Set<String> traceIdsDescendingByTimestamp(QueryRequest request) {
    if (!searchEnabled) return Collections.emptySet();

    Collection<TraceIdTimestamp> traceIdTimestamps =
      request.serviceName() != null
        ? traceIdTimestampsByServiceName(request.serviceName())
        : spansByTraceIdTimestamp.keySet();

    if (traceIdTimestamps == null || traceIdTimestamps.isEmpty()) return Collections.emptySet();

    return lowTraceIdsInRange(traceIdTimestamps, request.endTs, request.lookback);
  }
Used for testing. Returns all dependency links unconditionally.
zipkin/src/main/java/zipkin2/storage/InMemoryStorage.java