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======================================= Dogslow -- Django Slow Request Watchdog ======================================= Overview -------- Dogslow is a Django watchdog middleware class that logs tracebacks of slow requests. It started as an `internal project inside Bitbucket`_ to help trace operational problems. .. _internal project inside Bitbucket: http://blog.bitbucket.org/2011/05/17/tracking-slow-requests-with-dogslow/ Installation ------------ Install dogslow:: $ pip install dogslow Then add ``dogslow.WatchdogMiddleware`` to your list of middleware classes in your Django settings.py file:: MIDDLEWARE_CLASSES = ( 'dogslow.WatchdogMiddleware', ... ) For best results, make it one of the first middlewares that is run. Configuration ------------- You can use the following configuration properties in your ``settings.py`` file to tune the watchdog:: # Watchdog is enabled by default, to temporarily disable, set to False: DOGSLOW = True # Location where Watchdog stores its log files: DOGSLOW_OUTPUT = '/tmp' # Log requests taking longer than 25 seconds: DOGSLOW_TIMER = 25 # When both specified, emails backtraces: DOGSLOW_EMAIL_TO = 'errors@atlassian.com' DOGSLOW_EMAIL_FROM = 'no-reply@atlassian.com' # Also log to this logger (defaults to none): DOGSLOW_LOGGER = 'syslog_logger' DOGSLOW_LOG_LEVEL = 'WARNING' # Tuple of url pattern names that should not be monitored: # (defaults to none -- everything monitored) DOGSLOW_IGNORE_URLS = ('some_view', 'other_view') # Print (potentially huge!) local stack variables (on by default, use # False for less detailed, but more manageable reports) DOGSLOW_STACK_VARS = True Usage ----- Every incoming HTTP request gets a 25 second timeout in the watchdog. If a request does not return within that time, the watchdog activates and takes a peek at the request thread's stack and writes the backtrace (including all local stack variables -- Django style) to a log file. Each slow request is logged in a separate file that looks like this:: Undead request intercepted at: 16-05-2011 02:10:12 UTC GET http://localhost:8000/?delay=2 Thread ID: 140539485042432 Process ID: 18010 Started: 16-05-2011 02:10:10 UTC File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/management/commands/runserver.py", line 107, in inner_run run(self.addr, int(self.port), handler, ipv6=self.use_ipv6) File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/servers/basehttp.py", line 696, in run httpd.serve_forever() File "/usr/lib/python2.7/SocketServer.py", line 227, in serve_forever self._handle_request_noblock() File "/usr/lib/python2.7/SocketServer.py", line 284, in _handle_request_noblock self.process_request(request, client_address) File "/usr/lib/python2.7/SocketServer.py", line 310, in process_request self.finish_request(request, client_address) File "/usr/lib/python2.7/SocketServer.py", line 323, in finish_request self.RequestHandlerClass(request, client_address, self) File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/servers/basehttp.py", line 570, in __init__ BaseHTTPRequestHandler.__init__(self, *args, **kwargs) File "/usr/lib/python2.7/SocketServer.py", line 639, in __init__ self.handle() File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/servers/basehttp.py", line 615, in handle handler.run(self.server.get_app()) File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/servers/basehttp.py", line 283, in run self.result = application(self.environ, self.start_response) File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 68, in __call__ return self.application(environ, start_response) File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 273, in __call__ response = self.get_response(request) File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response response = callback(request, *callback_args, **callback_kwargs) File "/home/erik/work/middleware/middleware/sleep/views.py", line 6, in sleep time.sleep(float(request.GET.get('delay', 1))) Full backtrace with local variables: File "/home/erik/work/virtualenv/bit/lib/python2.7/site-packages/django/core/management/commands/runserver.py", line 107, in inner_run run(self.addr, int(self.port), handler, ipv6=self.use_ipv6) ...loads more... The example above shows that the request thread was blocked in ``time.sleep()`` at the time ``dogslow`` took its snapshot. Requests that return before ``dogslow``'s timeout expires do not get logged. Note that ``dogslow`` only takes a peek at the thread's stack. It does not interrupt the request, or influence it in any other way. Using ``dogslow`` is therefore safe to use in production. Sentry Integration ------------------ Dogslow natively integrates with Sentry. You can set it up by configuring Dogslow to use ``DOGSLOW_LOGGER`` and ``DOGSLOW_LOG_TO_SENTRY`` and by `configuring Raven`_ to collect Dogslow's reports. :: DOGSLOW_LOGGER = 'dogslow' # can be anything, but must match `logger` below DOGSLOW_LOG_TO_SENTRY = True DOGSLOW_LOG_LEVEL = 'WARNING' # optional, defaults to 'WARNING' # Add a new sentry handler to handle WARNINGs. It's not recommended to # modify the existing sentry handler, as you'll probably start seeing # other warnings unnecessarily sent to Sentry. LOGGING = { ... 'handlers': { ... 'dogslow': { 'level': 'WARNING', 'class': 'raven.contrib.django.handlers.SentryHandler', } ... } 'loggers': { ... 'dogslow': { 'level': 'WARNING', 'handlers': ['dogslow'], # or whatever you named your handler } ... } ... } .. _configuring Raven: http://raven.readthedocs.org/en/latest/config/django.html#integration-with-logging Caveats ------- Dogslow uses multithreading. It has a single background thread that handles the watchdog timeouts and takes the tracebacks, so that the original request threads are not interrupted. This has some consequences. Multithreading and the GIL ~~~~~~~~~~~~~~~~~~~~~~~~~~ In cPython, the GIL (Global Interpreter Lock) prevents multiple threads from executing Python code simultaneously. Only when a thread explicitly releases its lock on the GIL, can a second thread run. Releasing the GIL is done automatically whenever a Python program makes blocking calls outside of the interpreter, for example when doing IO. For ``dogslow`` this means that it can only reliably intercept requests that are slow because they are doing IO, calling sleep or busy waiting to acquire locks themselves. In most cases this is fine. An important cause of slow Django requests is an expensive database query. Since this is IO, ``dogslow`` can intercept those fine. A scenario where cPython's GIL is problematic is when the request's thread hits an infinite loop in Python code (or legitimate Python that is extremely expensive and takes a long time to execute), never releasing the GIL. Even though ``dogslow``'s watchdog timer does become runnable, it cannot log the stack. Co-routines and Greenlets ~~~~~~~~~~~~~~~~~~~~~~~~~ ``Dogslow`` is intended for use in a synchronous worker configuration. A webserver that uses dedicated threads (or single-threaded, dedicated worker processes) to serve requests. Django's built-in wsgi server does this, as does ``Gunicorn`` in its default sync-worker mode. When running with a "co-routines framework" where multiple requests are served concurrently by one thread, backtraces might become nonsensical.