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Migrating to Cloud Logging

With just a few lines of code, you can update your app to use Cloud Logging and achieve nearly the same filtering and log correlation features that were available with the App Engine logging service.

To migrate to Cloud Logging, attach the AppEngineHandler log handler to the Python logging module, then continue using the Python logging module to write log entries. You can make these updates in your Python 2 app and see the same logging behavior when you app runs in either the Python 2 or Python 3 runtimes.

Key differences between App Engine logging and Cloud Logging

  • App Engine logging requires you use the Python logging module to write app log entries. Cloud Logging supports all of the following approaches:

    • (Recommended) Use the Python logging module with AppEngineHandler as the logging handler. This idiomatic approach only requires a few additional lines of code to register the Cloud Logging handler, and therefore makes your app more portable to other Python environments.

    • Write entries to stdout or stderr. These entries appear in the Logs Explorer, but to enable filtering and correlation with request logs you need to format the entries as a JSON object and provide specific metadata. For more information about this approach, see Structured logging.

    • Use the Cloud Logging client library directly. If you use the AppEngineHandler, this approach achieves the same result as using the Python logging module, but changing from this library to a different logging framework could require significant updates to your code.

  • With App Engine logging, the Logs Explorer assigns a severity level to request logs, and the severity level reflects the highest severity of any app log entry that is correlated with the request. For example, if a request results in your app emitting a warning log entry, the Logs Explorer will display a warning icon next to the request log entry. When you expand the request entry, you see the warning log entry nested within the request entry.

    With Cloud Logging, the Log Viewer does not assign a severity level to request logs. When you expand a request entry, you'll see app entries nested within the request entry, and those app entries will show a severity icon. But the request entry itself does not show a severity icon and cannot be filtered by severity level.

  • Quotas and data retention are the same for both App Engine and Cloud Logging, but if you exceed the free quota of logs data, Cloud Logging pricing may be different from App Engine logs pricing.

Before you start migrating

  1. Enable the Cloud Logging API in the project that contains your app.

    Enable the API

  2. Make sure your app has permission to write logs.

    By default, your app's default service account has permission to write logs.

    If your app uses a different service account or user account, or if you have changed permissions for the default service account, make sure the account you use has permission to write logs.

Overview of the migration process

To migrate your Python app to use Cloud Logging instead of App Engine logging:

  1. Install the Cloud Client Library for Cloud Logging.

  2. Attach AppEngineHandler to the Python logging module.

  3. Deploy your app to App Engine and confirm logging works as expected.

Installing the Cloud Client Library for Cloud Logging

To make AppEngineHandler available to your app when it runs in App Engine:

  1. Create a requirements.txt file in the same folder as your app.yaml file and add the following lines:


    We recommend you use the 1.14.0 version of the Cloud Logging client library since it supports Python 2.7 apps.

  2. In your app's app.yaml file, specify the required Google RPC and setuptools libraries, and the optional SSL library in the libraries section:

    - name: grpcio
      version: 1.0.0
    - name: setuptools
      version: 36.6.0
    - name: ssl
      version: latest

    Some client libraries don't need the SSL library. If you don't include the SSL library for a client library that needs it, you'll see an SSL error in the Logs Explorer when your app receives a request.

  3. Create a directory to store your third-party libraries, such as lib/. Then use pip install to install the libraries into the directory. For example:

      pip install -t lib -r requirements.txt

  4. Create an file in the same folder as your app.yaml file. Add the following to your file:

    import pkg_resources
    from google.appengine.ext import vendor
    # Set path to your libraries folder.
    path = 'lib'
    # Add libraries installed in the path folder.
    # Add libraries to pkg_resources working set to find the distribution.

    The file in the preceding example assumes that the the lib folder is located in the current working directory. If you can't guarantee that lib will always be in the current working directory, specify the full path to the lib folder. For example:

    import os
    path = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'lib')

For local development, we recommend that you install dependencies in a virtual environment such as virtualenv for Python 2.

Attaching AppEngineHandler to the Python logging module

In each file that writes log entries:

  1. Import the Cloud Logging client library.
  2. Instantiate the Cloud Logging client.
  3. Retrieve the Cloud Logging handler that is appropriate for the current environment. When your app runs in App Engine, the Cloud Logging client retrieves AppEngineHandler.

    AppEngineHandler will format the entries as a JSON object and provide the metadata needed to support filtering by severity and correlating your app's entries with request log entries.

  4. Run the Cloud Logging client's setup_logging() method, which attaches its default listener as the logging handler for the Python root logger.

For example:

# Imports the Cloud Logging client library

# Instantiates a client
client =

# Retrieves a Cloud Logging handler based on the environment
# you're running in and integrates the handler with the
# Python logging module. By default this captures all logs
# at INFO level and higher

Once the handler is attached, any logs at, by default, INFO level or higher which are emitted in your application will be sent to Logging:

# Imports Python standard library logging
import logging

# The data to log
text = "Hello, world!"

# Emits the data using the standard logging module

Deploying your app

Once you are ready to deploy your app, you should:

  1. Test the app on App Engine.

    The only way to test your updates and confirm that severity levels are assigned and app entries are correlated with request entries is to deploy your app to the Python 3 runtime.

    If you deploy your updates to the Python 2 runtime, the App Engine logging service will listen for entries from the Python root logger and format them appropriately for the Logs Explorer.

    If you run your app locally, Cloud Logging does not use AppEngineHandler, and App Engine cannot write request entries.

  2. View the Logs Explorer and confirm that severity levels are assigned to your app's entries. Expand a request entry and confirm that entries written by your app while it was processing the request appear nested below the request entry.

  3. If the app runs without errors, use traffic splitting to slowly ramp up traffic for your updated app. Monitor the app closely for any issues before routing more traffic to the updated app.