private ContainerDocumentApi buildDocumentApi(DeployState deployState, ApplicationContainerCluster cluster, Element spec, ConfigModelContext context) {
        Element documentApiElement = XML.getChild(spec, "document-api");
        if (documentApiElement == null) return ContainerDocumentApi.createDummyApi(cluster);

        ContainerDocumentApi.HandlerOptions documentApiOptions = DocumentApiOptionsBuilder.build(documentApiElement);
        Element ignoreUndefinedFields = XML.getChild(documentApiElement, "ignore-undefined-fields");
        return new ContainerDocumentApi(deployState, cluster, documentApiOptions,
                                        "true".equals(XML.getValue(ignoreUndefinedFields)), portBindingOverride(deployState, context));
    }
Allocate a container cluster without a nodes tag
config-model/src/main/java/com/yahoo/vespa/model/container/xml/ContainerModelBuilder.java