在运行app.js的时候报出如下错误：
[root@VM_0_14_centos html]# node app.js
服务器启动成功
数据库连接成功
(node:12711) UnhandledPromiseRejectionWarning: MongoError: command find requires authentication
at Connection. (/var/www/html/node_modules/mongodb/lib/core/connection/pool.js:466:61)
at Connection.emit (events.js:305:20)
at processMessage (/var/www/html/node_modules/mongodb/lib/core/connection/connection.js:384:10)
at Socket. (/var/www/html/node_modules/mongodb/lib/core/connection/connection.js:553:15)
at Socket.emit (events.js:305:20)
at addChunk (_stream_readable.js:341:12)
at readableAddChunk (_stream_readable.js:316:11)
at Socket.Readable.push (_stream_readable.js:250:10)
at TCP.onStreamRead (internal/stream_base_commons.js:186:23)
(node:12711) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag --unhandled-rejections=strict (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:12711) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:12711) UnhandledPromiseRejectionWarning: MongoError: command find requires authentication
at Connection. (/var/www/html/node_modules/mongodb/lib/core/connection/pool.js:466:61)
at Connection.emit (events.js:305:20)
at processMessage (/var/www/html/node_modules/mongodb/lib/core/connection/connection.js:384:10)
at Socket. (/var/www/html/node_modules/mongodb/lib/core/connection/connection.js:553:15)
at Socket.emit (events.js:305:20)
at addChunk (_stream_readable.js:341:12)
at readableAddChunk (_stream_readable.js:316:11)
at Socket.Readable.push (_stream_readable.js:250:10)
at TCP.onStreamRead (internal/stream_base_commons.js:186:23)
(node:12711) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag --unhandled-rejections=strict (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
错误信息说是数据库需要验证，刚开始以为是权限不够，查了很久的资料但是在第一天还是没有解决，网站还是没法传输数据，知道第二天晚上继续这个问题的时候，突然看到一篇文章中的一段字
保存mongodb.conf文件，然后重启！
没错需要重启，因为我尝试过把 xshell 关闭然后重新打开过，发现还是不行，因为我的MongoDB是设置成开机自动启动的，于是就忽略了重启这个问题，但是请注意你把xshell关了，只是把这个软件关了，而服务器还处于开机状态，所以要输入MongoDB的重启命令才能重启MongoDB，也就是这个命令：
systemctl restart mongod.service
果然输入重启之后成功解决问题。
在这个解决问题的过程中也学到了很多其他东西，比如：
- 如何远程连接数据库
- 如何设置系统变量
- MongoDB的安全问题，如何连接加密数据库
- 对mongodb.conf 这个文件也有了更多的认识等等
