folly官方例子

Future<vector<LeafResponse>> fanout(const map<Leaf, LeafReq> &leafToReqMap,chrono::milliseconds timeout) {vector<Future<LeafResponse>> leafFutures;for (const auto &kv : leafToReqMap) {const auto &leaf = kv.first;const auto &leafReq = kv.second;leafFutures.push_back(// Get the client for this leaf and do the async RPCgetClient(leaf)->futureLeafRPC(leafReq)// If the request times out, use an empty response and move on..onTimeout(timeout, [=] { return LeafResponse(); })// If there's an error (e.g. RPC exception),// use an empty response and move on..onError([=](const exception &e) { return LeafResponse(); }));}// Collect all the individual leaf requests into one Futurereturn collect(leafFutures);
}
// Some sharding function; possibly dependent on previous responses.
map<Leaf, LeafReq> buildLeafToReqMap(const Request &request,const vector<LeafResponse> &responses);
// This function assembles our final response.
Response assembleResponse(const Request &request,const vector<LeafResponse> &firstFanoutResponses,const vector<LeafResponse> &secondFanoutResponses);
Future<Response> twoStageFanout(shared_ptr<Request> request) {// Stage 1: first fanoutreturn fanout(buildLeafToReqMap(*request, {}), FIRST_FANOUT_TIMEOUT_MS)// Stage 2: With the first fanout completed, initiate the second fanout..then([=](vector<LeafResponse> &responses) {auto firstFanoutResponses =std::make_shared<vector<LeafResponse>>(std::move(responses));// This time, sharding is dependent on the first fanout.return fanout(buildLeafToReqMap(*request, *firstFanoutResponses),SECOND_FANOUT_TIMEOUT_MS)// Stage 3: Assemble and return the final response..then([=](const vector<LeafResponse> &secondFanoutResponses) {return assembleResponse(*request, *firstFanoutResponses,secondFanoutResponses);});});
}