Traceflags

Traceflags are a small set of boolean switches encoded as bits that accompany a trace context in distributed tracing systems. They indicate optional or future-oriented behavior related to tracing, most commonly the sampling decision for a trace.

In the W3C Trace Context specification and its common implementations (including OpenTelemetry and many language SDKs), the trace context includes a traceflags field as part of the span context. Traceflags is an 8-bit field; the least significant bit, named SAMPLED (value 0x01), indicates whether the trace is sampled for export. Other bits are reserved for future use and should be ignored if unknown.

Operation and semantics: The trace flags are propagated across process boundaries via the traceparent header or equivalent, so that downstream services and collectors can decide whether to collect or export spans. If SAMPLED is 1, exporters may emit spans; if 0, spans are typically omitted or kept locally depending on configuration. Implementations may apply additional, more granular sampling policies at collectors or analyzers, but traceflags primarily convey the initial sampling decision.

Other uses and distinctions: In other domains, the term trace flag can refer to different mechanisms. For example, in database systems such as SQL Server, trace flags enable or alter diagnostic features via commands like DBCC TRACEON. Those flags are separate from distributed tracing tracecontext flags and are not standardized with the traceflags used in tracing protocols.

See also: W3C Trace Context, OpenTelemetry, distributed tracing, SQL Server trace flags.