private Set<String> resolveSourceSubset(Set<String> sources) {
        Set<String> candidates = new HashSet<>();
        for (String source : sources) {
            Cluster cluster = schemaInfo.clusters().get(source);
            if (cluster != null)
                candidates.addAll(cluster.schemas());
        }
        return (candidates.isEmpty() ? sources : candidates).stream()
                .filter(schema2Searcher::containsKey).collect(Collectors.toUnmodifiableSet());
    }
Do not use, for internal testing purposes only. *
container-search/src/main/java/com/yahoo/prelude/cluster/ClusterSearcher.java