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 environment logs.
Writing application logs
When your application handles a request, it can write its own logging messages
stderr. Write your application logs using
stdout for output
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.
To view logs written by apps running in the flexible environment, use the Logs Viewer.
To filter log entries by label or text search in the Logs Viewer interface, see Basic Logs Filters. To write advanced logs filters using expressions specifying a set of log entries from any number of logs, 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.
Reading logs via API
There is no API for reading App Engine logs created in the flexible environment.