private Result verifyRedundancy(DistributorNodeInfo distributorNodeInfo, Integer minReplication, Integer minReplicationIndex) {
        // Why test on != null? Missing min-replication is OK (indicate empty/few buckets on system).
        if (minReplication != null && minReplication < requiredRedundancy) {
            return disallow("Distributor " + distributorNodeInfo.getNodeIndex()
                                    + " says storage node " + minReplicationIndex
                                    + " has buckets with redundancy as low as "
                                    + minReplication + ", but we require at least " + requiredRedundancy);
        }

        return allow();
    }
Verifies that storage nodes and distributors are up (or retired).
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/NodeStateChangeChecker.java