生产环境MongoDB重启之后，服务连不上MongoDB了，这是其中一个问题，需要分析一下。

MongoDB设置了连接池，当我未配置readPreference参数，重启MongoDB时调用接口查询会报错， 当我配置了"readPreference": "secondaryPreferred"就不会报错。 这是为什么？

一.这是不会报错的代码， 假如注释readPreference，重启时调接口就会报错。

mongo: {
        mongourl: mongourl || "mongodb://127.0.0.1:37017/dgfhk",
        options: {
            "poolSize": 120,
            "auto_reconnect": true,
            "db": {
                "readPreference": "secondaryPreferred" // 先从后主， 优先从secondary读取，如果secondary不可用时就从primary读。
              },
        }
    },

调查询接口会报错
二.第一次报错，

list error MongoError: not master and slaveOk=false
    at Connection. (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/connection/pool.js:453:61)
    at Connection.emit (events.js:400:28)
    at processMessage (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/connection/connection.js:456:10)
    at Socket. (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/connection/connection.js:625:15)
    at Socket.emit (events.js:400:28)
    at addChunk (internal/streams/readable.js:290:12)

第二次报错

list error MongoError: pool destroyed
    at Pool.write (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/connection/pool.js:844:8)
    at _command (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/wireprotocol/command.js:149:10)
    at command (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/wireprotocol/command.js:28:5)
    at Object.query (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/wireprotocol/query.js:70:3)
    at Server.query (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/topologies/server.js:649:16)
    at FindOperation.execute (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/operations/find.js:38:12)
    at /Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/operations/execute_operation.js:148:15
    at Server.selectServer (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/core/topologies/server.js:837:3)
    at Server.selectServer (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/topologies/topology_base.js:342:32)
    at executeWithServerSelection (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/operations/execute_operation.js:131:12)
    at /Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/operations/execute_operation.js:70:9
    at maybePromise (/Users/liwenhao/Desktop/imagecreate/node_modules/mongodb/lib/utils.js:692:3)
    at executeOperation (/Users/liwenha

这似乎不是代码问题，你应该提供的是mongodb集群拓扑和健康状态以分析问题

我之前一直认为是代码问题，尝试修改代码解决。按你说的，我去监控MongoDB状态才发现服务重连MongoDB失败了；只要完善重连的机制就解决问题了。