,
    DEPLOY_FAILED,
    RUNTIME_FAILED 
  };
 
  Status previewStatus;
  @Nullable
  String failureMessage;
  // Represents the request with which the preview was started.
  AppRequest request;
}
 
  2. REST API exposed by platform:

    1. Start a preview

      POST /v3/namespaces/{namespace-id}/previews
where namespace-id is the name of the namespace
Response will contain the CDAP generated unique preview-id which can be used further to get the preview data.

    2. Get the status of preview

      GET /v3/namespaces/{namespace-id}/previews/{preview-id}/status
where namespace-id is the name of the namespace
      preview-id is the id of the preview for which status is to be requested

    3. Stop preview

      POST /v3/namespaces/{namespace-id}/previews/{preview-id}/stop
where namespace-id is the name of the namespace
      preview-id is the id of the preview which is to be stopped

    4. Get the list of loggers in the preview

      GET /v3/namespaces/{namespace-id}/previews/{preview-id}/loggers
where namespace-id is the name of the namespace
      preview-id is the id of the preview which is to be stopped

    5. Get the data associated with the preview

      GET /v3/namespaces/{namespace-id}/previews/{preview-id}/loggers/{logger-id}
where namespace-id is the name of the namespace
      preview-id is the id of the preview
      logger-id is the id of the logger for which logs to be fetched

    6. Get the logs generated for the preview

      GET /v3/namespaces/{namespace-id}/previews/{preview-id}/logs
where namespace-id is the name of the namespace
      preview-id is the id of the preview

    7. Get the metrics associated with the preview

      GET /v3/namespaces/{namespace-id}/previews/{preview-id}/metrics
where namespace-id is the name of the namespace
      preview-id is the id of the preview

  3. Preview specific configurations understood by CDAP: When preview is started, CDAP needs to know which program need to be executed. Following is a sample request json - 

    {
    "artifact":{
      "name":"cdap-data-pipeline",
      "version":"3.5.0-SNAPSHOT",
      "scope":"SYSTEM"
    },
    "name":"MyPipeline",  
    "config":{
    ..... application specific configurations
    },
    "preview": {
      "programId": "MyProgram",
      "programType": "workflow"
    }
}

    In the above config json, CDAP will look for "preview" key to figure out which program to be executed by preview.

Application responsibilities:

  1. Application can use the API exposed by CDAP for getting the logger and logging the data.

  2. Application specific configurations can be specified in the config section of the json. For example following are the preview related configurations for hydrator app - 

    {
    "artifact":{
      "name":"cdap-data-pipeline",
      "version":"3.5.0-SNAPSHOT",
      "scope":"SYSTEM"
    },
    "name":"MyPipeline",  
    "config":{
       "connections": {
          ...
       },
       "stages": {
          ...
       },
       "appPreviewConfig": {
          "startStages": ["MyCSVParser"], // stages from which pipeline execution is to be started
          "endStages": ["MyTable"], // stages till which pipeline need to be executed
          "useRealDatasets": ["FTP"], // list of datasets to be used from the real user space for READ only purpose
          "outputs": {
             "FTP": {
                "data": [
                    {"offset": 1, "body": "100,bob"},
                    {"offset": 2, "body": "200,rob"},
                    {"offset": 3, "body": "300,tom"}
                ]
            }
          }  
       }
    },
    "preview": {
    ..... CDAP specific preview configurations    
    }
}

  3. Handling application level preview configurations: Preview configurations mentioned in the above section "appPreviewConfig" are application level and are required to handle by the application. More details TBD.