Django access site name/url in template - views, contexts and settings - django

(It feels like "best practices" type of question, sorry if these don't belong here)
I'm building a website which has user registration and password reset feature, both of which would send email to the user with a link to click to complete the respective process. While testing on a staging box, I used a dirty workaround in order finish the Proof Of Concept - hardcoded the website address in the template ) Needless to say, I want to fix this sooner rather than later. Searching around, found references to django.contrib.sites, which looks like a possible solution, though one thing bothers me - technically there's but one site, but with two stages, staging and production. I don't want to swim against the current, potentially abusing "sites", where another solution might apply.
I've then considered another approach - add respective URLs to corresponding settings.py files, of which I already have a grasp - one for local, one for staging, and one for prod. The issue here is probably obvious - I don't think it's proper (or possible) to read the settings.py from templates, and trying to keep the solution as simple as possible, I'm using a few standard views, which means I don't have access to the code thereof, to add values to the context. Perhaps I am supposed to override these standard views? Just call 'super' and then add my custom data to the context? But I don't know if this is a right approach tbh.
The third option seems to be adding a custom context processor to achieve the same goal, i.e. making settings values available to the templates that are interested therein. Sorry about a long question of "design" variety.

ok, so it was a wrong path after all - the reason why {{ domain }} in my template didn't work (resolved to 'localhost') was a missing line in nginx config:
location / {
proxy_pass http://unix:/run/gunicorn.sock;
proxy_set_header Host $http_host;
}
(the proxy_set_header line, to be exact)
can't claim I understand 100% what's happening here, but at least now I can move forward; the biggest question is where this {{ domain }} directive comes from - can't tell which of the several context processors copied from a tutorial put it in:
"django.template.context_processors.debug",
"django.template.context_processors.request",
"django.contrib.auth.context_processors.auth",
"django.contrib.messages.context_processors.messages",

Related

Generate a complete URL within a Yesod application

I mean that for example
YC.getUrlRender >>= enact where
enact renderURL = renderURL HomeR
will result in "/", for example, and usually that's what you need. However, I'm trying to crack email verification where the email has to contain the https://yesod.domain.com bit as well so that the user can click on the link to verify the new account. Is it in fact possible to obtain that bit ? In my deployment for example, the Yesod application is behind nginx, with the configuration:
location / {
proxy_pass http://127.0.0.1:3000;
}
so would the Yesod app even know that it was reached via https://myvps.hosting.com ?
Maybe the answer is to pass the information in via nginx, thus:
location / {
proxy_pass http://127.0.0.1:3000?youEnteredVia=https://myvps.hosting.com;
}
although I'd want to use $1 or something to avoid that hard-coded URL, and I don't know how to pick up the value within the Yesod application but I'm sure MS's book has the information somewhere.
This is the entire purpose around having the approot class method, since it's not always possible to determine the correct application root based on the request itself. You may also want to look at ApprootMiddleware.

