Watch for Changes监视更改

Iteration

The following example opens a change stream on the haikus collection in the insertDB database and prints change events as they occur:以下示例在insertDB数据库中打开haikus集合上的更改流，并在更改事件发生时打印它们：

import { MongoClient } from "mongodb";

// Replace the uri string with your MongoDB deployment's connection string.将uri字符串替换为MongoDB部署的连接字符串。
const uri = "<connection string uri>";

const client = new MongoClient(uri);

let changeStream;
async function run() {
  try {
    const database = client.db("insertDB");
    const haikus = database.collection("haikus");

    // Open a Change Stream on the "haikus" collection打开“haikus(俳句)”集合的变更流
    changeStream = haikus.watch();

    // Print change events打印更改事件
    for await (const change of changeStream) {
      console.log("Received change:\n", change);
    }

    await changeStream.close();
    
  } finally {
    await client.close();
  }
}
run().catch(console.dir);

When you run this code and then make a change to the haikus collection, such as performing an insert or delete operation, you can see the change event document printed in your terminal.当您运行此代码，然后对haikus集合进行更改（例如执行插入或删除操作）时，您可以看到打印在终端中的更改事件文档。

For example, if you insert a document to the collection, the code prints the following output:例如，如果将文档插入集合，则代码将打印以下输出：

Received change:
{
  _id: {
    _data: '...'
  },
  operationType: 'insert',
  clusterTime: new Timestamp({ t: 1675800603, i: 31 }),
  fullDocument: {
    _id: new ObjectId("..."),
    ...
  },
  ns: { db: 'insertDB', coll: 'haikus' },
  documentKey: { _id: new ObjectId("...") }
}

const options = { fullDocument: "updateLookup" };
// This could be any pipeline.这可以是任何管道。
const pipeline = [];

const changeStream = myColl.watch(pipeline, options);

Listener Function侦听器函数

The following example opens a change stream on the haikus collection in the insertDB database. 以下示例在insertDB数据库中打开一个关于haikus集合的更改流。Let's create a listener function to receive and print change events that occur on the collection.让我们创建一个侦听器函数来接收和打印集合上发生的更改事件。

First, open the change stream on the collection and then define a listener on the change stream using the on() method. 首先，打开集合上的变更流，然后使用on()方法在变更流上定义一个侦听器。Once you set the listener, generate a change event by performing a change to the collection.设置侦听器后，通过对集合执行更改来生成更改事件。

To generate the change event on the collection, let's use the insertOne() method to add a new document. 要在集合上生成更改事件，让我们使用insertOne()方法添加一个新文档。Since insertOne() may run before the listener function can register, we use a timer, defined as simulateAsyncPause to wait 1 second before executing the insert.由于insertOne()可能在侦听器函数注册之前运行，因此我们使用一个计时器，定义为simulateAsyncPause，在执行插入之前等待1秒。

We also use simulateAsyncPause after the insertion of the document. 我们还在插入文档后使用simulateAsyncPause。This provides ample time for the listener function to receive the change event and for the listener to complete its execution before closing the ChangeStream instance using the close() method.这为侦听器函数提供了充足的时间来接收更改事件，并为侦听器在使用close()方法关闭ChangeStream实例之前完成其执行。

import { MongoClient } from "mongodb";

// Replace the uri string with your MongoDB deployment's connection string.将uri字符串替换为MongoDB部署的连接字符串。
const uri = "<connection string uri>";

const client = new MongoClient(uri);

const simulateAsyncPause = () =>
  new Promise(resolve => {
    setTimeout(() => resolve(), 1000);
  });

let changeStream;
async function run() {
  try {
    const database = client.db("insertDB");
    const haikus = database.collection("haikus");

    // open a Change Stream on the "haikus" collection打开“haikus(徘句)”集合的“变更流”
    changeStream = haikus.watch();

    // set up a listener when change events are emitted在发出更改事件时设置侦听器
    changeStream.on("change", next => {
      // process any change event处理任何更改事件
      console.log("received a change to the collection: \t", next);
    });

    await simulateAsyncPause();

    await myColl.insertOne({
      title: "Record of a Shriveled Datum",
      content: "No bytes, no problem. Just insert a document, in MongoDB",
    });

    await simulateAsyncPause();

    await changeStream.close();
    
    console.log("closed the change stream");
  } finally {
    await client.close();
  }
}
run().catch(console.dir);

Visit the following resources for additional material on the classes and methods mentioned on this page:有关本页中提到的类和方法的其他材料，请访问以下资源：

• Change streams更改流
• Change events更改事件
• Aggregation pipeline聚合管道
• Aggregation stages聚合阶段
• ChangeStream class API documentationChangeStream类API文档
• Collection.watch(),
• Db.watch(),
• MongoClient.watch() API documentation