MongoDB的多线程并行操作

在多线程中对MongoDB进行操作，需要确保各线程之间读写的安全性，需要为每个线程绑定独立的mongocxx::client对象。以下为官方标准例子：

Never do this：

mongocxx::instance instance{};
mongocxx::uri uri{};
mongocxx::client c{uri};
auto db1 = c["db1"];
auto db2 = c["db2"];
std::mutex db1_mtx{};
std::mutex db2_mtx{};

auto threadfunc = [](mongocxx::database& db, std::mutex& mtx) {
  mtx.lock();
  db["col"].insert_one({});
  mtx.unlock();
};

// BAD! These two databases are individually synchronized, but they are derived from the same
// client, so they can only be accessed by one thread at a time
std::thread t1([&]() { threadfunc(db1, db1_mtx); threadfunc(db2, db2_mtx); });
std::thread t2([&]() { threadfunc(db2, db2_mtx); threadfunc(db1, db1_mtx); });

t1.join();
t2.join();

A better version：

mongocxx::instance instance{};
mongocxx::uri uri{};
mongocxx::client c1{uri};
mongocxx::client c2{uri};
std::mutex c1_mtx{};
std::mutex c2_mtx{};

auto threadfunc = [](std::string dbname, mongocxx::client& client, std::mutex& mtx) {
  mtx.lock();
  client[dbname]["col"].insert_one({});
  mtx.unlock();
};

// These two clients are individually synchronized, so it is safe to share them between
// threads.
std::thread t1([&]() { threadfunc("db1", c1, c1_mtx); threadfunc("db2", c2, c2_mtx); });
std::thread t2([&]() { threadfunc("db2", c2, c2_mtx); threadfunc("db1", c1, c1_mtx); });

t1.join();
t2.join();

The best version：

(注：连接池大小可由uri中指定)

mongocxx::instance instance{};

mongocxx::pool pool{mongocxx::uri{"mongodb://localhost:27017/?minPoolSize=3&maxPoolSize=3"}};

auto threadfunc = [](mongocxx::client& client, std::string dbname) {
  auto col = client[dbname]["col"].insert_one({});
};

// Great! Using the pool allows the clients to be synchronized while sharing only one
// background monitoring thread.
std::thread t1 ([&]() {
  auto c = pool.acquire();
  threadfunc(*c, "db1");
  threadfunc(*c, "db2");
});

std::thread t2 ([&]() {
  auto c = pool.acquire();
  threadfunc(*c, "db2");
  threadfunc(*c, "db1");
});

t1.join();
t2.join();