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<document>

	<properties>
		<author email="nicko at apache dot org">Nicko Cadell</author>
		<title>log4net Manual - Contexts</title>
	</properties>

	<meta name="keywords" content="building log4net, log4net" />

	<body>
		<section name="log4net Manual - Contexts">
			<sectionMenu name="Contents" />

			<p>
				Most real-world systems have to deal with multiple clients simultaneously. In a 
				typical multithreaded implementation of such a system, different threads will 
				handle different clients. Logging is especially well suited to trace and debug 
				complex distributed applications. An approach to differentiate the 
				logging output of one client from another is to instantiate a new separate 
				logger for each client. However this promotes the proliferation of loggers and 
				increases the management overhead of logging.
			</p>
			<p>
				A lighter technique is to uniquely stamp each log request initiated from the 
				same client interaction.
			</p>
			<p>
				Log4net supports different types of contextual logging and contexts with different scopes.
			</p>
			
			<section name="Scopes">
				<p>
					Contextual data can be set in different scopes. These contexts have progressively narrower visibility.
					In the logging event itself the values from all of the contexts are combined together such that
					values specified in a lower scoped context hide values from a higher context.
				</p>
				
				<div class="table">
					<table cellspacing="0">
						<tr>
							<th>Scope</th>
							<th>Type</th>
							<th>Description</th>
						</tr>
						<tr align="left">
							<td>Global</td>
							<td><span class="code">log4net.GlobalContext</span></td>
							<td>
								The global context is shared by all threads in the current AppDomain.
								This context is thread safe for use by multiple threads concurrently.
							</td>
						</tr>
						<tr align="left">
							<td>Thread</td>
							<td><span class="code">log4net.ThreadContext</span></td>
							<td>
								The thread context is visible only to the current managed thread.
							</td>
						</tr>
						<tr align="left">
							<td>Logical Thread</td>
							<td><span class="code">log4net.ThreadLogicalContext</span></td>
							<td>
								The logical thread context is visible to a logical thread. Logical
								threads can jump from one managed thread to another. For more details
								see the .NET API <span class="code">System.Runtime.Remoting.Messaging.CallContext</span>.
							</td>
						</tr>
						<tr align="left">
							<td>Event</td>
							<td><span class="code">log4net.Core.LoggingEvent</span></td>
							<td>
								Each event captures the current contextual state at the time the event
								is generated. Contextual data can be set on the event itself. This context
								is only visible to the code generating the event itself.
							</td>
						</tr>
					</table>
				</div>				
			</section>
			
			<section name="Context Properties">
				<p>
					The log4net contexts store properties, i.e. name value pairs. The name is a string
					the value is any object. A property can be set as follows:
				</p>
				<source language="c#"><![CDATA[
log4net.GlobalContext.Properties["name"] = value;
]]></source>
				<p>
					If properties with the same name are set in more than one context scope then
					the value in the narrowest scope (lower down in the list above) will hide the
					other values.
				</p>
				<p>
					The property values are stored as objects within the <span class="code">LoggingEvent</span>. 
					The <span class="code">PatternLayout</span> supports rendering the value of a named
					property using the <span class="code">%property{name}</span> syntax. The value is 
					converted to a string by passing it to the <span class="code">log4net.ObjectRenderer.RendererMap</span>
					which will locate any custom renderer for the value type. The default behaviour for
					custom types is to call the object's <span class="code">ToString()</span> method.
				</p>
				
				<section name="Active Property Values">
					<p>
						An active property value is one who's value changes over time.
					</p>
					<p>
						For example, imagine a custom type that implemented the 
						<span class="code">ToString()</span> method to return the
						number of bytes allocated by the runtime garbage collector. 
					</p>
				<source language="c#"><![CDATA[
public class GCAllocatedBytesHelper
{
    public override string ToString()
    {
        return GC.GetTotalMemory(true).ToString();
    }
}]]></source>
					<p>
						An instance of this type can be added to the <span class="code">log4net.GlobalContext</span> 
						during application startup: 
					</p>
				<source language="c#"><![CDATA[
log4net.GlobalContext.Properties["GCAllocatedBytes"] = new GCAllocatedBytesHelper();
]]></source>
					<p>
						Once this property is set in the context all subsequent logging events will have a property
						called <i>GCAllocatedBytes</i>. The value of the property will be an instance of the
						<span class="code">GCAllocatedBytesHelper</span> type. When this value is rendered to a 
						string by calling the <span class="code">ToString</span> method the current number of bytes 
						allocated by the garbage collector will be returned and included in the output.
					</p>
					
