SLAB - Reactive Event Sourcing

In the previous post we saw how we can use ETW events generated using EventSource using PerfView. We can also use these events in our application using EventListener. This is another new abstract type provided in .net framework 4.5. The abstract member is OnEventWritten() method. Additionally, it has a virtual method OnEventSourceCreated(). Below is a custom event listener. Here we are overriding the two members. We are just writing the debug messages when these methods are called. During development, debug messages can be seen in the Visual Studio's Debug Output window.

	internal class MyEventListener : EventListener
	{
	protected override void OnEventSourceCreated(EventSource eventSource)
	{
	base.OnEventSourceCreated(eventSource);
	Debug.WriteLine("Listener attached to the source");
	}
	protected override void OnEventWritten(EventWrittenEventArgs eventData)
	{
	Debug.WriteLine("Event data: {0}", eventData.Payload[0]);
	}
	}

view raw MyEventListener.cs hosted with ❤ by GitHub

The listener's OnEventWritten method is called when EventSource writes an event. But before that we need to specify what level of events we are interested in from the EventSource. This is called Enabling the events where listener is specified with the EventSource and EventLevel.

	var listener = new MyEventListener();
	listener.EnableEvents(MyEventSource.Log, EventLevel.Informational);
	MyEventSource.Log.WriteLog("message logged");

view raw EventSourceListener.cs hosted with ❤ by GitHub

You can notice OnEventSourceCreated and OnEventWritten being used by the logs in the Output window. The above code wold result the following logs:



Semantic Logging Application Block
Semantic Logging Application Block is provided in Enterprise Library 6 for supporting structured logging. It enhances EventSource to support logging events in SQL Server and Azure tables. Like other application blocks, this is also available as a Nuget package.



SLAB provides custom implementation of EventListener. The listener is Rx's IObservable. This is named as ObservableEventListener. Since this implements IObservable, it allows subscriptions from Event sinks. The sinks are IObserver (s) to observe on these event listeners.



The application block also provides a specialization of EventSource. This is named as SemanticLoggingEventSource.



In-Process Vs Out-Process
SLAB can be used in-process or out-process. We can use the Nuget package to use the libraries for using it in process. For out of process, it is hosted as a windows service. The service installer is available as a separate download. In the previous post, we discussed how we can use out-process logging with SLAB.

Event Text Formatter
SLAB supports JSON and XML based text formatting. The types are available in Microsoft.Practices.EnterpriseLibrary.SemanticLogging.Formatters namespace available in Microsoft.Practices.EnterpriseLibrary.SemanticLogging assembly. The assembly is downloaded using EnterpriseLibrary.SemanticLogging nuget package.



Event Sinks & Factories
SLAB also provides Event Sinks and their factories. All of these event sinks are IObserver.



Let's install EnterpriseLibrary.SemanticLogging application block nuget package. The package should take care of all its dependencies installation as well, which is only comprised of Newtonsoft.Json package.



Let's also install Rx-Main nuget package for subscriptions using Reactive extensions based subscribers.



In the following code we are subscribing to ObservableEventListener. We can get the messages in the OnNext() method for the subscriber. Here we have also used OnCompleted() and OnError() of the subscriber.

	var listener = new ObservableEventListener();
	listener.EnableEvents(MyEventSource.Log, EventLevel.Informational);
	listener.Subscribe<EventEntry>(
	a => Debug.WriteLine(a.Payload[0]),
	e => Debug.WriteLine(e.Message),
	() => Debug.WriteLine("Completed!"));
	MyEventSource.Log.WriteLog("message logged");
	MyEventSource.Log.WriteLog("Another Message");

view raw semanticObserver.cs hosted with ❤ by GitHub

Running the above code would result in the following output. Please notice that the messages being logged using the event source as provided to the listener. Since we are subscribing to the listener, we are getting those messages in OnNext(). Here we are just writing those messages to Debug Window.


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