Tracing

Import from loggily (main entry).

ID Format

Format	Span ID	Trace ID	Use Case
"simple"	sp_1, sp_2	tr_1, tr_2	Development, testing
"w3c"	16-char hex	32-char hex	Distributed tracing

Via env var (recommended)

TRACE_ID_FORMAT=w3c bun run app

Via config object

const log = createLogger("myapp", [{ level: "debug", idFormat: "w3c" }, console])
const span = log.span("request")
// span.spanData.id    → "a1b2c3d4e5f6a7b8"
// span.spanData.traceId → "a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6"

Via setters (deprecated)

// @deprecated — use TRACE_ID_FORMAT env var or { idFormat } config instead
function setIdFormat(format: IdFormat): void
function getIdFormat(): IdFormat

type IdFormat = "simple" | "w3c"

traceparent

function traceparent(spanData: SpanData, options?: TraceparentOptions): string

interface TraceparentOptions {
  /** Whether this span is sampled. Defaults to true. */
  sampled?: boolean
}

Format a W3C traceparent header.

const span = log.span("http-request")
const header = traceparent(span.spanData)
// → "00-a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6-1a2b3c4d5e6f7a8b-01"

fetch(url, { headers: { traceparent: header } })

// Mark as not sampled
traceparent(span.spanData, { sampled: false })
// → "00-...-...-00"

Works with both "simple" and "w3c" ID formats. Simple IDs are zero-padded to spec length.

Sampling

Head-based sampling: the decision is made when a trace starts and inherited by all child spans.

• Rate must be between 0.0 and 1.0 (throws otherwise)
• Child spans always inherit the parent's sampling decision
• Unsampled spans still have valid spanData -- only output is suppressed

Via env var (recommended)

TRACE_SAMPLE_RATE=0.1 bun run app   # Sample 10% of traces

Via config object

const log = createLogger("myapp", [{ level: "debug", sampleRate: 0.1 }, console])

Via setters (deprecated)

// @deprecated — use TRACE_SAMPLE_RATE env var or { sampleRate } config instead
function setSampleRate(rate: number): void
function getSampleRate(): number