How to Set Up Logging in MongoDB: Syntax and Examples

To set up logging in MongoDB, configure the systemLog section in the mongod.conf file, specifying the destination, path, and logLevel. Then restart the MongoDB server to apply the changes and start logging as configured.

Syntax

The logging configuration in MongoDB is done in the mongod.conf file under the systemLog section. Key options include:

• destination: Where logs are sent, e.g., file or console.
• path: File path for log storage when destination is file.
• logAppend: Whether to append to existing log file (true) or overwrite (false).
• logLevel: Verbosity level from 0 (default) to 5 (most verbose).

yaml

systemLog:
  destination: file
  path: /var/log/mongodb/mongod.log
  logAppend: true
  logLevel: 1

Example

This example shows how to configure MongoDB to log to a file with verbose level 2 and append logs instead of overwriting.

yaml

systemLog:
  destination: file
  path: /var/log/mongodb/mongod.log
  logAppend: true
  logLevel: 2

Output

2024-06-01T12:00:00.000+0000 I CONTROL  [initandlisten] MongoDB starting : pid=12345 port=27017 dbpath=/data/db 64-bit host=server
2024-06-01T12:00:00.001+0000 I CONTROL  [initandlisten] options: { systemLog: { destination: "file", path: "/var/log/mongodb/mongod.log", logAppend: true, logLevel: 2 } }

Common Pitfalls

Common mistakes when setting up MongoDB logging include:

• Not restarting the mongod service after changing mongod.conf, so changes don't take effect.
• Setting destination to file but not specifying a valid path, causing logging to fail.
• Using too high logLevel in production, which can generate large log files and affect performance.
• Incorrect file permissions on the log file path, preventing MongoDB from writing logs.

yaml

Incorrect example:
systemLog:
  destination: file
  logLevel: 3

# Missing 'path' causes no logs to be saved to file.

Correct example:
systemLog:
  destination: file
  path: /var/log/mongodb/mongod.log
  logLevel: 3

Quick Reference

Option	Description	Example Value
destination	Where logs are sent	"file" or "console"
path	File path for logs (required if destination is file)	"/var/log/mongodb/mongod.log"
logAppend	Append to existing log file if true	true
logLevel	Verbosity level (0-5)	1
component	Set log level per component (optional advanced)	systemLog.component.accessControl=2

Key Takeaways

Configure logging in the mongod.conf file under the systemLog section.

Always specify a valid log file path when logging to a file.

Restart the mongod service after changing logging settings.

Use appropriate logLevel to balance detail and performance.

Check file permissions to ensure MongoDB can write logs.