nodejs 连接 mongodb 正确的姿势是什么？现在 mongo 经常挂掉

nodejs 连接的 mongodb 方式是用完就关

MongoClient.connect(DB_CONN, {useNewUrlParser: true}, function (err, db) {
        if (err) throw err;
        db.db(DB_DB).collection(DB_COL).replaceOne(
            {title: where.getTitle(), $or: [{...}, {...}]},
            data,
            {upsert: true},
            callback);
        db.close();
    });

然后最近发现 mongodb 的容器经常挂掉，好像是连接太多的原因？接口的读写的确会很频繁。 所以想问下是不是这样写不行？是不是应该用连接池来写？那么 nodejs 里面怎么写 mongo 的连接池啊，能麻烦给个例子吗。

日志：

[ftdc] serverStatus was very slow: { after basic: 286, after asserts: 690, after backgroundFlushing: 690, after connections: 690, after dur: 690, after extra_info: 783, after freeMonitoring: 876, after globalLock: 876, after locks: 876, after logicalSessionRecordCache: 876, after network: 876, after opLatencies: 990, after opcounters: 990, after opcountersRepl: 990, after repl: 990, after security: 990, after storageEngine: 990, after tcmalloc: 990, after transactions: 990, after wiredTiger: 990, at end: 1001 }

日志也不懂什么意思，能给解释一下吗？这里的 after 什么意思？