Understanding Tagged vs. Structured Logging

What's Tagged Logging?

Tagged logging allows you to prepend tags to your log messages. For instance, using ActiveSupport::TaggedLogging, you can tag logs with "Auth API" or user IDs.

class Permissions
  def get(user)
    logger.tagged("Auth API") do
      uri = URI("https://auth.example.com/permissions/?user=#{user}")

      logger.info { "Fetching #{uri}" }
      permissions = Permissions.parse(Net::HTTP.get(uri))
      logger.info { "Got permissions #{permissions.to_s}" }
    end
  end
end

# Log Output
#[Auth API] Fetching https://auth.example.com/permissions/?user=bob
#[Auth API] Got permissions admin, writer, reader

Pros:

• Enhances log readability.
• Avoids repetition by tagging once for multiple log lines.

Cons:

• Not machine-friendly. While the text version looks good, the JSON format can be confusing, especially when searching for specific tags.

Structured Logging: A Better Alternative?

Structured logging provides a more organized way to log messages. Instead of tagging, you structure your logs with key-value pairs.

Example:

log.info "hello", user: "1", team: "The best team"

Output:

Text Log:
INFO hello user=1, team=The Best Team

JSON Log:
{severity: INFO, message: "hello", user: 1, team: "The Best Team"}

This approach is more machine-readable and fits better with modern log aggregators.

Libraries Supporting Structured Logging:

• logcraft
• ougai
• semantic_logger

Note: Lograge offers structured logs for Rails requests but lacks custom structuring.

Why I Wrote This

I'm building a dynamic log library and wanted to understand the best approaches to structured and tagged logging.

In Summary:

• Tagged logging is user-friendly but not machine-friendly.
• Structured logging offers a more organized and machine-readable approach.
• Tagged logging does have nice nesting behavior, though you can achieve that with the semantic logger gem as well.