@Test void cannotLinkSingleSpanWithoutBothServiceNames() {
    List<Span> incompleteRootSpans = List.of(
      span("a", null, "a", Kind.SERVER, null, null, false),
      span("a", null, "a", Kind.SERVER, "server", null, false),
      span("a", null, "a", Kind.SERVER, null, "client", false),
      span("a", null, "a", Kind.CLIENT, null, null, false),
      span("a", null, "a", Kind.CLIENT, "client", null, false),
      span("a", null, "a", Kind.CLIENT, null, "server", false)
    );

    for (Span span : incompleteRootSpans) {
      assertThat(new DependencyLinker(logger)
        .putTrace(List.of(span)).link())
        .isEmpty();
    }
  }
A dependency link is between two services. We cannot link if we don't know both service names.
zipkin/src/test/java/zipkin2/internal/DependencyLinkerTest.java