How to Debug the Logstash File Plugin

The Logstash file plugin is commonly used to ingest log files, but issues can arise with log reading, file monitoring, or sincedb persistence. Here’s a quick guide to debugging the file plugin effectively.

1. Enable Debug Logging

Logstash provides detailed logs in debug mode, which can help diagnose problems. Start Logstash with debug logging by running:

bin/logstash --log.level debug

This will output verbose logs, showing plugin activity and helping identify issues with the file input plugin.

2. Check File Path and Permissions

Make sure the file path is correct and that Logstash has permission to read the file. Incorrect paths or permissions are common causes of ingestion failures.

• Verify the path in the plugin configuration:

   

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  path => "/path/to/your/log/file.log"
  

• Ensure the Logstash user has read access:

   

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  ls -l /path/to/your/log/file.log
  

3. Monitor Sincedb File

The file plugin tracks read positions using a .sincedb file. If you notice Logstash not processing new entries, the sincedb file may be outdated or corrupted. For testing, reset the sincedb file by setting:

sincedb_path => "/dev/null"

Or delete the file manually to force Logstash to reread the file from the beginning.

4. Check start_position Setting

The start_position option controls where Logstash begins reading the file. If set incorrectly, it may ignore existing data. For new files, use:

start_position => "beginning"

Note: This only applies to files that Logstash has not processed before.

5. Watch Logstash Logs for Errors

Look for messages related to file rotation, read permissions, or plugin warnings. Errors like “file is already closed” or “failed to open file” indicate issues that may require adjusting your configuration or addressing filesystem access.

By following these steps, you should be able to pinpoint issues and ensure the file plugin is correctly ingesting logs in your Logstash pipeline.

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