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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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Fatal: The highest priority, indicating that the program cannot continue. After generating a fatal error message, the program should exit. In the generated logging output, the medium in which the message is transmitted reflects the priority of the message. The high-level idea is to define which priorities are logged, and the destinations of those priorities. For example, let s say that when an application sends a fatal log message, you want to receive that message using a pager or telephone. However, an info message could be stored in a database for later perusal. It s important to understand that using log4net doesn t invoke some other mechanism. This means that a fatal error message won t exit your application or generate an exception. Your application needs to do those things manually. Log4net is intended to be used in conjunction with other error-handling mechanisms. In the example source code, the property IsDebugEnabled was tested before calling the method Log. This is necessary for performance reasons. Otherwise, there are performance ramifications, and an application is slowed down unnecessarily. The log4net framework doesn t verify that the proper enabled property is called before calling a logging method. For example, if you can call the property IsErrorEnabled to test for an Error priority, you can also call the Debug method. You must remember that log4net expects you to execute the correct method call. From a performance perspective, calling the enabled methods doesn t incur a large overhead.
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Managing a Configuration File
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When an application uses the ILog interface, this doesn t mean that logging messages are generated. Whether a message is generated depends on the configuration information. You can define log4net configuration information in three ways: within an application configuration file, within a custom configuration file, or using source-code commands. The simplest and probably the most common way is to use the application configuration file. The application configuration file defines the configuration identifier, priority, destination, and format of the message. Retrieving an ILog interface instance using the method call GetLogger also retrieves a configuration identifier. A configuration identifier defines different logging configurations within an application. For example, often in my own applications I have a tracing mode and an application-logging mode. In tracing mode, the generated output is nested and extensive. In application-logging mode, a specific logging format is generated, so that it s easy to inspect what the application is doing. When defining a configuration identifier, there s a default definition and a specific definition. The default definition provides default functionality that the custom configuration definition does or doesn t inherit. An example of defining a configuration identifier follows: <root> <level value="ERROR" /> <appender-ref ref="ConsoleAppender" /> </root> <logger name="logger.tracer" additivity="false"> <level value="ALL" /> <appender-ref ref="TracerAppender" /> </logger> The root XML tag defines the default configuration. Contained within the root XML tag are the XML tags level and appender-ref. The XML tag level defines which priority is logged,
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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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and the XML tag appender-ref references a message destination. The logger XML tag is a custom configuration definition, where the identifier is defined by the attribute name. When the attribute additivity is false, it means that the custom configuration doesn t inherit the default configuration details. The default is for the custom configuration to inherit from the root configuration. Log4net supports inheritance using the additivity attribute, allowing an administrator to log messages to multiple destinations simultaneously. Let s reconsider the previously defined custom configuration logger.tracer without the additivity attribute. When a message is generated, that message is generated in both the TracerAppender destination and the ConsoleAppender destination. For every message logged, two messages are generated. In contrast, when the additivity attribute is assigned a value of false, a single message is generated for the TracerAppender destination. For this example, the default configuration has a defined priority of ERROR, meaning that only messages having a priority of ERROR and higher are generated. Whenever an inherited custom configuration is defined, then the default priority of the custom configuration would be ERROR. In the preceding example, the custom definition logger.tracer has assigned the attribute additivity a false value, meaning that the default message priority level isn t inherited, and the custom configuration must define its own priority. For the example, the priority is ALL, indicating that all logging messages are to be generated. Configuration definitions reference a message destination, which is called a log4net appender. An appender defines the layout and filter for a message. Following is an example definition of the configuration reference appender-ref TracerAppender: <appender name="TracerAppender" type="log4net.Appender.ConsoleAppender"> <layout type="log4net.Layout.PatternLayout"> <param name="ConversionPattern" value="Tracer %m%n" /> </layout> </appender> The XML element appender requires two attributes: name and type. The name attribute is a user-friendly name of the message destination that s referenced in the logger or root XML tags. The type attribute is a class name that s loaded when a message is generated. For most cases, you ll use a log4net-defined implementation, but you could reference a custom class by implementing the proper interfaces. In the example, the class name log4net.Appender. ConsoleAppender references an appender that generates a message on the console. The following appenders are defined within the log4net package (note that all class names are prefixed with the namespace log4net.Appender): ADONetAppender: Appends the data to a SQL database using the ADO.NET API. You append the data using a prepared SQL statement or stored procedure. ASPNetTraceAppender: Appends messages to an ASP.NET trace context. BufferingForwardingAppender: A special type of forwarder that propagates the messages as events. The events are sent when you define a condition. ColoredConsoleAppender: The messages are appended to the standard output, or standard error output, but you can color or highlight specific message priorities. Remember that not all consoles support all color codes.
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