private Tuple2<String, Chain<Searcher>> resolveChain(String explicitChainName) {
        String chainName = explicitChainName;
        if (chainName == null) {
            chainName = defaultSearchChainName;
        }

        Chain<Searcher> searchChain = executionFactory.searchChainRegistry().getChain(chainName);
        if (searchChain == null && explicitChainName == null) { // explicit chain not found should cause error
            chainName = fallbackSearchChain;
            searchChain = executionFactory.searchChainRegistry().getChain(chainName);
        }
        return new Tuple2<>(chainName, searchChain);
    }
If present, responses from this will set the HTTP response header with this key to the host name of this
container-search/src/main/java/com/yahoo/search/handler/SearchHandler.java