Checklist

• User stories documented (Shankar)
• User stories reviewed (Nitin)
• Design documented (Shankar/Vinisha)
• Design reviewed (Terence/Andreas)
• Feature merged ()
• Examples and guides ()
• Integration tests () 
• Documentation for feature ()
• Blog post

Usecase

• User wants to group log messages at application level and write multiple separate log files for each application. Example: application-dir/{audit.log, metrics.log, debug.log}
• User wants to write these log files to a configurable path in HDFS.
• User also wants to be able to configure rolling policy for these log files similar to log-back. 

User Stories

1. For each application, user wants to  collect the application's logs into multiple logs files based on log level 

2. For each application, user wants to configure a location in HDFS to be used to store the collected logs. 
3. For each application, User wants the application log files stored in text format. 
4. For each application, User would wants to configure the RotationPolicy of log files. 
5. For each application, user wants to configure different layout for formatting log messages for the different log files generated.

Design

Introduce Log Processor, FileWriter and RotationPolicy interfaces. Pluggable in CDAP Log-saver. 

Programmatic API

public interface LogProcessor {

  /**
   * Called during initialize, passed properties for log processor.
   *
   * @param properties
   */
  void initialize(Properties properties);

  /**
   * Process method will be called with iterator of log messages, log messages received will be in sorted order,
   * sorted by timestamp. This method should not throw any exceptions. If any unchecked exceptions are thrown,
   * log.saver will log an error and the processor will not receive messages.
   * Will start receiving messages on log.saver startup.
   * 
   * @param events list of {@link LogEvent}
   */
  void process(Iterator<LogEvent> events);

  /**
   * stop logprocessor
   */
  void destroy();
}

class LogEvent {
  /**
   * Logging event
   */
  ILoggingEvent iLoggingEvent;
 
  /**
   * CDAP program entity-id
   */
  EntityId entityId;
}

Currently, we only have AvroFileWriter in Log.saver; we can create an interface for users to configure the FileWriter if needed. This provides the option to abstract certain common logic for file rotation, maintaining created files, etc. in Log saver and a custom file writer can implement the other methods specific to its logic,

Example: Creating files in HDFS and maintaining the size of events processed is maintained by custom FileWriter extension.

public interface MultiFileWriter {
  /**
   * get File manager for the log event. This file manager will be used to create, append-events, flush and close the file for the logging 
   * events of entityId (logging-context)
   */
   getFileManager(LogEvent event); 

}

interface FileManager {
  /**
   * Based on the logEvent, get entityId and use that information to create the file.
   **/
  File createFile(LogEvent logEvent); 

 /**
  * append log events to the currently active file belonging to the entityId represented by these log events. 
  * Logic : on the first append, we determine if the file has to be rotated or not using the RotationPolicy#shoudRotateFile(File file, LogEvent 
  * logEvent). If it has to be rotated, we will use RotationPolicy#rotateFile(File file, LogEvent logEvent) to rotate the file (close the old  
  * file) and append to the new file
  **/
  void appendEvents(Iterator<LogEvent> logEvents);  
 
  /**
   * close the currently active file.
   **/
  void close();

  /**
   * flush the contents of the currently active file
   **/
  void flush();
}

public interface RotationPolicy {
  /**
   * For the logEvent, decide if we should rotate the current file corresponding to this event or not.
   */
  boolean shouldRotateFile(File file, LogEvent logEvent);
 
  /**
   * For the logEvent, rotate the log file based on rotation logic and return the newly created File.
   */
  File rotateFile(File file, LogEvent logEvent);
 
  /**
   * For the logEvent, get the currently active file used for appending the log events.
   */ 		
  File getActiveFile(LogEvent logEvent);
}

Approach

Option-1

Log Processor/File Writer Extensions run in the same container as log.saver. 

Lifecycle

1) Log saver will load and initialize the log processor plugin.

2) As Log saver processes the messages, the log processor's process will also be called with logging events.

3) if log processor extension throws an error :

• we can Stop the plugin (or) 
• we can log an error and continue and stop the plugin after an error threshold.

4) FileWriterExtension will be used for file system operations (create, append, close) and RotationPolicyExtension will be used for deciding when to rotate the file.

5) stop the log processor when log.saver stops.

Class-Loading Isolation

1) Should the log processor plugins have separate class-loaders (or) can they share the same ClassLoader as the log.saver system. 

2)  If we use same Class-loader as log.saver, dependencies of extensions can be added to the classpath, and the classes available in log.saver system (hadoop, proto, ec) can be filtered out from the extension, so we use the classes provided by log.saver.

3) However if there are multiple log.processor extensions, say one for writing to s3 and another for writing to splunk, the classes from their dependencies could  potentially conflict with each other if we use the system class-loader ?

4) If we create separate Class loader for each extension to provide class loader isolation - we need to expose the following 

• cdap-watchdog-api
• cdap-proto
• hadoop
• logback-classic ( we need ILoggingEvent)
• should we expose more classes ? 
  
  

Sample Custom Log Plugin Implementation 

1) Log Processor would want to process the ILoggingEvent, format it to a log message string (maybe using log-back layout classes) and write it to a destination.

2) However the configuration for this log.processor cannot be a logback.xml.

• as there can only be one logback.xml in a JVM and the logback is already configured for the log.saver container.
• logback doesn't existing implementation for writing to HDFS. 

3) The properties required for extensions could be provided through cdap-site.xml.

4) Log processor extension could provide an implementation of FileWriter and RotationPolicy interfaces for HDFSFileWriter logic for the events it has received from LogProcessor. 

4) Future implementation for other policies have to be implemented at the extensions and can be configured through cdap-site.xml

Pros

1) Leverages scalability of log.saver

2) Utilization of existing resources , logic and processing.  Leveraging sorted messaging capability of log.saver for plugins.

3) Makes log saver extendable, to have option to store in different format and have custom logic on filtering.

Cons

1) As number of extensions increases and if a processor extension is slow, this could cause performance of log.saver to drop, which will affect the CDAP log.saver performance

Option-2 (or) Improvement 

Configure and Run a separate container for every log.processor plugin. 

Log.saver could have capability to launch system and user plugin containers. Scalability of these plugin containers could be managed separately.