Registering Cleanup Code¶
This document describes how to go about registering callbacks to perform cleanup tasks at the end of a request and when an application process is being shutdown.
Cleanup At End Of Request¶
To perform a cleanup task at the end of a request a couple of different approaches can be used dependent on the requirements. The first approach entails wrapping the calling of a WSGI application within a Python ‘try’ block, with the cleanup code being triggered from the ‘finally’ block:
def _application(environ, start_response): status = '200 OK' output = b'Hello World!' response_headers = [('Content-type', 'text/plain'), ('Content-Length', str(len(output)))] start_response(status, response_headers) return [output] def application(environ, start_response): try: return _application(environ, start_response) finally: # Perform required cleanup task. ...
This might even be factored into a convenient WSGI middleware component:
class ExecuteOnCompletion1: def __init__(self, application, callback): self.__application = application self.__callback = callback def __call__(self, environ, start_response): try: return self.__application(environ, start_response) finally: self.__callback(environ)
The WSGI environment passed in the ‘environ’ argument to the application could even be supplied to the cleanup callback as shown in case it needed to look at any configuration information or information passed back in the environment from the application.
The application would then be replaced with an instance of this class initialised with a reference to the original application and a suitable cleanup function:
def cleanup(environ): # Perform required cleanup task. ... application = ExecuteOnCompletion1(_application, cleanup)
Using this approach, the cleanup function will actually be called prior to the response content being consumed by mod_wsgi and written back to the client. As such, it is probably only suitable where a complete response is returned as an array of strings. It would not be suitable where a generator is being returned as the cleanup would be called prior to any strings being consumed from the generator. This would be problematic where the cleanup task was to close or delete some resource from which the generator was obtaining the response content.
In order to have the cleanup task only executed after the complete response has been consumed, it would be necessary to wrap the result of the application within an instance of a purpose built generator like object. This object needs to yield each item from the response in turn, and when this object is cleaned up by virtue of the ‘close()’ method being called, it should in turn call ‘close()’ on the result returned from the application if necessary, and then call the supplied cleanup callback:
class Generator2: def __init__(self, iterable, callback, environ): self.__iterable = iterable self.__callback = callback self.__environ = environ def __iter__(self): for item in self.__iterable: yield item def close(self): try: if hasattr(self.__iterable, 'close'): self.__iterable.close() finally: self.__callback(self.__environ) class ExecuteOnCompletion2: def __init__(self, application, callback): self.__application = application self.__callback = callback def __call__(self, environ, start_response): try: result = self.__application(environ, start_response) except: self.__callback(environ) raise return Generator2(result, self.__callback, environ)
Note that for a successfully completed request, since the cleanup task will be executed after the complete response has been written back to the client, if an error occurs there will be no evidence of this in the response seen by the client. As far as the client will be concerned everything will look okay. The only indication of an error will be found in the Apache error log.
Both of the solutions above are not specific to mod_wsgi and should work with any WSGI hosting solution which complies with the WSGI specification.
Cleanup On Process Shutdown¶
To perform a cleanup task on shutdown of either an Apache child process when using ‘embedded’ mode of mod_wsgi, or of a daemon process when using ‘daemon’ mode of mod_wsgi, the standard Python ‘atexit’ module can be used:
import atexit def cleanup(): # Perform required cleanup task. ... atexit.register(cleanup)
Such a registered cleanup function will also be called if the ‘Interpreter’ reload mechanism is enabled and the Python sub interpreter in which the cleanup function was registered was destroyed.
Note that although mod_wsgi will ensure that cleanup functions registered using the ‘atexit’ module will be called correctly, this solution may not be portable to all WSGI hosting solutions.
Also be aware that although one can register a cleanup function to be called on process shutdown, this is no absolute guarantee that it will be called. This is because a process may crash, or it may be forcibly killed off by Apache if it takes too long to shutdown normally. As a result, an application should not be dependent on cleanup functions being called on process shutdown and an application must have some means of detecting an abnormal shutdown when it is started up and recover from it automatically.