Logging

Logging in Weave is done using a third party library. Prior to Weave 2.6.7 Log4j 1.2 was used. From 2.6.7 onwards Logback is used. It is beyond the scope of this document to explain how the logging infrastructure works with Log4j and Logback so the reader is encouraged to visit the Log4j or Logback websites to find out more information about the inner workings of these libraries.

The following links provide details about Logback.

Main Site:

https://logback.qos.ch/

Documentation:

https://logback.qos.ch/manual/index.html

Architecture

From an architectural point of view, software application modules and logging components reside in two separate layers. The application makes a call to the logging components in the logging layer and delegates the logging responsibility to those components. The logging components receive the logging request and publish the logging information at preferred destinations.

The figure below represents the collaboration of a software module and its logging components.

The architecture of the Logback API is layered. Each layer consists of different objects performing different tasks. The top layer captures the logging information, the middle layer is involved in analyzing and authorizing the logging information, and the bottom layer is responsible for formatting and publishing the logging information to a destination. In essence, Logback consists of three types of primary objects:

Logger: The Logger object is responsible for capturing logging information. Logger objects are stored in a namespace hierarchy.
Appender: The Appender object is responsible for publishing logging information to various preferred destinations. Each Appender object will have at least one target destination attached to it. For example, a ConsoleAppender object is capable of printing logging information to a console. https://logback.qos.ch/manual/appenders.html
Layout: The Layout object is used to format logging information in different styles. Appender objects utilize Layout objects before publishing logging information. Layout objects play an important role in publishing logging information in a way that is human-readable and reusable. https://logback.qos.ch/manual/layouts.html

The preceding core objects are central to the architecture of Logback. Apart from them, there are several auxiliary objects that can plug and play to any layer of the API. These objects help manage the different Logger objects active within an application and fine-tune the logging process.

The principal auxiliary components in the Logback framework that play a vital role in the logging framework are:

Level: The Level object defines the granularity and priority of any logging information. Each piece of logging information is accompanied by its appropriate Level object, which tells the Logger object about the priority of the information. The levels of logging defined within the API are: TRACE, DEBUG, INFO, WARN and ERROR. The Level values can be arranged in an ascending manner as:
TRACE < DEBUG < INFO < WARN < ERROR
During normal operation of Weave you’re likely to only require log messages with a level of INFO or higher (WARN and ERROR) which will tell you what Weave is doing and if there are any problems. But when you’re trying to figure out a problem, or are actively configuring the system, then seeing the log level of DEBUG can also be useful. It’s unlikely that a Weave instance would require a log level of TRACE. This log level produces a large amount of low level detail and is mainly intended to help developers troubleshoot issues with the Weave code.
Additionally, when filtering log output, you can specify OFF as a log level to disable all log output from a specific logger. For example a number of third party libraries used by Weave also produce log output but this can be unnecessary noise when trying to view the Weave log output, so the default Weave log configuration disables the log output from a number of these libraries by setting their log level to OFF (or in some cases ERROR or WARN).

Filter: The Filter object is used to analyze logging information and make further decisions on whether that information should be logged or not. In the Logback context, Appender objects can have several Filter objects associated with them. If logging information is passed to a particular Appender object, all the Filter objects associated with that Appender need to approve the logging information before it can be published to the preferred destination attached to the Appender. Filter objects are very helpful in filtering out unwanted logging information based on any application-specific criteria.
Pattern: The Pattern object is specialized in providing a string representation of different objects passed to the logging framework. More precisely, when the application passes a custom Object to the logging framework, the logging framework will use the corresponding Pattern to obtain a string representation of the passed Object. This is used by Layout objects to prepare the final logging information.

Configuring Logging

The configuration below defines the level of the root logger as DEBUG and specifies the appender to use as testAppender. Next, the appender testAppender is defined as referencing the ch.qos.logback.core.ConsoleAppender object and the layout of the appender as the pattern is specified. A pattern requires that a conversion pattern or a format be supplied. In this case the conversion pattern of %m%n is supplied, which means the logging message will be printed followed by a newline character. More information about these patterns are available at https://logback.qos.ch/manual/layouts.html#conversionWord

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

	<appender name="testAppender" class="ch.qos.logback.core.ConsoleAppender">
		<encoder>
			<pattern>%message%n</pattern>
		</encoder>
	</appender>

	<root level="DEBUG">
		<appender-ref ref="testAppender"/>
	</root>

</configuration>

A more complex configuration can attach multiple appenders to a particular logger. Each appender, in turn, can have a different layout, and that layout can have a conversion pattern associated with it.

