private static FederationOptions applyDefaultSourceGroupOptions(FederationOptions targetOptions) {
            FederationOptions defaultSourceGroupOption = new FederationOptions().setUseByDefault(true);
            return targetOptions.inherit(defaultSourceGroupOption);
        }
One or more search chains that are handled as a single group,
which can be federated to as a single entity.
config-model/src/main/java/com/yahoo/vespa/model/container/search/searchchain/FederationSearcher.java