LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT in VB.NET

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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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ConsoleAppender: The messages are appended to the standard output, or standard error output. EventLogAppender: The messages are appended to the Application log of the Windows event log. FileAppender: The messages are sent to a file. ForwardingAppender: This Appender is similar to the BufferingForwardingAppender, except that messages aren t filtered and are sent immediately. MemoryAppender: The messages are appended to an in-memory Appender. NetSendAppender: The messages are sent using the Windows Messenger Service. OutputDebugStringAppender: The messages are sent to a Windows debug facility. RemotingAppender: The messages are sent to a remote sink. RollingFileAppender: This Appender is similar to a FileAppender, except that a new file is used when certain conditions arise. For example, if the output file exceeds a certain size or date, a new file is generated. For production scenarios, use RollingFileAppender. SmtpAppender: Sends a message using the SMTP e-mail protocol. SmtpPickupDirAppender: Writes an SMTP-formatted message to a directory. An SMTP engine can then pick up the message for forwarding to an e-mail address. TextWriterAppender: This Appender writes to the standard .NET TextWriter class. TraceAppender: Appends all messages to a Trace system. UdpAppender: Sends a message to a specific client or broadcast group, using the connectionless User Datagram Protocol (UDP). The details of each appender are beyond the scope of this book, and I recommend that you read the log4net documentation for further details. Within each appender is the layout XML element, which specifies the format of the message that is output. There are two main layouts: log4net.Layout.PatternLayout and log4net.Layout.XMLLayout. The layout PatternLayout generates a single line, formatted using a set of escape sequences. You can find the individual escape sequences and their purposes in the log4net PatternLayout class documentation. The XMLLayout engine generates an XMLcompliant log-output file suitable for later processing. Within the layout XML element are other XML parameter elements that define how the layout appears.
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The purpose of logging is to understand what an application is doing, and when it s doing it. A simple way to generate logging messages would be to use the priority levels to generate the errors and everything else. Generating errors isn t the problem, because errors are easy to locate. The rest of the priorities are more difficult. For example, when generating debug messages, you might wonder what constitutes a debug message. If the debug message is overused, then the
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CHAPTER 2 LOGGING, ERRORS, AND TEST-DRIVEN DEVELOPMENT
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generated messages are too numerous, and they make it hard to know what the application is doing. In a nutshell, you must define a strategy when generating logging messages.
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Defining the Configuration File
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Let s define a scenario of logging messages in an application. When an application is running in production, you should log fatal and error priorities. Logging these messages in production or during development would be a good default behavior. Therefore, to assign a default behavior, you would define the priorities in the root XML element. Following is an example configuration: <root> <level value="ERROR" /> <appender-ref ref="ErrorAppender" /> <appender-ref ref="PanicAppender" /> </root> In the example configuration, the priority logged is ERROR, meaning that all ERROR and FATAL messages will be processed. There are also two message destinations. The ErrorAppender message destination is a generic error-handler message destination. The PanicAppender message destination is a special destination used to broadcast high-priority messages. For example, if an application has crashed, the administrator will want to know as quickly as possible that the application doesn t work anymore. The PanicAppender broadcasts a high-priority message via the Windows event log or via e-mail, allowing for immediate attention. Using two logging destinations seems illogical and wasteful of resources, as two logging messages are generated. However, you define two destinations so that two messages can be sent to two different destinations. If an application in production generates a FATAL priority message, then the you-know-what has hit the fan, and somebody has to react quickly before managerial problems arise. When an application generates an ERROR priority message, that message is important, but probably not as important as a FATAL priority message. Having both messages sent to the same destination is generally a bad idea, because the receiver has to decipher whether the message is bad, or really bad. Therefore, it s necessary to distinguish between fatal and error exceptions. Another way to separate the two error levels is to define a default and custom configuration for each error level. The default configuration would log all FATAL priorities, and a custom configuration would assign a priority to log all ERROR priorities. There s nothing wrong with this approach, except that the default wouldn t be to log both priorities, and the administrator would need to inform the developer of his or her deployment solution. What hasn t yet been covered, and would clarify the reasoning of the decision, is the ability of a destination to filter messages, making it possible to define a priority filter in two places. Following is an example of an appender that can filter messages: <appender name="PanicAppender" type="log4net.Appender.SmtpAppender"> <filter type="log4net.Filter.LevelRangeFilter"> <levelMin value="FATAL" /> <levelMax value="FATAL" /> </filter> <layout type="log4net.Layout. SimpleLayout " /> </appender>
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