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Since the release of 3.4.0 parts of NAV have been using the Python web framework Django, starting with the new IP Device Info app.
As NAV has not used Django from the very beginning, NAV does not strictly follow the Django convention of multiple apps with their own models, views, etc. To plug NAV into the Django framework some glue is needed. This glue and other common Django-related code is located in the
nav.django Python package, which is located in
The usual Django settings file is located at
nav.django.settings. The end-users do not need to modify this settings file when deploying NAV, as all options are derived from existing NAV configuration in i.e. the files
Note that there is no
INSTALLED_APPS setting. NAV does not use this setting at all, due to a different file organization than what Django expects. Following from this, Django does not have a concept of what is an app in NAV. An app is just defined in the minds of the developers. This also means that NAV to a very little degree may take advantage of the
django-admin.py executable, including features like
syncdb for creating database tables for new apps.
nav.django.urls the root URL configuration for all things Django in NAV is located. The URLconf is roughly divided in two. The first part delegates various URL namespaces, like
/ipdevinfo/, to the corresponding Django apps and their own URLconfs. The second part is URL patterns for non-Django NAV subsystems, like the report subsystem. The second part is used by Django apps to link to non-Django apps, through the use of the
url template tag or the
reverse() function, instead of using the archaic
Since before Django, NAV has been using the Cheetah template system. To enable the use of Django templates for Django apps, while still integrating with the existing template hierarchy of NAV, some shortcut functions has been created in
nav.django.shortcuts. At the time of writing, the shortcuts are
render_to_response() is analogous to the well known
object_detail() are analogous to functions in
The difference between the original functions from Django and the ones provided in
nav.django.shortcuts is that the NAV versions take an additional first argument, namely
cheetah_template_func is assumed to be a Cheetah template function, which returns a Cheetah template with a
content_string variable. The shortcuts takes the content which Django normally would have returned, and inserts it into the
content_string variable of the Cheetah template. In other words, a Django template is rendered as usual, and then the result are wrapped into a Cheetah template.
At the time of writing,
nav.django.context_processors only contains one context processors:
debug. If Django is run in debug mode, this context processors appends a
sql_queries variable to the context of all templates, containing all SQL queries executed to generate the current page. This is useful for optimizing the use of the Django ORM.
The Django convention places models in the apps where they belong the most. In NAV, all models are located centrally in the
nav.models package. Usually, one creates the models first and then generates the database from the models using the previously mentioned
syncdb feature. Since NAV existed long before Django, our models was created for an existing legacy database through the use of Django's database introspection and numerous hours of hard manual labor to clean up the models, adding
__unicode__() methods, etc.
The models was split in as many modules as possible, only limited by interdependencies between the models. This resulted in the following seven modules:
cabling: models for mapping switch ports to physical rooms.
event: models related to event and alert queues.
manage: all models from the old
managedatabase which were not possible to split out to own modules, including the most important models like
msgmaint: models related to the message and maintenance subsystems.
oid: models related to OID mappings.
rrd: models related to RRD files and data sources.
service: models related to services.
Of these the
manage module is by far the most used and probably of the highest quality. All users of the models should have a look through their definitions to be familiar with how they are mapped to the legacy database using the
db_table arguments, and what methods like
__unicode__() are available to the user.
DJANGO_SETTINGS environment variable is set to
nav.django.settings. As a result of this, one can simply import the models one wants to work with and just use them, without regard to database connections, etc. An example follows:
>>> from nav.models.manage import Netbox >>> Netbox.objects.filter(sysname__endswith='uninett.no').count() 189L >>> n = Netbox.objects.filter(sysname__endswith='uninett.no') >>> n <Netbox: afs-gw.uninett.no> >>> n.up_since datetime.datetime(2008, 5, 30, 15, 36, 50, 350000) >>> n.organization <Organization: uninett (UNINETT)> >>>
To create a new NAV tool/Django app, the easiest approach is probably to start with copying an existing app, like
subsystem/ipdevinfo/. This section explains what the different parts of the
ipdevinfo app is, and how it is plugged into the rest of NAV, by following the life cycle of a HTTP request and response.
