Problems with logging while using Gevent’s WSGI Server

Dated: February 01, 2015

This article is about the logging problem with Gevent’s WSGI Server and how a simple hack can actually help you solve that problem and make sure you continue to use Gevent for serving WSGI applications (Flask in this article) without any compromises.

Flask comes really handy for writing internal services in Python because it is batteries excluded, there is not too much of opinionated design decisions that are pushed down your throat and that just makes it easy to do things your way. I started working on a project a while back, for which I chose to use Flask for precisely the same reasons. The project was an internal service that other web applications in the stack would use to do a certain type of task.

This service is heavy on network and disk I/O. It would query large amounts of data from another data store which would have data stored on the disk, and the data is transferred to this application over the network. The service would then do some processing of this data before it could send it in the response.

Since this application is mostly bound by I/O, every request would block the next request. With a WSGI server like Gunicorn, you can only get as far as the number of workers would take you. So I thought I can give a try to a non-blocking WSGI web server.

First consideration was Tornado. You can use Flask over Tornado. But the issue with that is that Tornado needs you to use it’s own networking API instead of the stdlib’s networking API. For example, if you use the PyMySQL client (which is the pure Python MySQL client) with Tornado, it’s no good as Tornado’s IOLoop will not be able to handle network requests unless you use Tornado’s networking API and all requests to MySQL will block the IOLoop. So any client that you are using with Tornado, has to make use of Tornado’s networking API or it cannot be a drop-in solution.

Next in the list was Gevent. The awesome thing about Gevent is that it allows you to monkey-patch Python stdlib’s modules that can be made non-blocking with its own versions of those modules. That way you don’t have to adopt anything new to make your applications non-blocking. Just keep using stdlib’s modules and then monkey patch them and you have a non-blocking version of your application ready. Its pretty much a drop-in replacement. Yes, things can be a little more different and difficult rather than how simply they are described above. So now was the time to try Flask over Gevent. Gevent provides a WSGIServer that can run any WSGI compliant app like Flask’s apps. This worked like a charm and the problem of handling multiple concurrent request was handled without much of an effort. Note: if we did used MySQLdb instead of PyMySQL with Gevent, we will still have the same problem as we had in case of PyMySQLdb + Tornado, because MySQLdb is written in C and is not Gevent friendly.

But as I said, things are not so easy. You only wish that they were. Everything worked fine with the server implementation, except one — capturing access logs. Gevent’s WSGIServer prints all access logs to stderr and provides no way to override this behaviour via its APIs. Since access logs are critical for debugging production issues and any other ad-hoc requirement that may arise in the future, you have to capture them no matter what. In this case, the only way you are left with is redirecting stderr to a file like so:

python app.py 2> /var/log/app.log

This works fine and you may actually use it if it works for you. However, while building new applications and servers, you must consider in what ways and conditions can your code get deployed. If you know that there can be changing situations and environments from deployment-to-deployment, then you have to make sure that the variables that change are configurable in some way or another — most standard being configuration files. So a nice way to deal with the situation would be to have the path for access logs file in your configuration file and let your application send all access logs to a file at that path, but how do you redirect logs to file whose path is in your configuration file? There can be many ways, like if you plan to run your server using init scripts, then have your init script parse the configuration file and start the server with redirecting stderr stream to the correct access log file according to your configuration file. But that seemed to much of unnecessary work.

Here is a solution that I propose. Since Gevent writes all the logs to stderr, what we need to do is somehow redirect everything that is written to stderr to another file. Since sys.stderr is a file object, we can monkey-patch sys.stderr with an object that overrides write method:

class RedirectStderr(object):

def __init__(self, cfg):
self._log_file = open(cfg['access_log_path'], 'a')

def write(self, msg):
self._log_file.write(msg)
self._log_file.flush()

import sys
sys.stderr = RedirectStderr(cfg)

This solution will work just fine and solve almost all our problems. The problem is that it gives rise to other problems. Since sys.stderr is nothing but a file object, we cannot be sure if Gevent is not using other methods on sys.stderr elsewhere in the code. And yes we can find it, but there is no point in doing all that. So a better solution would be subclassing file class and overriding all that we need to override.

class RedirectStderr(file):

def __init__(self, *args, **kwargs):
self._cfg = kwargs.pop('cfg')
self._log_file = open(self._cfg['access_log_path'], 'a')
super(RedirectStderr, self).__init__(*args, **kwargs)

def write(self, msg):
# Just in-case you want to write to both stderr and another log file
super(RedirectStderr, self).write(msg)

# Write to another access log file
self._log_file.write(msg)
self._log_file.flush()

# Or use Python's logging facility to log the standard way you log
# across your project
log.info(msg)

def close(self):
super(RedirectStderr, self).close()
self._log_file.close()

import sys
sys.stderr = RedirectStderr('/dev/stderr', 'w', cfg=cfg)

This implementation will override only sys.stderr.write leave everything intact so that any other use of sys.stderr does not break Gevent, and your application.

The RedirectStderr class is not meant to just work with stderr. If you notice, we open /dev/stderr at the time of instantiating the class. So you can use this hack to monkey-patch anything that follows the file interface in Python.

Hope this is a solution elegant enough for solving this problem. What do you think? Can there be an improvement to this? Or do you have a different better soltution? Would love to hear them in comments.


Originally published at vaidik.in.

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