Running multiple sites on the same python process

In our company we make news portals for a pretty big number of local newspapers (currently 13, going to 30 next month and more in the future), each with 2k to 100k page views/day. Since we are evolving from a situation where each site was heavily customized to one where each difference is a matter of configuration or custom template, our software is already pretty much the same for all sites. Right now our deployment strategy is one gunicorn instance for each site (with 1-17 workers each, depending on the site traffic), on a 16-core server and 12GB RAM. The problem with this setup is that each worker (regular pre-forked gunicorn) takes 110MB, whether its being used or not. Now with the new sites we would need to add more RAM to serve not that much many requests, so basically it doesn't scale. Also, since we are moving from this model where each site is independent, each site has its own database and I quite like it that way, especially since we are using relational databases (mysql, but migrating to pgsql), so its much easier to shard this way.
I'm doing some research and experimenting with running all sites on one gunicorn instance, so I could use the servers fully and add more servers behind a load balancer when it came to it. The problem is that django assumes in a lot of places that only one site is running per process, so for what I've thought of so far I'd have to implement:
A middleware that takes the HTTP_HOST from the request and places an identifier on a threadlocal variable.
A template loader that uses that variable to load custom templates accordingly.
Monkey patch django.db.model.Model, probably adding a metaclass (not even sure that's possible, but I think I would need it because of the custom managers we sometimes need to use) that would overwrite the managers for one that would first call db_manager(identifier) on the original manager and then call the intended method. I would also need to overwrite the save and delete methods to always include the using=identifier parameter.
I guess I would need to stop using inclusion_tag decorators, not a big problem, but I need to think of other cases like this.
Heavy and ugly patching of urlresolvers if I need custom or extra urls for each site. I don't need them now, but probably will at some point.
And this is just is what I came up with without even implementing it and seeing where it breaks, I'm sure I'd need many more changes for it to work. So I really don't want to do it, especially with the extra maintenance effort I'll need, but I don't see any alternatives and would love to learn that someone already solved this in a better way. Of course I could also stop using django altogether (I already have many reasons to do so) but that would mean a major rewrite and having two maintain two incompatible branches of the software until the new one reached feature parity with the django version, so to me it seems even worse than all the ugly hacks.
I've recently developed an e-commerce system with similar requirements -- many instances running from the same project sharing almost everything. The previous version of the system was a bunch of independent installations (~30) so it was pretty unmaintainable. I'm sure the requirements still differ from yours (for example, all instances shared the same models in my case), but it still might be useful to share my experience.
You are right that Django doesn't help with scenarios like this out of the box, but it's actually surprisingly easy to work it around. Here is a brief description of what I did.
I could see a synergy between what I wanted to achieve and django.contrib.sites. Also because many third-party Django apps out there know how to work with it and use it, for example, to generate absolute URLs to the current site. The major problem with sites is that it wants you to specify the current site id in settings.SITE_ID, which a very naive approach to the multi host problem. What one naturally wants, and what you also mention, is to determine the current site from the Host request header. To fix this problem, I borrowed the hook idea from django-multisite: https://github.com/shestera/django-multisite/blob/master/multisite/threadlocals.py#L19
Next I created an app encapsulating all the functionality related to the multi host aspect of my project. In my case the app was called stores and among other things it featured two important classes: stores.middleware.StoreMiddleware and stores.models.Store.
The model class is a subclass of django.contrib.sites.models.Site. The good thing about subclassing Site is that you can pass a Store to any function where a Site is expected. So you are effectively still just using the old, well documented and tested sites framework. To the Store class I added all the fields needed to configure all the different stores. So it's got fields like urlconf, theme, robots_txt and whatnot.
The middleware class' function was to match the Host header with the corresponding Store instance in the database. Once the matching Store was retrieved, It would patch the SITE_ID in a way similar to https://github.com/shestera/django-multisite/blob/master/multisite/middleware.py. Also, it looked at the store's urlconf and if it was not None, it would set request.urlconf to apply its special URL requirements. After that, the current Store instance was stored in request.store. This has proven to be incredibly useful, because I was able to do things like this in my views:
def homepage(request):
featured = Product.objects.filter(featured=True, store=request.store)
...
request.store became a natural additional dimension of the request object throughout the project for me.
Another thing that was defined on the Store class was a function get_absolute_url whose implementation looked roughly like this:
def get_absolute_url(self, to='/'):
"""
Return an absolute url to this `Store` or to `to` on this store.
The URL includes http:// and the domain name of the store.
`to` can be an object with `get_absolute_url()` or an absolute path as string.
"""
if isinstance(to, basestring):
path = to
elif hasattr(to, 'get_absolute_url'):
path = to.get_absolute_url()
else:
raise ValueError(
'Invalid argument (need a string or an object with get_absolute_url): %s' % to
)
url = 'http://%s%s%s' % (
self.domain,
# This setting allowed for a sane development environment
# where I just set it to ".dev:8000" and configured `dnsmasq`.
# The same value was also removed from the `Host` value in the middleware
# before looking up the `Store` in database.
settings.DOMAIN_SUFFIX,
path
)
return url
So I could easily generate URLs to objects on other than the current store, e.g.:
# Redirect to `product` on `store`.
redirect(store.get_absolute_url(product))
This was basically all I needed to be able to implement a system allowing users to create a new e-shop living on its own domain via the Django admin.

