Observability

MediatorLite provides built-in observability through logging and OpenTelemetry tracing.

v2 Note

In v2, observability has moved to compile time. The source generator emits ILogger calls and ActivitySource events inline into every generated Send_* and Publish_* method. Both are on by default; you opt out at compile time with assembly-level attributes.

Logging Configuration

Enable / Disable Built-in Logging

Logging is on by default. To disable it entirely, add this to any .cs file in the consuming assembly:

[assembly: DisableMediatorLogging]

When the attribute is present the generator simply omits the logging calls — there is no runtime branch or dead code.

Controlling the Log Level

Generated code always calls LogDebug under the MediatorLite.IMediator category. Control verbosity through standard Microsoft.Extensions.Logging filters:

services.AddLogging(builder =>
{
    builder.AddFilter("MediatorLite.IMediator", LogLevel.Information);
});

Or via appsettings.json:

{
  "Logging": {
    "LogLevel": {
      "MediatorLite.IMediator": "Information"
    }
  }
}

OpenTelemetry Integration

Enable / Disable Tracing

Tracing is on by default. To disable it entirely, add to any .cs file in the consuming assembly:

[assembly: DisableMediatorTracing]

Configure OpenTelemetry

services.AddOpenTelemetry()
    .WithTracing(builder =>
    {
        builder.AddSource("MediatorLite");  // Add MediatorLite source
        builder.AddConsoleExporter();
    });

Activity Tags

MediatorLite activities include these tags:

Tag	Description
mediatorlite.request.type	Full type name of request
mediatorlite.response.type	Full type name of response
mediatorlite.notification.type	Full type name of notification
mediatorlite.handler.type	Handler type name
mediatorlite.handler.count	Number of notification handlers
mediatorlite.execution.strategy	Notification execution strategy
error	Boolean indicating error occurred
error.message	Error message if applicable

Diagnostic Events (reserved — not currently emitted)

MediatorDiagnostics.Listener (a DiagnosticListener named "MediatorLite") and the event-name constants under MediatorDiagnostics.Events (MediatorLite.RequestStarted, MediatorLite.RequestCompleted, MediatorLite.RequestFailed, MediatorLite.NotificationPublished, MediatorLite.NotificationHandlerStarted, MediatorLite.NotificationHandlerCompleted) are a reserved surface: no MediatorLite code writes to the listener today, so subscribing to it observes nothing.

Use the two live instrumentation channels instead:

• Logging — the MediatorLite.IMediator logger category (see above).
• Tracing — the "MediatorLite" ActivitySource (see above).

Custom Logging Behavior

public class DetailedLoggingBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private readonly ILogger<DetailedLoggingBehavior<TRequest, TResponse>> _logger;

    public DetailedLoggingBehavior(ILogger<DetailedLoggingBehavior<TRequest, TResponse>> logger)
    {
        _logger = logger;
    }

    public async ValueTask<TResponse> HandleAsync(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken = default)
    {
        using var scope = _logger.BeginScope(new Dictionary<string, object>
        {
            ["RequestType"] = typeof(TRequest).Name,
            ["CorrelationId"] = Guid.NewGuid()
        });

        _logger.LogInformation("Handling {RequestType}: {@Request}", 
            typeof(TRequest).Name, request);

        var stopwatch = Stopwatch.StartNew();

        try
        {
            var response = await next();
            stopwatch.Stop();

            _logger.LogInformation(
                "Handled {RequestType} in {ElapsedMs}ms: {@Response}",
                typeof(TRequest).Name, stopwatch.ElapsedMilliseconds, response);

            return response;
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error handling {RequestType}", typeof(TRequest).Name);
            throw;
        }
    }
}