Question

Can someone show me an example of logging client-side events in JavaScript to AppEngine’s server-side Python logging module? I’d like to log events of the same five severity levels, but from client-side code, not just server-side code as is described in the App Engine logging docs.

If there’s a built-in handler for logging, excellent. If not and I must write a server-side handler, I would love to see an example.

For those that are curious, I’m trying to build a UI entirely in the browser that only communicates with the server for data. It would be nice to log things that happen in the browser that wouldn’t otherwise trigger a call to the server.

Answer 1

The JavaScript isn’t too complicated. Here is a JavaScript module modeled after Python’s logging module. It uses jQuery to POST the log message to the server at URLs like /log/debug or /log/info.

// logging.js
var logging = {};

logging.log = function log(lvl, msg) {
  $.ajax({
    url: "/log/" + lvl,
    type: "POST",
    contentType: "text/plain",
    data: msg,
  });
};

logging.debug = function debug(msg) {
  logging.log("debug", msg);
};

logging.info = function info(msg) {
  logging.log("info", msg);
};

logging.warning = function warning(msg) {
  logging.log("warning", msg);
};

logging.error = function error(msg) {
  logging.log("error", msg);
};

logging.critical = function critical(msg) {
  logging.log("critical", msg);
};

return logging;

To use it, simply make calls like logging.info("log message here").

The server-side Python code to handle these requests isn’t complicated. Here is the URL mapping.

# main.py (some stuff omitted for clarity)
import logging
import wsgiref.handlers
from google.appengine.ext import webapp

urlmap = [(r"/log/(.+)", handlers.LoggingHandler),]
application = webapp.WSGIApplication(urlmap)
wsgiref.handlers.CGIHandler().run(application)

And here is the logging handler.

# handlers.py
import logging
from google.appengine.ext import webapp

class LoggingHandler(webapp.RequestHandler):
  def post(self, lvl):
    if lvl not in ("debug", "info", "warning", "error", "critical",):
      self.error(404)
      self.response.out.write("Unknown log level %s" % lvl)
      return
    msg = self.request.body
    getattr(logging, lvl)(msg)
    response = "Message logged: %s %s" % (lvl.upper(), msg))
    self.response.out.write(response)