Tags, logs and baggage

Tags

Tags are key:value pairs that enable user-defined annotation of spans in order to query, filter, and comprehend trace data.

Span tags should apply to the whole span. There is a list available at semantic_conventions.md listing conventional span tags for common scenarios. Examples may include tag keys like db.instance to identify a database host, http.status_code to represent the HTTP response code, or error which can be set to True if the operation represented by the Span fails.

Logs

Logs are key:value pairs that are useful for capturing timed log messages and other debugging or informational output from the application itself. Logs may be useful for documenting a specific moment or event within the span (in contrast to tags which should apply to the span regardless of time).

Baggage Items

The SpanContext carries data across process boundaries. Specifically, it has two major components:

• An implementation-dependent state to refer to the distinct span within a trace
  • i.e., the implementing Tracer’s definition of spanID and traceID
• Any Baggage Items
  • These are key:value pairs that cross process-boundaries.
  • These may be useful to have some data available for access throughout the trace.