Trace and span — stitching one user request

1def on_inbound_request(req):
2    tp = parse(req.headers.get('traceparent'))
3    trace_id = tp.trace_id if tp else new_trace_id()
4    parent_id = tp.span_id if tp else None
5    span = start_span(
6        name=req.path,
7        trace_id=trace_id,
8        parent_id=parent_id,
9    )
10    resp = forward_to_app(req)
11    span.duration = elapsed(span.start)
12    emit_to_collector(span)  # span is fire-and-forget
13    return resp

1def on_outbound_request(req, current_span):
2    # W3C format: 00-<32 hex trace>-<16 hex span>-<2 hex flags>
3    req.headers['traceparent'] = (
4        f'00-{current_span.trace_id}'
5        f'-{current_span.span_id}'
6        f'-{flags_byte(sampled=current_span.sampled)}'
7    )
8    return req  # sidecar can only stamp what's on the wire

1def emit_to_collector(span):
2    # RED metrics: counted on EVERY request, no sampling.
3    metrics.requests.inc()
4    metrics.duration.observe(span.duration)
5    if span.errored: metrics.errors.inc()
6    # Traces: only written if the sampled bit is set.
7    if span.sampled:  # head decision from flags byte
8        trace_store.write(span)
9    # else: span is dropped at emit time.