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.
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.
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 = '[email protected]' DOGSLOW_EMAIL_FROM = '[email protected]' # Also log to this logger (defaults to none): DOGSLOW_LOGGER = 'syslog_logger' # 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
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.
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.
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.
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.