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# \$1changeStream
<a name="changeStream"></a>

弹性集群不支持。

`$changeStream`聚合阶段会打开更改流游标，以监视集合的实时更改。当进行插入、更新、替换或删除操作时，它会返回更改事件文档。

**参数**
+ `fullDocument`：指定是否返回完整文档以进行更新操作。选项有 `default` 或 `updateLookup`。
+ `resumeAfter`: 可选。恢复令牌以从变更流中的特定点继续。
+ `startAtOperationTime`: 可选。开始变更流的时间戳。
+ `allChangesForCluster`: 可选。布尔值。何时`true`，监视集群中的所有更改（对于管理员数据库）。当`false`（默认）时，仅监视指定的集合。

## 示例（MongoDB 外壳）
<a name="changeStream-examples"></a>

以下示例演示了如何使用`$changeStream`舞台来监视集合的更改。

**查询示例**

```
// Open change stream first
const changeStream = db.inventory.aggregate([
  { $changeStream: { fullDocument: "updateLookup" } }
]);

// In another session, insert a document
db.inventory.insertOne({ _id: 1, item: "Widget", qty: 10 });

// Back in the first session, read the change event
if (changeStream.hasNext()) {
  print(tojson(changeStream.next()));
}
```

**输出**

```
{
  _id: { _data: '...' },
  operationType: 'insert',
  clusterTime: Timestamp(1, 1234567890),
  fullDocument: { _id: 1, item: 'Widget', qty: 10 },
  ns: { db: 'test', coll: 'inventory' },
  documentKey: { _id: 1 }
}
```

## 代码示例
<a name="changeStream-code"></a>

要查看使用`$changeStream`聚合阶段的代码示例，请选择要使用的语言对应的选项卡：

------
#### [ Node.js ]

```
const { MongoClient } = require('mongodb');

async function example() {
  const client = await MongoClient.connect('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');
  const db = client.db('test');
  const collection = db.collection('inventory');

  // Open change stream
  const changeStream = collection.watch([]);

  changeStream.on('change', (change) => {
    console.log('Change detected:', change);
  });

  // Simulate insert in another operation
  setTimeout(async () => {
    await collection.insertOne({ _id: 1, item: 'Widget', qty: 10 });
  }, 1000);

  // Keep connection open to receive changes
  // In production, handle cleanup appropriately
}

example();
```

------
#### [ Python ]

```
from pymongo import MongoClient
import threading
import time

def example():
    client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
    db = client['test']
    collection = db['inventory']

    # Open change stream
    change_stream = collection.watch([])

    # Insert document in separate thread after delay
    def insert_doc():
        time.sleep(1)
        collection.insert_one({'_id': 1, 'item': 'Widget', 'qty': 10})

    threading.Thread(target=insert_doc).start()

    # Watch for changes
    for change in change_stream:
        print('Change detected:', change)
        break  # Exit after first change

    client.close()

example()
```

------