// file: test.cpp
// build: c++ --std=c++11 test.cpp -o test $(pkg-config --cflags --libs libmongocxx)  -lpthread
#include <iostream>
#include <ostream>
#include <sstream>
#include <thread>
#include <vector>

#include <bsoncxx/builder/basic/document.hpp>
#include <bsoncxx/builder/basic/kvp.hpp>
#include <bsoncxx/json.hpp>
#include <bsoncxx/types.hpp>
#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
#include <mongocxx/pool.hpp>
#include <mongocxx/uri.hpp>

using bsoncxx::builder::basic::kvp;

int main() {
    // The mongocxx::instance constructor and destructor initialize and shut down the driver,
    // respectively. Therefore, a mongocxx::instance must be created before using the driver and
    // must remain alive for as long as the driver is in use.
    mongocxx::instance inst{};
    mongocxx::uri uri{"mongodb://localhost:27017/?minPoolSize=5&maxPoolSize=5"};

    mongocxx::pool pool{uri};

    std::vector<std::string> collection_names = {"foo", "bar", "baz"};
    std::vector<std::thread> threads{};

    for (auto i : {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}) {
        auto run = [&](std::int64_t j) {
            // Each client and collection can only be used in a single thread.
            auto client = pool.acquire();
            auto coll = (*client)["test"][collection_names[static_cast<std::size_t>(j)]];
            coll.delete_many({});

            bsoncxx::types::b_int64 index = {j};
            coll.insert_one(bsoncxx::builder::basic::make_document(kvp("x", index)));

            if (auto doc =
                    (*client)["test"][collection_names[static_cast<std::size_t>(j)]].find_one({})) {
                // In order to ensure that the newline is printed immediately after the document,
                // they need to be streamed to std::cout as a single string.
                std::stringstream ss;
                ss << bsoncxx::to_json(*doc) << std::endl;

                std::cout << ss.str();
            }

            // The client goes out of scope at the end of the lambda and is returned to the pool.
        };

        std::thread runner{run, i};

        threads.push_back(std::move(runner));
    }

    for (auto&& runner : threads) {
        runner.join();
    }
}

m 4.4.9-ent
rm -rf data && mkdir data 
mongod --dbpath data --maxConns 5

terminate called after throwing an instance of 'mongocxx::v_noabi::bulk_write_exception'
  what():  Failed to send "isMaster" command with database "admin": socket error or timeout: generic server error
Aborted (core dumped)