dvTraced function

Future<Response> dvTraced(DVTracer tracer, Request request, Future<Response> handler(Request request))

Runs handler inside a span for the request.

Joins the caller's trace when the request carries a valid traceparent, and starts a new one when it does not.

Implementation

Future<Response> dvTraced(
  DVTracer tracer,
  Request request,
  Future<Response> Function(Request request) handler,
) async {
  final DVSpan span = tracer.startSpan(
    '${request.method} ${request.url.path}',
    context: DVTraceContext.parse(request.headers.get('traceparent')),
  );
  span
    ..setAttribute('http.method', request.method)
    ..setAttribute('http.path', request.url.path);

  try {
    final Response response = await dvInSpan(span, () => handler(request));

    span.setAttribute('http.status', '${response.status}');
    // 5xx only. A 404 is the client asking for something that is not there,
    // and counting it as a server error makes an error rate that alerts on
    // ordinary traffic.
    if (response.status >= 500) {
      span.status = DVSpanStatus.error;
      span.setAttribute('error', 'HTTP ${response.status}');
    }

    // Sent back so a support ticket can quote a trace id. Without it the id
    // exists only in the backend and nobody outside can name one.
    response.headers.set('traceparent', span.context.toHeader());
    return response;
  } on Object catch (error, stack) {
    span.recordError(error, stack);
    rethrow;
  } finally {
    // Either way. A span left open is a span never exported, so an endpoint
    // that always fails would be the one that never appears in the trace
    // view.
    span.end();
  }
}