Are Django template tags cached?

I have gone through the (painful) process of writing a custom template tag for use in Django. It is registered as an inclusion_tag so that it renders a template. However, this tag breaks as soon as I try to change something.
I've tried changing the number of parameters and correspondingly changing the parameters when it's called. It's clear the new tag code isn't being loaded, because an error is thrown stating that there is a mismatch in the number of parameters, and it's evident that it's attempting to call the old function.
The same problem occurs if I try to change the name of the template being rendered and correspondingly change the name of the template on disk. It continues to try to call the old template. I've tried clearing old .pyc files with no luck.
Overall, the system is acting as though it's caching the template tags, likely due to the register command. I have dug through endless threads trying to find out if this is so, but all could find it James Bennett stating here that register doesn't do anything. Please help!
I have gone through the (painful) process of writing a custom template tag for use in Django
I agree that the process for writing the template tag in django is more elaborate than it needs to be.
But let me point you towards some of the third party apps, that when installed, a template tag is just another python function (or class).
http://github.com/alex/django-templatetag-sugar
http://github.com/codysoyland/django-template-repl
Firstly, I can't imagine what's complicated about inclusion tags. Writing a completely custom tag from scratch, yes: that's complicated. But inclusion tags are simply three lines of code and a template - what's complicated about that?
Secondly, all Python code in your Django project is loaded once by the server [*], and remains until it is restarted. The dev server usually detects changes and restarts itself to reload the code, but this doesn't always work. You should get into the habit of checking the console to see that it does restart, and doing it manually if necessary. Note that this has nothing whatsoever to do with caching.
[*] strictly speaking, once per process, but the dev server is single-process anyway.
In case anybody else encounters this: the exact caching mechanism is unclear, but restarting the django dev server fixes the problem.

