private static TargetSelector resolveSelector(String selectorId,
                                                  ComponentRegistry<TargetSelector> targetSelectors) {
        if (selectorId.isEmpty()) return null;
        return checkNotNull(targetSelectors.getComponent(selectorId),
                "Missing target selector with id '" + selectorId + "'");
    }
The name of the query property containing the source name added to the query to each source by this
container-search/src/main/java/com/yahoo/search/federation/FederationSearcher.java