Question

I can enable tracing by replacing the default No Op trace writer.

GlobalConfiguration.Configuration.Services.Replace(typeof(ITraceWriter), new SimpleTracer());

But how can I disable (and enable it again) while the server is running?

The Enum “TraceLevel” has the option TraceLevel.Off.
Is there a way to set the tracing-framwork form web api to TraceLevel.Off?

Answer 1

Take a look at this article here.

WebApi does not provide any way to configure trace writers regarding their current category or level.

And

Determining which categories and levels are currently enabled is the
responsibility of the ITraceWriter implementation. The Web API
framework has no concept of trace configuration or determining which
categories and levels are enabled.

The intent of this approach is to rely on the ITraceWriter
implementation’s own mechanisms to configure what gets traced and
where.

For example, if you use an NLog-specific ITraceWriter, you should
configure tracing through NLog configuration mechanisms.

In short if you want to enable and disable your tracing or chose not to log a trace for a particular level it is up to you to implement inside your SimpleTracer.

You could create your own threadsafe singleton TraceConfiguration (akin to GlobalConfiguration) implementation to control the trace configuration to allow toggling on or off within the code.

public void Trace(HttpRequestMessage request, string category, TraceLevel level, Action<TraceRecord> traceAction)
{
     if(TraceConfiguration.Configuration.IsEnabled)
     {
        //trace
     }
}

Or you could set and access properties on the request object to determine whether to enable tracing or not. This could be set by ActionFilters on actions and controllers or even set within the controller.

public void Trace(HttpRequestMessage request, string category, TraceLevel level, Action<TraceRecord> traceAction)
{
     if(request.Properties["SupressTracing"] as string != "true")
     {
        //trace
     }
}

Or as the article suggests if you are using NLog or Log4Net etc it is up to you set the levels enabled via config etc.