public Set<ClusterId> statefulClusters() {
        return stream().filter(node -> node.allocation().isPresent() &&
                                       node.allocation().get().membership().cluster().isStateful())
                       .map(node -> new ClusterId(node.allocation().get().owner(),
                                                  node.allocation().get().membership().cluster().id()))
                       .collect(Collectors.toUnmodifiableSet());

    }
Returns the stateful clusters on nodes in this
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/NodeList.java