Writing Application Logs

When a request is sent to your application running in App Engine, request and response details are logged automatically, and can be viewed in the Google Cloud Platform Console Stackdriver Logs Viewer.

The request and application logs for your app are collected by a Stackdriver Logging agent. See Quota Policy for the logs retention policy and for the maximum size of log entries. If you want to store your logs for a longer period, you can export your logs to Cloud Storage. You can also export your logs to BigQuery and Pub/Sub for further processing.

For a list of available logs you can use, see App Engine flexible logs.

Writing application logs

When your application handles a request, it can write its own logging messages to stdout and stderr. Write your application logs using stdout for output and stderr for errors. These files are automatically collected and can be viewed in the Logs Viewer. Note that this does not provide log levels that you can use for filtering in the Logs Viewer; however, the Logs Viewer does provide other filtering, such as text, timestamp, etc. Only the most recent entries in the Logs Viewer are retained in order to limit their size.

You can write log entries using the psrLogger method, which fetches a PSR-3 compliant logger:

$logging = new LoggingClient([
    'projectId' => $projectId
]);
$logger = $logging->psrLogger('app');
$logger->notice($text);

Viewing application logs

You can view your application logs using the Logs Viewer:

  1. Go to the Stackdriver > Logging page in the GCP Console:

    Go to the Logs Viewer page

  2. Select an existing GCP project at the top of the page.

  3. From the drop-down menus, select GAE Application as your resource type to see your App Engine logs.

You can use the drop-down menus and other components of the Logs Viewer to filter or search the logs for relevant entries. See Logs Viewer filter interfaces for more details.

Filtering application logs

To filter log entries by label or text search in the Logs Viewer, see Basic Logs Filters.

To write advanced logs filters to further specify a set of log entries, see Advanced Logs Filters.

Tip: To view the logs for specific instances, services, or versions, filter by that resource's ID in the Logs Viewer. On the App Engine dashboard in the GCP Console, service and version IDs are listed in the drop-down menus at the top. Your instance IDs are listed in the GCP Console Instances page.

Go to the App Engine dashboard

In the Logs Viewer, log entries correlated by the same trace can be viewed in a "parent-child" format.

Following are instructions for correlating log entries:

  1. Decide which app logs you want to correlate with your request logs. Note that, for this correlation to work, you must choose app logs with a type that is not the same as the request log type.

  2. In your application code, look for the X-Cloud-Trace-Context HTTP header of incoming requests. Extract the trace identifier from the header. For details, see Stackdriver Trace support.

  3. Set the trace identifier in the LogEntry trace field of your app log entries. The expected format is projects/[PROJECT_ID]/traces/[TRACE_ID].

Once you have correlated the log entries, following are instructions for viewing correlated log entries:

  1. Open your App Engine logs in the Logs Viewer; for details, see Viewing application logs.

  2. Expand a request log by clicking the small black triangle to the left of the log entry. You see your request and app logs displayed in a nested format.

    Note that there is no inherent parent-child relationship between logs. For example, if the correlated logs have the log types nginx.request and app, then in the Logs Viewer, if you open an nginx.request log with many correlated app logs, the app logs display underneath. If you open an app log that correlates to one nginx.request log, the nginx.request log displays underneath that app log.

Additionally:

  • The highest severity from the "child" log entries does not automatically apply to the top-level entry. If that behavior is desired, manually set the highest severity in the top-level entry.
  • Set the httpRequest.requestUrl field for the top-level log.

Reading logs via API

There is no API for reading App Engine logs created in the flexible environment.

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