ipdevinfo app as an example. The
nav/ folder contains all runnable Python code, and nothing else. It is installed to the Python path.
nav.web.ipdevinfo Python package after installation) contains all the Django code for the app, including views, forms, etc., but not models, as they are centralized in
subsystem/lib-python in the
nav.models package. The URLconf for the apps URL scope is also located in
nav/web/ipdevinfo/. For Django to find it, it must be referenced in the root URLconf as mentioned in the
nav.django section above.
nav/web/templates/ contains a Cheetah template (not runnable Python code, but it will be compiled to a runnable Python file during installation) which is used to wrap around the Django templates. The Django templates themselves are not runnable Python code, but HTML files with some additional syntax, and are thus available directly in the
templates/ folder, not within the
nav/ folder. The Django templates are installed to a
templates/ folder next to the
python/ folder where all installed Python code goes. Typically this is
Further, the file
ipdevinfo.tool defines the name, description, icon and URL of the IP Device Info tool in NAV's toolbox. The file
htaccess is the only file which is installed into the
ipdevinfo/ folder in the web server's document root. It simply states that all URLs starting with i.e.
http://nav.example.com/ipdevinfo/ are to be handled by a Python program, namely Django's mod_python handler, using
nav.django.settings as configuration. In other words, the htaccess file should be identical for all Django apps in NAV.
How to install everything mentioned here is defined in
/ipdevinfo/subfolder within its document root which contains the mentioned
.htaccessfile stating that this URL scope is controlled by Django using
nav.django.urls, as configured in
nav.django.settings, to identify what Django view should process the request.
nav.web.ipdevinfo.urlsis identified as responsible for the
/ipdevinfo/is stripped and the rest is passed to
nav.web.ipdevinfo.urlsfor further lookup.
searchview, as imported from
searchview does its job, i.e. either creating a search form, and if a query already has been issued, looking up IP devices matching the query.
nav.django.shortcuts.render_to_response()with the following arguments: a Cheetah template, a Django template, the form, the query string, the search results and a list of error mesages if any.
render_to_response()uses Django's own
render_to_response()implementation to render the Django template. Then the result is inserted into the
content_stringvariable of the given Cheetah template. Finally, the result of the Cheetah template is extracted and inserted into a Django
HttpResponse, which is returned to the
search()view returns the
HttpResponseobject which it gets from
In addition to the steps mentioned here, Django middleware may plug into the life cycle and do work at inbetween most of these steps, except those happening inside the view function. This is not NAV specific in any way, so the reader is referenced to the Django documentation for futher details.
The integration between NAV and Django is rather new and basic. Given increased usage of Django in NAV one should strive to follow Django's conventions and, where applicable, use more of Django's capabilities instead of NAV's old homebrewn solutions. All NAV developers are encouraged to sketch out ideas for improvements below.
Django 1.0 is targeted for release September 2 2008. To jump from the version NAV are currently using (SVN r7534) to 1.0, some changes has to be made to the Django code in NAV. http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges contains details of the about 30 backwards incompatible changes done to Django between our current version and Django 1.0. IMHO, we should do this jump before the release of NAV 3.5.0 in September 2008.
NAV uses sessions for keeping track of logged in users. Currently, we are not using
django.contrib.sessions, but are still using plain old
mod_python sessions. For now, the easiest way to get hold of the
mod_python session in views is to use
request._req is the traditional
mod_python request object. In the future, one should look into using Django's authentication and session frameworks.
As more parts of NAV are (re)implemented in Django, one should look into replacing the wrapping of Django templates in Cheetah templates with pure Django templates throughout the stack. Even if this may require maintenance of base templates in both Cheetah and Django for quite some time, it would result in much more flexibility with regard to page titles, bread crumb paths, etc. when using Django templates.
NAV should certainly start using unit testing. Tests for i.e.
nav.web.ipdevinfo could for example go into
nav.web.ipdevinfo.tests, but this should be synchronized with Django conventions, and one must also find a way to run the tests, most probably through a custom test runner or something. The point is, one needs to give this some thought, and the sooner the better. New Django apps in NAV should really have unit tests from the start.