@Override
    public Set<TopologyCluster> getClusters(Topology topology) {
        return defaultTopology(topology).getClusters();
    }
@Test
    public void testGetClusters() {
        VirtualNetwork virtualNetwork = setupVirtualNetworkTopology();

        TopologyService topologyService = manager.get(virtualNetwork.id(), TopologyService.class);

        Topology topology = topologyService.currentTopology();

        // test the getClusters() method.
        assertNotNull("The clusters should not be null.", topologyService.getClusters(topology));
        assertEquals("The clusters size did not match.", 2, topologyService.getClusters(topology).size());
    }