=======================
What is Logging & Why?
=======================

=> The process of storing app execution details into a file/console is called as Logging.

=> Log messages will help us in understanding runtime behaviour of the application.

=> Using log msgs we can identify root cause of the exceptions.

=> To implement logging there are several logging frameworks available in the market.

		1) Log4J

		2) Log4J2

		3) Logback

		4) Logstash

=====================
Logging Architecture
=====================	

1) Logger

2) Layout

3) Appender


=> Logger is a predefined class and it is providing several methods to generate log msgs.

ex: trace(), debug(), info(), warn(), error()

Note: For every java class we will create one logger object.	

=> Layout represents log msg pattern.

	<date> <time> <class-name> <msg>


=> Appender is used to write the log msg to destination.

	Ex : ConsoleAppender
		 FileAppender
		 JdbcAppender	

----------------------------------
@Service
public class ReportService {

	private Logger logger =
			LoggerFactory.getLogger(ReportService.class);

	public void generateReport() {
		logger.trace("this is trace msg");
		logger.debug("this is debug msg");
		logger.info("this is info msg");
		logger.warn("this is warn msg");
		logger.error("this is error msg");
	}

	public void deleteUser() {
		logger.debug("deleteUser() - execution started");
		try {
			// logic to delete user
			logger.info("User deleted successfully");
		} catch (Exception e) {
			logger.error("Exception : " + e.getMessage());
		}
		logger.debug("deleteUser() - execution completed");
	}

}

===============
LOG Levels
===============

=> To write log msgs to the destination we have several LOG LEVELS.

	1) TRACE

	2) DEBUG

	3) INFO  (DEFAULT log LEVEL IN SPRING BOOT)

	4) WARN

	5) ERROR		 

Note: Log msgs will be printed based on log level of our application.


Note: From current log level, it will print all the higher log level msgs.

ex-1: if log level is info, it will print 'info + warn + error' msgs.

ex-2: if log level is warn, it will print 'warn + error' msgs.

ex-3: if log level is debug, it will print 'debug + info + warn + error' msgs.

ex-4: if log level is error, it will print only 'error' msgs.


=> We can change application log level by using below property in application.properties file

	logging.level.root = ERROR


=> By default, spring boot will use console appender hence log msgs will be printed on console.

=> To print log msgs in log file, we can configure below property

		logging.file.name=ashokit.log		

============================
Logging with Rolling Policy	
============================

=> It is used to create multiple log files to store log msgs of our application.

=> Rolling Policy we can implement in 2 ways

		1) Size based rolling 

		2) Time based rolling 

=> Size Based rolling means after reaching given limit then new log file will be created.
		
		Ex : 1 GB

=> Time Based rolling means every day new log file should be created.

=> To configure rolling policy we can use logback.xml file.		

=========================
What is logback.xml ?
=========================

=> logback.xml is used to customize logging in our application.

=> In logback.xml we will configure below components

	1) Rolling File Appender with policy

	2) Log Msg Pattern

	3) Log Level

=> We will keep "logback.xml" under "src/main/resources" folder	


---------------------
<configuration>

	<appender name="RollingFile"
		class="ch.qos.logback.core.rolling.RollingFileAppender">
		<file>ashokit.log</file>
		<encoder>
			<pattern>%d [%thread] %-5level %-50logger{40} - %msg%n</pattern>
		</encoder>
		<rollingPolicy
			class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
			<fileNamePattern>ashokit-%d{yyyy-MM-dd}.%i.log</fileNamePattern>
			<maxFileSize>1GB</maxFileSize>
			<maxHistory>30</maxHistory>
		</rollingPolicy>
	</appender>
	<root level="INFO">
		<appender-ref ref="RollingFile" />
	</root>

</configuration>
-------------------------------------------------------

===============
Log Monitoring
===============

=> The process of getting log msgs from the log file is called as Log Monitoring.

=> To monitor log files we have several tools in the market

		1) Splunk (licensed)

		2) ELK (free) 

		3) Putty / mobaxterm / gitbash / winscp


-------------------------------------------------

@@ Project Lombok Tutorials : https://www.youtube.com/watch?v=8tDym-FxU0A

@@ Log Monitoring with ELK Video : https://www.youtube.com/watch?v=n2HHAvpn6Jo

-------------------------------------------------

1) What is Logging & Why Logging

2) Logging Frameworks

3) Log4J vs SLF4J

4) Logging Architecture

5) LOG LEVELS

6) Rolling Policies (Time and size)

7) What is logback.xml

8) What is @SLF4J annotation

9) Log Monitoring

10) Log Monitoring Tools

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