When working with MongoDB Change Streams in Node.js, it's essential to understand that change events can be emitted multiple times for the same change. This behavior is expected due to the nature of MongoDB Change Streams and how they handle certain scenarios. Below are some reasons why you might experience multiple change events and how to handle them:
Resume Tokens: MongoDB Change Streams use a mechanism called "resume tokens" to keep track of the last processed change. When you start a change stream, you receive a resume token for the current point in the stream. If your application crashes or disconnects and then reconnects, the change stream will resume from the last seen resume token. This can result in receiving some change events multiple times, especially after a reconnection.
To handle this, your application should keep track of the last processed resume token and use it to resume the change stream after a crash or disconnect.
Concurrency: Multiple instances of your application or multiple connections to the MongoDB server might consume the same change stream. In this case, you may receive the same change event multiple times, once for each consumer.
To prevent duplicate processing, you can use a message queue or distributed lock to ensure that only one instance of your application processes each change event.
Network or Latency Issues: If there are network or latency issues between your application and the MongoDB server, the change events might be re-emitted. This can happen when acknowledgments from the server are not received promptly.
To handle this, you can implement retry logic in your application to reprocess the change events if they are not acknowledged successfully.
Multi-document Operations: Some MongoDB operations can result in multiple change events. For example, if you update multiple documents in a single operation, you might receive a separate change event for each document updated.
To handle this, you can check the operation type and other information in the change event to determine if it corresponds to a single or multi-document operation.
Resumable Errors: MongoDB Change Streams may emit certain errors (e.g., "ResumableChangeStreamError") that indicate the change stream can be resumed after the error is handled. When your application recovers from such errors, it may receive the same change event again.
To handle resumable errors, follow the guidelines in the MongoDB documentation for resuming change streams after encountering specific errors.
In summary, multiple change events are a normal behavior of MongoDB Change Streams, and your application should be designed to handle duplicate events gracefully. By keeping track of resume tokens, handling concurrency, and implementing appropriate retry and error handling strategies, you can ensure that your application processes change events accurately and efficiently.