如何使用统一的拓扑和promise从NodeJS连接到MongoDB

Question

我是新的MongoDB-抱歉提前为任何pebkac。

根据本文，统一拓扑范式远离了“连接”的概念，而是简单地设置一个连接字符串并开始执行操作。然后，每个操作都会失败或成功，这取决于驱动程序在执行该操作时是否可以到达服务器。

我试图实现以下示例代码，以在测试机器上建立到本地MongoDB服务器的“连接”，但它不起作用：节点抛出未捕获的promise拒绝消息，显然是因为它在某个地方出错。

然而，示例代码没有显示如何使用promise实现新的范例。我假设这是主要的问题，所以我尝试了下面显示的第二个代码，但仍然没有得到允许我检索记录的对象。你能告诉我我做错了什么，以及在Express应用程序中建议/最好的方法来确保它尽可能稳定吗？

第一次尝试：索引。js:

async function init(){
    console.log("Starting...");
    const MongoClient = require('mongodb');
    const client = MongoClient('mongodb://test:test@localhost:27017', { useUnifiedTopology: true });
    const coll = client.db('test').collection('foo');
    await coll.insert({ test: 'document' });
    const docs = coll.find({ test: 1 }, { readPreference: 'secondary' }).toArray();
    console.dir({ docs });
    await client.close();
}


init();

错误：

Starting...
(node:15328) UnhandledPromiseRejectionWarning: TypeError: client.db is not a function
    at init (index.js:5:22)
    at Object.<anonymous> (index.js:13:1)
    at Module._compile (internal/modules/cjs/loader.js:1128:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1167:10)
    at Module.load (internal/modules/cjs/loader.js:983:32)
    at Function.Module._load (internal/modules/cjs/loader.js:891:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47
(node:15328) 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(). (rejection id: 1)
(node:15328) [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.

所以很明显，这是试图使用Promises，但是这个例子没有以这种方式实现。

下面是第二次尝试：

index.js：

async function init(){
    console.log("Starting...");
    const MongoClient = require('mongodb');
    const client = MongoClient('mongodb://test:test@localhost:27017', { useUnifiedTopology: true })
        .then(async (result) => {
            console.log("Connected: result = ", result);

            const db = result('test');  // Get the 'test' database
            const coll = db.collection('foo');  // Ge the 'foo' collection
            await coll.insert( { test: 'document' });   // Create a new document
            const docs = coll.find({}, { readPreference: 'secondary'})  // Retrieve all docs
                .then((results) => {
                    const data = results.toArray();
                    console.log('Results: ', data);
                });
            await client.close();

        })
        .catch((err) => {
            console.log('Caught error: ', err);
        });
}


init();

输出：

Starting...
Connected: result =  MongoClient {
  _events: [Object: null prototype] { newListener: [Function (anonymous)] },
  _eventsCount: 1,
  _maxListeners: undefined,
  s: {
    url: 'mongodb://test:test@localhost:27017',
    options: {
      servers: [Array],
      caseTranslate: true,
      useUnifiedTopology: true,
      checkServerIdentity: true,
      sslValidate: true,
      auth: [Object],
      authSource: 'admin',
      dbName: 'test',
      socketTimeoutMS: 360000,
      connectTimeoutMS: 30000,
      retryWrites: true,
      useRecoveryToken: true,
      readPreference: [ReadPreference],
      credentials: [MongoCredentials],
      promiseLibrary: [Function: Promise]
    },
    promiseLibrary: [Function: Promise],
    dbCache: Map {},
    sessions: Set {},
    writeConcern: undefined,
    namespace: MongoDBNamespace { db: 'admin', collection: undefined }
  },
  topology: NativeTopology {
    _events: [Object: null prototype] {
      authenticated: [Function (anonymous)],
      error: [Function (anonymous)],
      timeout: [Function (anonymous)],
      close: [Function (anonymous)],
      parseError: [Function (anonymous)],
      fullsetup: [Function],
      all: [Function],
      reconnect: [Function (anonymous)],
      serverOpening: [Function (anonymous)],
      serverDescriptionChanged: [Function (anonymous)],
      serverHeartbeatStarted: [Function (anonymous)],
      serverHeartbeatSucceeded: [Function (anonymous)],
      serverHeartbeatFailed: [Function (anonymous)],
      serverClosed: [Function (anonymous)],
      topologyOpening: [Function (anonymous)],
      topologyClosed: [Function (anonymous)],
      topologyDescriptionChanged: [Function (anonymous)],
      commandStarted: [Function (anonymous)],
      commandSucceeded: [Function (anonymous)],
      commandFailed: [Function (anonymous)],
      joined: [Function (anonymous)],
      left: [Function (anonymous)],
      ping: [Function (anonymous)],
      ha: [Function (anonymous)]
    },
    _eventsCount: 24,
    _maxListeners: Infinity,
    s: {
      id: 0,
      options: [Object],
      seedlist: [Array],
      state: 'connected',
      description: [TopologyDescription],
      serverSelectionTimeoutMS: 30000,
      heartbeatFrequencyMS: 10000,
      minHeartbeatFrequencyMS: 500,
      Cursor: [Function: Cursor],
      bson: BSON {},
      servers: [Map],
      sessionPool: [ServerSessionPool],
      sessions: Set {},
      promiseLibrary: [Function: Promise],
      credentials: [MongoCredentials],
      clusterTime: null,
      iterationTimers: Set {},
      connectionTimers: Set {},
      clientInfo: [Object]
    }
  }
}

Caught error:  TypeError: result is not a function
    at index.js:8:15
    at processTicksAndRejections (internal/process/task_queues.js:97:5)

Answer 1

下面是从Node连接到MongoDB数据库的完整示例。JS使用统一拓扑范例（没有显式的连接命令）。感谢@MahamhBhatnagar指出我的错误。

本例使用promise，在建立连接后，开始以每5秒插入一条记录的频率插入记录。如果禁用或断开MongoDB数据库，它将缓存插入，直到（a.超时，默认为30秒，或b.连接恢复）。如果超时，插入将丢失，但如果服务器重新联机，后续请求将得到正确处理。

如果我做错了，或者有什么建议，请告诉我。

index.js：

const url = 'mongodb://test:test@localhost:27017';


async function init(){
    console.log("init...");
    const MongoClient = require('mongodb');

    return MongoClient(url, { useUnifiedTopology: true}); // Return a Promise
}


async function go(db_result) {
    // Do an insert and output the last-inserted document.
    console.log("Go...");

    const db = db_result.db('test');  // Get the 'test' database
    const coll = db.collection('foo');  // Ge the 'foo' collection
    // Create a new document:
    await coll.insertOne({ test: 'document' }, (err, response) => {
        if (err) {
            console.log("Error inserting", err);
        }
        else
        {
            console.log("New record: ", response.ops[0])  // Output newly inserted record (thanks to https://stackoverflow.com/questions/40766654)
        }
    });   


}

// Sleep for a specified time:
function sleep(ms) {
    return new Promise( resolve => {
        setTimeout(resolve,ms)
    })
}


init()  // Set up connection, then start doing inserts.
    .then( async (db_result) => {
        // When MongoClient is instantiated:
        while(true) {
            console.log("Going...")
            go(db_result)
            console.log("Sleeping")
            await sleep(5000);
        }
    })
    .catch((err) => {
        console.log("Caught error: ", err)
    })