Function linkTraces

packages/browser/src/tracing/linkedTraces.ts:49–126

(
  client: Client,
  {
    linkPreviousTrace,
    consistentTraceSampling,
  }: {
    linkPreviousTrace: 'session-storage' | 'in-memory';
    consistentTraceSampling: boolean;
  },
)

 * @param client - Sentry client
 */
export function linkTraces(
  client: Client,
  {
    linkPreviousTrace,
    consistentTraceSampling,
  }: {
    linkPreviousTrace: 'session-storage' | 'in-memory';
    consistentTraceSampling: boolean;
  },
): void {
  const useSessionStorage = linkPreviousTrace === 'session-storage';

  let inMemoryPreviousTraceInfo = useSessionStorage ? getPreviousTraceFromSessionStorage() : undefined;

  client.on('spanStart', span => {
    if (getRootSpan(span) !== span) {
      return;
    }

    const oldPropagationContext = getCurrentScope().getPropagationContext();
    inMemoryPreviousTraceInfo = addPreviousTraceSpanLink(inMemoryPreviousTraceInfo, span, oldPropagationContext);

    if (useSessionStorage) {
      storePreviousTraceInSessionStorage(inMemoryPreviousTraceInfo);
    }
  });

  let isFirstTraceOnPageload = true;
  if (consistentTraceSampling) {
    /*
    When users opt into `consistentTraceSampling`, we need to ensure that we propagate
    the previous trace's sample rate and rand to the current trace. This is necessary because otherwise, span
    metric extrapolation is inaccurate, as we'd propagate too high of a sample rate for the subsequent traces.

    So therefore, we pretend that the previous trace was the parent trace of the newly started trace. To do that,
    we mutate the propagation context of the current trace and set the sample rate and sample rand of the previous trace.
    Timing-wise, it is fine because it happens before we even sample the root span.

    @see https://github.com/getsentry/sentry-javascript/issues/15754
    */
    client.on('beforeSampling', mutableSamplingContextData => {
      if (!inMemoryPreviousTraceInfo) {
        return;
      }

      const scope = getCurrentScope();
      const currentPropagationContext = scope.getPropagationContext();

      // We do not want to force-continue the sampling decision if we continue a trace
      // that was started on the backend. Most prominently, this will happen in MPAs where
      // users hard-navigate between pages. In this case, the sampling decision of a potentially
      // started trace on the server takes precedence.
      // Why? We want to prioritize inter-trace consistency over intra-trace consistency.
      if (isFirstTraceOnPageload && currentPropagationContext.parentSpanId) {
        isFirstTraceOnPageload = false;
        return;
      }