When data is updated in MongoDB, why does Logstash not reflect the changes?

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Logstash by default relies on reading data from MongoDB's oplog (operations log), which captures all changes made to the database. If Logstash is not reflecting the changes made in MongoDB, it could be due to one of the following reasons:

1. The changes made to MongoDB were not written to the oplog yet, which means Logstash has not yet received the updates.

2. The Logstash pipeline might be misconfigured or not configured to use the correct database or collection.

3. The Logstash input plugin for MongoDB may need to be restarted to refresh the connection.

4. The Logstash output plugin may not have been configured correctly to write data to the correct location.

5. There might be a connection issue between Logstash and MongoDB, which could be caused by network issues or firewall rules blocking the connection.

To troubleshoot the issue, one can check the Logstash logs or MongoDB logs for any errors or warning messages. Additionally, one can also verify the configuration files and ensuring that the pipeline is set up correctly.