@Test void doesntLinkUnrelatedSpansWhenMissingRootSpan() {
    String missingParentId = "a";
    List<Span> trace = List.of(
      span("a", missingParentId, "b", Kind.SERVER, "service1", null, false),
      span("a", missingParentId, "c", Kind.SERVER, "service2", null, false)
    );

    assertThat(new DependencyLinker(logger)
      .putTrace(trace).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