private static VirtualSourceResolver createVirtualSourceResolver(FederationConfig config) {
        return VirtualSourceResolver.of(config.target().stream().map(FederationConfig.Target::id).collect(Collectors.toUnmodifiableSet()));
    }
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