private NodeSpec getNextSearchNodeSpec(StorageGroup parentGroup) {
        NodeSpec spec = groupToSpecMap.get(parentGroup);
        if (spec == null) {
            spec = new NodeSpec(groupToSpecMap.size(), 0);
        } else {
            spec = new NodeSpec(spec.groupIndex(), spec.partitionId() + 1);
        }
        groupToSpecMap.put(parentGroup, spec);
        return spec;
    }
Translates group ids to continuous 0-base "row" id integers
config-model/src/main/java/com/yahoo/vespa/model/content/ContentSearchCluster.java