README.md

Log Correlation

The getting started docs for logs and traces showed how to emit logs and traces independently, and export them to console exporter.

This doc explains how logs can be correlated to traces.

Logging Data Model support for correlation

Logging Data Model defines fields which allow a log to be correlated with span (Activity in .NET). The fields TraceId and SpanId allow a log to be correlated to corresponding Activity.

Correlation in OpenTelemetry .NET

The good news is that, in OpenTelemetry .NET SDK, there is no user action required to enable correlation. i.e the SDK automatically enables logs to Activity correlation, by populating the fields TraceId, SpanId and TraceFlags from the active activity (i.e Activity.Current), if any.

The example Program.cs shows how to emit logs within the context of an active Activity. Running the application will show the following output on the console:

LogRecord.Timestamp:               2024-01-26T17:55:39.2273475Z
LogRecord.TraceId:                 aed89c3b250fb9d8e16ccab1a4a9bbb5
LogRecord.SpanId:                  bd44308753200c58
LogRecord.TraceFlags:              Recorded
LogRecord.CategoryName:            Program
LogRecord.Severity:                Info
LogRecord.SeverityText:            Information
LogRecord.Body:                    Food `{name}` price changed to `{price}`.
LogRecord.Attributes (Key:Value):
    name: artichoke
    price: 9.99
    OriginalFormat (a.k.a Body): Food `{name}` price changed to `{price}`.
LogRecord.EventId:                 344095174
LogRecord.EventName:               FoodPriceChanged

...

Activity.TraceId:            aed89c3b250fb9d8e16ccab1a4a9bbb5
Activity.SpanId:             bd44308753200c58
Activity.TraceFlags:         Recorded
Activity.ActivitySourceName: MyCompany.MyProduct.MyLibrary
Activity.DisplayName:        SayHello
Activity.Kind:               Internal
Activity.StartTime:          2024-01-26T17:55:39.2223849Z
Activity.Duration:           00:00:00.0361682
...

As you can see, the LogRecord automatically had the TraceId, SpanId fields matching the ones from the Activity. In the logs getting started doc, the logging was done outside of an Activity context, hence these fields in LogRecord were not populated.

Learn more

Check ASP.NET Core example application which shows how all the logs done within the context of request are automatically correlated to the Activity representing the incoming request.