@Override
    public List<HostSpec> prepare(ClusterSpec cluster, Capacity capacity, ProvisionLogger logger) {
        List<HostSpec> hostsAlreadyAllocatedToCluster = 
                allocatedHosts.getHosts().stream()
                                         .filter(host -> host.membership().isPresent() && matches(host.membership().get().cluster(), cluster))
                                         .toList();
        if ( ! hostsAlreadyAllocatedToCluster.isEmpty()) 
            return hostsAlreadyAllocatedToCluster;
        else
            return fallback.prepare(cluster, capacity, logger);
    }
Creates a static host provisioner which will fall back to using the given provisioner
if a request is made for nodes in a cluster which is not present in this allocation.
configserver/src/main/java/com/yahoo/vespa/config/server/provision/StaticProvisioner.java