Semantic Logging

Recently a customer asked me about semantic logging. This is a concept promoted by Microsoft. It is a concept promoted by Microsoft to put more structure in logging. Generally, people log by creating an unstructured message. In semantic logging, the messages have a clear structure so that later they can be processed by tools. Clearly an attractive goal looking at the overload of logging that most applications exhibit. (I do believe that there is such a thing as over-logging.)

Processing by tools requires an accuracy for which we mere mortals have very little talent. I am always full of awe for Javascript and Python developers that can survive without completion, refactoring, or in general something more intelligent than me watching my fingers. However, logging in Java consists of writing strings …

Coincidentally, a few years ago I added a feature to bnd that could provide very useful. It defined the log messages in an interface. A proxy that implemented that interface was then used to construct the log message.

For example:

interface DeviceCatalog extends Catalog {
	TRACE measurement(String sensor, int temperature);
	INFO comparing(String sensor, int temperature, int maxTemperature);
	WARN tempTooHigh(String sensor, int temperature);
	ERROR fire(String sensor, String location);
}

Observant readers will probably say: “What the hack?” before they realize that using the return type is a very elegant way of marking the message level. Yes, it is a bit of a hack but its readability is hard to deny.

So how could we use this?

final static DeviceCatalog catalog = CatalogLogger.catalog(DeviceCatalog.class);

…

	catalog.measurement("room", 25);
	catalog.comparing("room", 25, 23);
	catalog.tempTooHigh("room", 25);
	catalog.measurement("room", 54);
	catalog.comparing("room", 54, 23);
	catalog.fire("room", "Floor 4,\toffice 45\n");

The CatalogLogger class creates a proxy and uses SLF4J for logging. In Java 7 and later you can access the name of the parameters which allows us to log the information automatically:

[main] INFO DeviceCatalog1 - comparing sensor=room temperature=25 maxTemperature=23
[main] WARN DeviceCatalog - tempTooHigh sensor=room temperature=25
[main] INFO DeviceCatalog - comparing sensor=room temperature=54 maxTemperature=23
[main] ERROR DeviceCatalog - fire sensor=room location="Floor 4,\toffice 45\n"

Semantic Logging

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