Using Cloud Logging in App Engine Apps

Python SDK

You may find it useful to read the documentation for the standard Python logging module at Python.org.

The Python logging module allows a developer to log 5 levels of severity:

• Debug
• Info
• Warning
• Error
• Critical

The following example shows how to use some of these levels to log events in a simple Guestbook application:

import logging
import os
import datetime
import wsgiref.handlers

from google.appengine.api import users
from google.appengine.ext import db
from google.appengine.ext import webapp
from google.appengine.ext.webapp import template

class Post(db.Model):
    author = db.StringProperty()
    content = db.StringProperty()
    date = db.DateTimeProperty()https://docs.google.com/a/google.com/spreadsheet/ccc?hl=en_US&hl=en_US&key=tw1ZFtv5Lbbr5G7i5GnS47A#gid=72

class MainPage(webapp.RequestHandler):

  def get(self):
      # Retrieve existing posts from the datastore when getting the Main Page
      try:
          post_query = db.GqlQuery('SELECT * FROM Post ORDER BY date DESC')
          posts = [post for post in post_query]
      except:
          logging.error('There was an error retrieving posts from the datastore')

      template_values = {
          'posts': posts,
      }
      path = os.path.join(os.path.dirname(__file__), 'index.html')
      self.response.out.write(template.render(path, template_values))

class Guestbook(webapp.RequestHandler):
    # The user has posted a comment to the guest book
    def post(self):
        logging.debug('Start guestbook signing request')

        post = Post()

        if users.get_current_user():
            logging.info('Signed by user %s', users.get_current_user().nickname())
            post.author = users.get_current_user().user_id()
        else:
            logging.info('Signed by anonymous user')

        post.content = self.request.get('content')
        post.date = datetime.datetime.now()

        try:
            post.put()
        except:
            logging.error('There was an error saving comment %s', self.request.get('content'))

        logging.debug('Finish guestbook signing')
        self.redirect('/')

application = webapp.WSGIApplication([('/', MainPage),
                                      ('/sign', Guestbook)],
                                     debug=True)

def main():
    # Set the logging level in the main function
    # See the section on <a href="/appengine/docs/python/#Python_App_caching">Requests and App Caching</a> for information on how
    # App Engine reuses your request handlers when you specify a main function
    logging.getLogger().setLevel(logging.DEBUG)
    webapp.util.run_wsgi_app(application)

if __name__ == '__main__':
    main()

In our example, we call debug() to provide information about the requests coming into our handler. We call info() to provide information about the users of our application, and we call error() to tell us where our application is experiencing errors.

Later, we can use this information to filter out only the actions we are interested in. This will help us troubleshoot our code, improve the user experience, or add additional features to our application.

Java SDK

The App Engine Java SDK allows a developer to log 5 levels of severity:

• DEBUG
• INFO
• WARNING
• ERROR
• CRITICAL

In order to add logging to your Java app, you'll first need to add the appropriate system properties to your project's appengine-web.xml file, and you may also need to modify the logging.properties file to set the desired log level. Both of these files are created for you when you create a new App Engine Java project using Maven. You can find these files at the following locations:

Edit the appengine-web.xml file to add the following inside the <appengine-web-app> tags:

    <system-properties>
        <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
    </system-properties>

Notice that the default log level in logging.properties is WARNING, which suppresses INFO messages from the output. To change the log level for all classes in your app, edit the logging.properties file to change the level. For example change:

.level = WARNING

to:

.level = INFO

Inside your application code, log messages as desired using the java.util.logging.Logger API. When your app runs, App Engine records the messages and makes them available for browsing in the Admin Console, and available for downloading using the AppCfg tool. The Admin Console lets you browse messages by log level.