public ApplicationReindexing withPending(String cluster, String documentType, long requiredGeneration) {
        Cluster current = clusters.getOrDefault(cluster, Cluster.empty());
        Cluster modified = new Cluster(with(documentType, requirePositive(requiredGeneration), current.pending),
                                       current.ready);
        return new ApplicationReindexing(enabled, with(cluster, modified, clusters));
    }
Returns a copy of this with a pending reindexing at the given generation, for the given document type.
configserver/src/main/java/com/yahoo/vespa/config/server/application/ApplicationReindexing.java