nrjohnstone/AmbientContext.LogService.Serilog

AmbientContext.LogService.Serilog

The Serilog AmbientLogService provides a way to address the cross cutting concern of Logging throughout an application, using the logging framework Serilog.

The use of such an ambient service removes the need for cross cutting concerns to pollute the constructors of classes throughout a project.

It provides an ambient logging context behind the facade of an instance based reference.

Usage

The AmbientLogService should be added to each class requiring logging as a read only public property.

It should be read only to prevent the reference to the instance of the AmbientLogService being changed and it should be a public property to allow setting of the ILogger instance for testing purposes.

For those who feel their "spidey sense" tingling at making a dependency such as this public, I suggest you remember the fact that you were probably passing it in on the constructor anyway, so you already had access to the instance.

As Mark Seemaans states regarding TDD,

People unused to TDD often react in one of two ways:

• "I don't like adding members only for testing"
• "It breaks encapsulation"

The excellent blog post of his on Structural Inspection is here for further reading

public class Foo
{
    public AmbientLogService Logger {get;} = new AmbientLogService();
}

Extra Log Handlers

To handle cases where you may want to do some extra logic for particular types of logging levels, it is possible to create handlers and register them with the AmbientLogService.

The AmbientLogService evaluates the registered handlers for each call to a logging method and will call the same method on the handlers.

Writing a handler involves extending the abstract class LogHandlerBase and overriding the methods that the desired behavior must be invoked on.

An example of using an extra log handler might be the need to record metrics in an external Statsd client on the amount of log events being seen at the FATAL level.

 public class StatsdLogHandler : LogHandlerBase
    {
        private const string MetricNameLogEventsFatal = "Serilog.Example.LogEventsFatal";
        private readonly Statsd _statsd;

        public StatsdLogHandler(Statsd statsd)
        {
            if (statsd == null) throw new ArgumentNullException(nameof(statsd));
            _statsd = statsd;
        }

        public override void Fatal(string messageTemplate)
        {
            ReportFatal();
        }

        public override void Fatal(Exception exception, string messageTemplate, params object[] propertyValues)
        {
            ReportFatal();
        }

        public override void Fatal(string messageTemplate, params object[] propertyValues)
        {
            ReportFatal();
        }

        private void ReportFatal()
        {
            _statsd.LogCount(MetricNameLogEventsFatal);
        }
    }

The repo has an example demonstrating writing just such a handler and it's use in a console application.

Testing

If a unit test is required to document logging requirements, the instance on the AmbientLogService can be set to a mock rather than the real Serilog instance.

// inside a unit test
sut.Logger.Instance = _mockLogger;

The repo contains a unit test project with an example unit test being written against a class within a console application that is using the AmbientLogService.