private void setExistingIndexingCluster(Content content, IndexingDocproc indexingDocproc, Collection<ContainerModel> containers) {
            String indexingClusterName = indexingDocproc.getClusterName(content.getCluster().getName());
            ContainerModel containerModel = findByName(indexingClusterName, containers);
            if (containerModel == null)
                throw new IllegalArgumentException("Content cluster '" + content.getCluster().getName() + "' refers to docproc " +
                                                   "cluster '" + indexingClusterName + "', but this cluster does not exist.");
            addIndexingChainsTo(containerModel.getCluster(), content, indexingDocproc);
        }
Select/creates and initializes the indexing cluster coupled to this
config-model/src/main/java/com/yahoo/vespa/model/content/Content.java