Invalidating a path from the Django cache recursively

I am deleting a single path from the Django cache like this:
from models import Graph
from django.http import HttpRequest
from django.utils.cache import get_cache_key
from django.db.models.signals import post_save
from django.core.cache import cache
def expire_page(path):
request = HttpRequest()
request.path = path
key = get_cache_key(request)
if cache.has_key(key):
cache.delete(key)
def invalidate_cache(sender, instance, **kwargs):
expire_page(instance.get_absolute_url())
post_save.connect(invalidate_cache, sender = Graph)
This works - but is there a way to delete recursively? My paths look like this:
/graph/123
/graph/123/2009-08-01/2009-10-21
Whenever the graph with id "123" is saved, the cache for both paths needs to be invalidated. Can this be done?
You might want to consider employing a generational caching strategy, it seems like it would fit what you are trying to accomplish. In the code that you have provided, you would store a "generation" number for each absolute url. So for example you would initialize the "/graph/123" to have a generation of one, then its cache key would become something like "/GENERATION/1/graph/123". When you want to expire the cache for that absolute url you increment its generation value (to two in this case). That way, the next time someone goes to look up "/graph/123" the cache key becomes "/GENERATION/2/graph/123". This also solves the issue of expiring all the sub pages since they should be referring to the same cache key as "/graph/123".
Its a bit tricky to understand at first but it is a really elegant caching strategy which if done correctly means you never have to actually delete anything from cache. For more information here is a presentation on generational caching, its for Rails but the concept is the same, regardless of language.
Another option is to use a cache that supports tagging keys and evicting keys by tag. Django's built-in cache API does not have support for this approach. But at least one cache backend (not part of Django proper) does have support.
DiskCache* is an Apache2 licensed disk and file backed cache library, written in pure-Python, and compatible with Django. To use DiskCache in your project simply install it and configure your CACHES setting.
Installation is easy with pip:
$ pip install diskcache
Then configure your CACHES setting:
CACHES = {
'default': {
'BACKEND': 'diskcache.DjangoCache',
'LOCATION': '/tmp/path/to/directory/',
}
}
The cache set method is extended by an optional tag keyword argument like so:
from django.core.cache import cache
cache.set('/graph/123', value, tag='/graph/123')
cache.set('/graph/123/2009-08-01/2009-10-21', other_value, tag='/graph/123')
diskcache.DjangoCache uses a diskcache.FanoutCache internally. The corresponding FanoutCache is accessible through the _cache attribute and exposes an evict method. To evict all keys tagged with /graph/123 simply:
cache._cache.evict('/graph/123')
Though it may feel awkward to access an underscore-prefixed attribute, the DiskCache project is stable and unlikely to make significant changes to the DjangoCache implementation.
The Django cache benchmarks page has a discussion of alternative cache backends.
Disclaimer: I am the original author of the DiskCache project.
Checkout shutils.rmtree() or os.removedirs(). I think the first is probably what you want.
Update based on several comments: Actually, the Django caching mechanism is more general and finer-grained than just using the path for the key (although you can use it at that level). We have some pages that have 7 or 8 separately cached subcomponents that expire based on a range of criteria. Our component cache names reflect the key objects (or object classes) and are used to identify what needs to be invalidated on certain updates.
All of our pages have an overall cache-key based on member/non-member status, but that is only about 95% of the page. The other 5% can change on a per-member basis and so is not cached at all.
How you iterate through your cache to find invalid items is a function of how it's actually stored. If it's files you can use simply globs and/or recursive directory deletes, if it's some other mechanism then you'll have to use something else.
What my answer, and some of the comments by others, are trying to say is that how you accomplish cache invalidation is intimately tied to how you are using/storing the cache.
Second Update: #andybak: So I guess your comment means that all of my commercial Django sites are going to explode in flames? Thanks for the heads up on that. I notice you did not attempt an answer to the problem.
Knipknap's problem is that he has a group of cache items that appear to be related and in a hierarchy because of their names, but the key-generation logic of the cache mechanism obliterates that name by creating an MD5 hash of the path + vary_on. Since there is no trace of the original path/params you will have to exhaustively guess all possible path/params combinations, hoping you can find the right group. I have other hobbies that are more interesting.
If you wish to be able to find groups of cached items based on some combination of path and/or parameter values you must either use cache keys that can be pattern matched directly or some system that retains this information for use at search time.
Because we had needs not-unrelated to the OP's problem, we took control of template fragment caching -- and specifically key generation -- over 2 years ago. It allows us to use regexps in a number of ways to efficiently invalidate groups of related cached items. We also added a default timeout and vary_on variable names (resolved at run time) configurable in settings.py, changed the ordering of name & timeout because it made no sense to always have to override the default timeout in order to name the fragment, made the fragment_name resolvable (ie. it can be a variable) to work better with a multi-level template inheritance scheme, and a few other things.
The only reason for my initial answer, which was indeed wrong for current Django, was because I have been using saner cache keys for so long I literally forgot the simple mechanism we walked away from.

Django url tag gives wrong values

My URLconf has:
url(r'^view-item$', 'appname.views.view_item', name='view-item'),
Now, if I go to http://myhost/path_to_django_app/view-item/, it works. However, {% url view-item %} returns '/view-item/'. Why is it doing this?
This problem occurred when I moved the application to a new server, so I'm guessing something must be configured wrong, but I don't even know where to look.
This may be due to the SCRIPT_NAME variable not being set correctly. The url tag will use this variable to compose the final absolute path to return.
You should check what request.META['SCRIPT_NAME'] is set to in one of your views. If it is not set correctly, then you may need to look into your backend configuration. If you are using mod_python, this usually involves making sure that django.root is set in the apache config. Check the installation docs for more info.
If you still can't get it to work, you can try adding this to settings.py:
FORCE_SCRIPT_NAME = '/path_to_django_app/'
The usual way to write your URl in django is with a trailing '/':
url(r'^view-item/$', 'appname.views.view_item', name='view-item')
or:
url(r'^view-item/', include('view.urls')),
Life will be much easier if you follow this convention.