Activity and message logs are an essential part of a web application. They are your view into what happens with your application over time as it is being used, who is using it and how, and what problems, if any, your users are having.
App Engine logs all incoming requests for your application, including application requests, static file requests, and requests for invalid URLs (so you can determine whether there is a bad link somewhere). For each request, App Engine logs the date and time, the IP address of the client, the URL requested (including the path and parameters), the domain name requested, the browser’s identification string (the “user agent”), the referring URL if the user followed a link, and the HTTP status code in the response returned by the app or by the frontend.
App Engine also logs several important statistics about each request: the amount of time it took to handle each request, the amount of “CPU time” that was spent handling the request, and the size of the response. The CPU time measurement is particularly important to watch because requests that consistently consume a large amount of CPU may be throttled, such that the CPU use is spread over more clock time.
Your application code can log the occurrence of notable events and data by using a logging API. Logging a message associates a line of text with the request that emitted it, including all the data logged for the request. Each message has a log level indicating the severity ...