@Override
    public Region createRegion(RegionId regionId, String name, Region.Type type,
                               Annotations annots, List<Set<NodeId>> masterNodeIds) {
        return regionsRepo.compute(regionId, (id, region) -> {
            checkArgument(region == null, DUPLICATE_REGION);
            return new DefaultRegion(regionId, name, type, annots, masterNodeIds);
        }).value();
    }
@Test(expected = IllegalArgumentException.class)
    public void duplicateCreate() {
        store.createRegion(RID1, "R1", METRO, NO_ANNOTS, MASTERS);
        store.createRegion(RID1, "R2", CAMPUS, NO_ANNOTS, MASTERS);
    }