Below is an example of a more complex configuration file. This configuration file defines the root logger as having level DEBUG and attaches two appenders named console and file to it.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

	<appender name="console" class="ch.qos.logback.core.ConsoleAppender">
		<encoder>
			<pattern>%message%n</pattern>
		</encoder>
	</appender>

	<appender name="file" class="ch.qos.logback.core.rolling.RollingFileAppender">
		<file>logs/weave.log</file>
		<encoder>
			<pattern>%date{yyyy-MM-dd HH:mm:ss.SSS} %level [%thread] %logger: "%message"%n</pattern>
		</encoder>
		<rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
			<fileNamePattern>logs/weave-%d{yyyy-MM-dd}.log</fileNamePattern>
			<maxHistory>10</maxHistory>
		</rollingPolicy>
	</appender>

	<root level="DEBUG">
		<appender-ref ref="console"/>
		<appender-ref ref="file"/>
	</root>

</configuration>

The default logging.xml file included with Weave is similar to the above example but has some additional configuration that enables the different types of log output depending upon how Weave was started. For example, if you use startup.cmd to start Weave you only get the console output, but if you started Weave as a service then you only get the weave.log file output. This is done by including conditions around the appender-ref tags in the root section of the file, but if you want to explicitly determine which log output is generated you can remove those additional condition tags and have the same output regardless of how Weave was started.

Note that the default log level set in logging.xml does not effect the log information recorded in a Support Dump. A Support Dump generated from Weave will now always include all log messages generated during startup of Weave and all log messages generated (up to a certain number) before the dump was produced regardless of the log level set in logging.xml.

Reading Log Files

When Weave has been installed without any modifications to the logging.xml file, Weave logs the following information to the weave.log file within the $WEAVE_HOME/logs/ directory when started as a service/daemon. The weave.log file contains the current log messages that Weave has produced in the following format.

TIME LOGGING_LEVEL THREAD USER_ID SESSION_ID MESSAGE_ORIGIN MESSAGE

Note that you can also use the dump logs OSGi command or the Admin UI to see additional log output if Weave was not started as a service/daemon.

Examples

The following examples help explain each element in each item.

Example 1


2002-01-06 00:06:45.405 DEBUG [pool-1-thread-1] system 00000000-0000-0000-0000-000000000000 com.cohga.client.weave.Activator "Removed script source from ..."

Given the above example, the following items can be determined:

Item	Value	Description

Item	Value	Description
TIME	2002-01-06 00:06:45.405	States the time the message was logged to the file. Format is YEAR-MONTH-DAY HOURS:MINUTES:SECONDS.MILLISECONDS
LOGGING_LEVEL	DEBUG	The Level of the message logged. Possible values are TRACE, DEBUG, WARN, INFO or ERROR
THREAD	[pool1-thread-1]	The thread that the message was logged in (See below for more information about reading logs).
USER_ID	system	The username of the user who made the request that generated the log message, or “system” if it wasn’t generated by a user
SESSION_ID	00000000-0000-0000-0000-000000000000	This is a GUID that remains constant across a single Weave session for each client, or all zeros if the request log wasn’t generated on behalf of a user. This can be used to group all log messages related to a single user session.
MESSAGE_ORIGIN	com.cohga.client.weave.Activator	The class that the message was logged against. e.g. The class Activator in the package com.cohga.client.weave
MESSAGE	"Removed script source from ..."	The message that is being set to the log. Typically this is in human readable form, however sometimes the messages may not be that intuitive to the user.

Example 2

Item	Value	Description

Item	Value	Description
TIME	00:06:45,405	States the time the message was logged to the file. Format is HOURS:MINUTES:SECONDS,MILLISECONDS
LOGGING_LEVEL	DEBUG	The Level of the message logged. Possible values are DEBUG, WARN, INFO or ERROR
THREAD	[OSGi Console]	The thread that the message was logged in (See below for more information about reading logs).
MESSAGE_ORIGIN	com.cohga.search.indexer.internal.Activator	The class that the message was logged against. e.g. The class Activator in the package com.cohga.search.indexer.internal
MESSAGE	"Stopping Weave Index ..."	The message that is being set to the log. Typically this is in human readable form, however sometimes the messages may not be that intuitive to the user.

The main items that change, and that should be noted are the Thread, Message Origin and the Message. These three items define when and where the message was logged from. The Thread component can cause some confusion to many first time readers if they have not familiar with multithreaded applications. A key example is that the same message may be logged three times in succession. This happens due the the WebService, that Weave runs under, which is handling requests from multiple different locations at the same time. Each thread is being executed concurrently and working in isolation within the Weave application. An example of what you would see in the log file for this is:

The above example illustrates that there were four threads being executed. btpool0-0,btpool0-4,btpool0-7 and btpool0-9. it shows that btpool0-4 has added a logging message between a message logged by btpool0-0 and btpool0-7. When looking for a particular sequence messages that have been logged by a particular user, make sure that the thread name is the same throughout the request.

Common Issues

Some of the underlying libraries used within Weave also produce logging information. Each library determines what it sees as being relevant to log at various debugging levels (i.e. ERROR, INFO, etc). These messages can sometimes confuse the reader about where and when an error has occurred. Also, an error message may be in server parts, this is called a nested exception. Typically the exact error location and message will be the first in

e.g. The error below was caused by the Database closing the connection. The first exception is the most detailed as it explains that the error's original location was in the class oracle.jdbc.driver.SQLStateMapping which is core to the Oracle communications libraries back to the database. The next error nested exception is the code that called the Oracle class com.cohga.server.stats.export.internal.ExportEvents. This class re-threw the exception which was logged in the logging file.

When reading a log file you will typically work your way from the bottom to the top. If you do and you encounter an error, please go to the first location in the log file that it occurs. It will generally be the most detailed.

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