I think we should add logging to Django in version 1.2, implemented as
a light-weight wrapper around the Python logging module
(django.core.log maybe?) plus code to write errors to the Apache error
log under the mod_python handler and environ['wsgi.errors'] under WSGI
(meaning mod_wsgi will write to the Apache error log as well).
Benefits of logging as a core Django service
============================================
Adding logging to Django core would provide the following benefits:
1. We'll be able to de-emphasise the current default "e-mail all
errors to someone" behaviour, which doesn't scale at all well.
2. Having logging in the core framework will mean people start
actually using it, which will make it easier for people to debug their
Django apps. Right now adding "print" statements to a Django app is a
common debugging technique, but it's messy (you have to remember to
take them out again) and error prone - some production environments
throw errors if an app attempts to write to stdout. It's also not
obvious - many developers are surprised when I show them the
technique.
3. Logging in Django core rather than a 3rd party app will encourage
reusable applications to log things in a predictable way, standard
way.
4. 3rd party debugging tools such as the debug toolbar will be able to
hook in to Django's default logging behaviour. This could also lead to
plenty of additional 3rd party innovation - imagine a tool that looks
out for logged SQL that took longer than X seconds, or one that groups
together similar log messages, or streams log messages to IRC...
5. Built-in support for logging reflects a growing reality of modern
Web development: more and more sites have interfaces with external web
service APIs, meaning there are plenty of things that could go wrong
that are outside the control of the developer. Failing gracefully and
logging what happened is the best way to deal with 3rd party problems
- much better than throwing a 500 and leaving no record of what went
wrong.
6. Most importantly from my point of view, when a sysadmin asks where
Django logs errors in production we'll have a good answer for them!
7. As a general rule, I believe you can never have too much
information about what's going on with your web application. I've
never thought to myself "the problem with this bug is I've got too
much information about it". As for large log files, disk space is
cheap - and pluggable backends could ensure logs were sensibly
rotated.
Places logging would be useful
==============================
- Unhandled exceptions that make it up to the top of the Django stack
(and would cause a 500 error to be returned in production)
- The development web server could use logging for showing processed
requests (where currently these are just printed to stdout).
- Failed attempts at signing in to the admin could be logged, making
security audits easier.
- We could replace (or complement) django.connection.queries with a
log of executed SQL. This would make the answer to the common question
"how do I see what SQL is being executed" much more obvious.
- Stuff that loads things from INSTALLED_APPS could log what is being
loaded, making it much easier to spot and debug errors caused by code
being incorrectly loaded.
- Likewise, the template engine could log which templates are being
loaded from where, making it easier to debug problems stemming from an
incorrectly configured TEMPLATE_DIRS setting.
- We could use logging to address the problems with the template
engine failing silently - maybe some template errors (the ones more
likely to be accidental than just people relying on the fail-silent
behaviour deliberately) should be logged as warnings.
Most of the above would be set to a low log level which by default
would not be handled, displayed or stored anywhere (logging.info or
similar). Maybe "./manage.py runserver --loglevel=info" could cause
such logs to be printed to the terminal while the development server
is running.
Problems and challenges
=======================
1. The Python logging module isn't very nicely designed - its Java
heritage shines through, and things like logging.basicConfig behave in
unintuitive ways (if you call basicConfig twice the second call fails
silently but has no effect). This is why I suggest wrapping it in our
own higher level interface.
2. There may be some performance overhead, especially if we replace
mechanisms like django.connection.queries with logging. This should be
negligble: here's a simple benchmark:
# ("hello " * 100) gives a 600 char string, long enough for a SQL
statement
>>> import timeit, logging
>>> t = timeit.Timer('logging.info("hello " * 100)', 'import logging')
>>> t.timeit(number=100) # one hundred statements
0.00061702728271484375
>>> t.timeit(number=1000000) # one million statements
6.458014965057373
That's 0.0006 of a second overhead for a page logging 100 SQL
statements. The performance overhead will go up if you attach a
handler, but that's fine - the whole point of a framework like
'logging' is that you can log as much as you like but only act on
messages above a certain logging level.
3. We risk people using logging where signals would be more
appropriate.
4. We might go too far, and make Django a "noisy" piece of software
which logs almost everything that happens within it. Let's be tasteful
about this.
5. People might leave logging on, then find their server disk has
filled up with log files and caused their site to crash.
6. Logging levels are confusing - what exactly is the difference
between warn, info, error, debug, critical and fatal? We would need to
document this and make decisions on which ones get used for what
within the framework.
What would it look like?
========================
Here's what I'm thinking at the moment (having given the API very
little thought). In your application code:
from django.core import log
# Log to the default channel:
log.debug('Retrieving RSS feed from %s' % url)
# Log to a channel specific to your app:
log.debug('Retrieving RSS feed from %s' % url, channel='myapp.rss')
try:
feed = httpfetch(url, timeout=3)
except socket.timeout:
log.info('Timeout fetching feed %s' % url)
In settings.py:
MIDDLEWARE_CLASSES = (
...
'django.middleware.LogErrorsToWSGI', # write exceptions to
wsgi.errors
)
LOGGING_MIDDLEWARE_LEVEL = 'info'
# If you want custom log handlers - not sure how these would interact
with
# channels and log levels yet
LOG_HANDLERS = (
'django.core.log.handlers.LogToDatabase',
'django.core.log.handlers.LogToEmail',
)
What do people think? I'd be happy to flesh this out in to a full spec
with running code.
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