				</section>
				
			</section>
			
			<section name="Context Stacks">
				<p>
					Sometimes simple key value pairs are not the most convenient way of capturing contextual
					information. A stack of information is a very convenient way of storing data especially
					as our applications tend to be stack based.
				</p>
				<p>
					The <span class="code">ThreadContext</span> and <span class="code">LogicalThreadContext</span>
					also support storing contextual data in a stack. The stack is stored in context property,
					therefore stacks have names and more than one stack can exist in the same context. A property
					value set in a narrower context would override a stack with the same property name set in a
					wider scoped context.
				</p>
				<p>
					The stack supports <span class="code">Push</span> and <span class="code">Pop</span> methods.
					As more contextual data is pushed onto the stack the stack grows. When the stack is rendered
					all the data pushed onto the stack is output with the most recent data to the right hand
					end of the string.
				</p>
				<p>
					As the stack is just an object stored in the context properties it is also rendered
					using the same <span class="code">PatternLayout</span> syntax: <span class="code">%property{name}</span>.
					Where <i>name</i> is the name of the stack.
				</p>
				<p>
					Calls the the stack's <span class="code">Push</span> and <span class="code">Pop</span>
					methods must be matched up so that each push has a corresponding pop. The 
					<span class="code">Push</span> method also returns an <span class="code">IDisposable</span>
					object that will perform the required pop operation when it is disposed. This allows
					the C# <i>using</i> syntax to be used to automate the stack management.
				</p>
				<source language="c#"><![CDATA[
using(log4net.ThreadContext.Stacks["NDC"].Push("context"))
{
    log.Info("Message");
}
]]></source>
				<p>
					The INFO level log has a stack stored in its <i>NDC</i> property. The top item in the
					stack is the string <i>context</i>.
					The <i>using</i> syntax ensures that the value <i>context</i> is popped off the stack
					at the end of the block.
				</p>
				<p>
					The <span class="code">using</span>
					syntax is recommended because it removes some work load from the developer and 
					reduces errors in matching up the Push and Pop calls, especially when exceptions
					can occur.
				</p>
			</section>
			
			<section name="Nested Diagnostic Contexts">
				<p>
					The <span class="code">NDC</span> (Nested Diagnostic Context) exists for compatibility
					with older versions of log4net. This helper class implements a stack which is stored
					in the thread context property named <i>NDC</i>.
				</p>
			</section>
			
			<section name="Mapped Diagnostic Contexts">
				<p>
					The <span class="code">MDC</span> (MappedDiagnostic Context) exists for compatibility
					with older versions of log4net. This helper class implements a properties map which is 
					mapped directly through to the thread context properties.
				</p>
			</section>
				
			<p>
				To illustrate this point, let us take the example of a web service delivering 
				content to numerous clients. The web service can build the <span class="code">NDC</span> at the very 
				beginning of the request before executing other code. The contextual 
				information can be the client's host name and other information inherent to the 
				request, typically information contained in cookies. Hence, even if the web 
				service is serving multiple clients simultaneously, the logs initiated by the 
				same code, i.e. belonging to the same logger, can still be distinguished 
				because each client request will have a different <span class="code">NDC</span> stack. Contrast this with 
				the complexity of passing a freshly instantiated logger to all code exercised 
				during the client's request.
			</p>
			<p>
				Nevertheless, some sophisticated applications, such as virtual hosting web 
				servers, must log differently depending on the virtual host context and also 
				depending on the software component issuing the request. Log4net supports 
				multiple logger repositories. This would allow each virtual host to possess its own copy 
				of the logger hierarchy. Configuring multiple logger hierarchies is beyond the 
				scope of this document.
			</p>

		</section>
	</body>
</document>
