node.to

NOTE: this is really only for CRITICAL logging you want to persist separately from google’s wrap of logging. You will burn up your quota otherwise. Main advantages: implements logging handler, maintains support for identities. Good for audit trails.

UPDATE: Appears the module level variable magic is not liked by appengine-patch. I switched to that from appengine helper because of hybrid auth convenience. I have updated to remove that part to prevent any module level variable wierdness. Updated code below. Alternative: just stick it inside your django tree.
— end UPDATE

I provide here a module that extends the Python logging framework to allow you to write messages to your Google appengine database. A database LogRecord model, Handler, Formatter, and logger manager are implemented.

( jump to the code )

Since you can’t store write access files on appengine, you have no easy way to separate your logging to different file handlers. The built in logging is nifty in the dev cycle. But a long term store is desireable, with the ability to set different log levels for different parts of your system.

So the goals I set out with were:

• Persist all the usual log record info, including file, line, module, stack, etc. as available
• Allow multiple logger identities for easy separation and grouping
• Don’t break anything about logging module’s expected behavior
• Provide a convience wrapper for zero-effort use

Now, in principle I’d generally avoid logging to a database, since log records are inherently denormalized, sequentially added, and typically read only after insert. They’re the perfect candidate for a flat file. But that is not an option here. Plus, I have confidence that the Appengine table structure should allow this to perform more or less like a flat file. Caveat: depending how you use this, you could hit your quotas substantially faster.

Quick note: this Handler provides its own formatter, which just shapes the data for the Appengine table. It doesn’t make sense to provide this a format string since it is not serializing the record. Similarly, I didn’t think it useful to trim off the lineno and funcname detail, since you can just select what you want from the table.

Otherwise, you use it just like any other logger from the logging module. When you actually write a message, a db entry is created for you. The default logger, which is created when getLogger() is called with no arguments. You can interact with the logger to set levels etc. via the object, as in
log = Log2DB.getLogger()
log.setLevel(logging.DEBUG).

Records created by this logger set the identity value as ‘Log2DB’. You can filter identities from the log table by setting identity = whatever you want.

For example:

import Log2DB
log = Log2DB.getLogger()
log.error('fun times')

import logging
log.logger.setLevel(logging.DEBUG)
log.debug('unfun detail')


All the above go to the default logger identity, “Log2DB”.
To create another logger to use, say for example you want one just for mailer status, you use the getLogger proxy function.

For example:

import Log2DB
maillogger = Log2DB.getLogger('MyMailLogger')
malllogger.error('oh noes')

try:
  raise HellException
except HellException, e:
  maillogger.exception('Error: %s stack to follow', e)


A record from the above is in the same table as the default log, but has identity = ‘MyMailLogger’. I thought about actually dynamically changing the log record model class name so you would get separate tables, but given the google architecture this has limited perf gain at cost of brittleness and complexity. I may add this anyway as an option.

Later I may paste a django view for managing the resulting records.

here is the code via pastie.org:

this replaces the old version at http://pastie.org/432156
moved _LogRecordHelper to inner class to fix an issue

This entry was posted on Thursday, February 11th, 2010 at 2:31 am and is filed under code or systems, migrated from old. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.