Event Logging for Google AppEngine

In production its often useful to gather statistics on the events that happen in your system. For example, how many successes were there vs. login failures, or how many AJAX calls in the last hour vs. an hour ago?

This project install a Logger that can be used to help collect that data, and display results back in chart form.

The logging is:

• Fast - Logs are stored in memcache, so its safe to make many calls
• Comparable - Easy to compare success vs. failure.
• Non-Persistant - Never stored in BigTable, so history is limited to 2 hours.

Installation

Copy the eventlogging .jar file into you lib directory (on AppEngine, in war/WEB-INF/lib). Edit your web.xml file to include the following:

<servlet>
	<servlet-name>EventImager</servlet-name>
	<servlet-class>ca.sheepdoginc.eventlogging.EventImager</servlet-class>
</servlet>
<servlet-mapping>
	<servlet-name>EventImager</servlet-name>
	<url-pattern>/eventimager</url-pattern>
</servlet-mapping>

This will add the chart handler to your project. Feel free to edit the actual endpoint for the chart (currently set to /eventimager)

There are a few bugs in AppEngines logging facilities, so to work around them we need to initialize the logger statically:

class MyFancyClass {
	private static final Logger log = Logger.getLogger(MyFancyClass.class.getName());
		
	static {
		MyFancyClass.log.addHandler(new EventLogger());
	}
	
	// ....
}

Usage

To actually use the logger, you'll just need to do something like:

log.log(EventLevel.EVENT, "rpc.success");

Notice that we're using the EVENT log level -- EventLogging will ignore log levels that don't match the EventLevel.EVENT log level. The above captures a successful RPC request -- you'd probably also want to catch failures for comparison:

log.log(EventLevel.EVENT, "rpc.failure");

Now that we have logged the successes and failures, we want to see the data! In your status dashboard or reports view, load an image with the URL:

<img src="http://app.gtraxapp.com/eventimager?series=rpc.success:rpc.failure" width="600" height="200">

(Replace app.gtraxapp.com with your application's domain name)

You should now see a 2 hour history of success events vs. failure events! The encoding for the image format is simple -- its simply a colon separated list of event namespaces.

Remember, the data is designed to be fast, rather then durable. If Google decides to flush memcache you'll lose data. In practice though, you shouldn't see this happen very often!