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logging are possible simply by changing the configuration after deployment and at run time. This means that administrators and operators can vary the logging behavior as they manage the application, including when using Group Policy.
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What Does the Logging Block Do
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The Logging application block allows you to decouple your logging functionality from your application code. The block can route log entries to a Windows Event Log, a database, or a text (or XML) file. It can also generate an e-mail message containing the logging information, a message you can route through Windows Message Queuing (using a distributor service provided with the block), or a Windows Management Instrumentation (WMI) event. And, if none of these built-in capabilities meets your requirements, you can create a provider that sends the log entry to any other custom location or executes some other action. In your application, you simply generate a log entry using a suitable logging object, such as the LogWriter class, and then call a method to write the information it contains to the logging system. The Logging block routes the log message through any filters you define in your configuration, and on to the listeners that you configure. Each listener defines the target of the log entry, such as Windows Event Log or an e-mail message, and uses a formatter to generate suitably formatted content for that logging target. You can see from this that there are many objects involved in this multi-step process, and it is important to understand how they interact and how the log message flows through the pipeline of processes. Figure 1 shows the overall process in more detail, and provides an explanation of each stage.
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Log Filter
Priority Filter
Log Enabled Filter
Category Filter
Trace Source
All Events Log Source
Not Processed Log Source
Errors Log Source
Category Source
Trace Listener
Formatted Event Log Trace Listener MSMQ Trace Listener
Flat File Trace Listener E-mail Trace Listener
WMI Trace Listener Formatted Database Trace Listener
Log Formatter
figure 1 An overview of the logging process and the objects in the Logging block
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Stage Creating the Log Entry
Description The user creates a LogWriter instance, uses it to create a new LogEntry, and passes it to the Logging block for processing. Alternatively, the user can create a new LogEntry explicitly, populate it with the required information, and use a LogWriter to pass it to the Logging block for processing. The Logging block filters the LogEntry (based on your configuration settings) for message priority, or categories you added to the LogEntry when you created it. It also checks to see if logging is enabled. These filters can prevent any further processing of the log entries. This is useful, for example, when you want to allow administrators to enable and disable additional debug information logging without requiring them to restart the application. Trace sources act as the link between the log entries and the log targets. There is a trace source for each category you define in the logging block configuration; plus, there are three built-in trace sources that capture all log entries, unprocessed entries that do not match any category, and entries that cannot be processed due to an error while logging (such as an error while writing to the target log). Each trace source has one or more trace listeners defined. These listeners are responsible for taking the log entry, passing it through a separate log formatter that translates the content into a suitable format, and passing it to the target log. Several trace listeners are provided with the block, and you can create your own if required. Each trace listener can use a log formatter to format the information contained in the log entry. The block contains log message formatters, and you can create your own formatter if required. The text formatter uses a template containing placeholders that makes it easy to generate the required format for log entries.
Filtering the Log Entry
Selecting Trace Sources
Selecting Trace Listeners
Formatting the Log Entry
Categories allow you to specify the target(s) for log entries processed by the block. You can define categories that relate to one or more targets. For example, you might create a category named General containing trace listeners that write to text files and XML files, and a category named Auditing for administrative information that is configured to use trace listeners that write to one or more databases. Then you can assign a log entry to one or more categories, effectively mapping it to multiple targets. The three log sources shown in the schematic in Figure 1 (all events log source, not processed log source, and errors log source) are themselves categories for which you can define trace listeners. Logging is an added-value service for applications, and so any failures in the logging process must be handled gracefully without raising an exception to the main business processes. The Logging block achieves this by sending all logging failures to a special category (the errors log source) which is named Logging Errors & Warnings. By default, these error messages are written to Windows Event Log, though you can configure this category to write to other targets using different trace listeners if you wish.
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