Message Passing For Anonymous Users

Proposal for 1.2: Django should include a contrib app, independent of contrib.auth, that facilitates message passing to anonymous users.

For more information see:

• Ticket #4604
• ​http://groups.google.com/group/django-developers/browse_thread/thread/eba93088c649022b
• ​http://www.caktusgroup.com/blog/2009/06/19/towards-a-standard-for-django-session-messages/

Justification

Reasons why an alternative to the existing functionality (​user.message_set.create) is needed:

• It's not possible to create messages for anonymous users
• If the same Django user is logged in twice on different computers simultaneously, they see each others' messages
• User messages may get wiped even if they're not actually displayed to the user
• High-load sites want to avoid the unneeded database or cache usage

Reasons why a standard needs to be endorsed by Django and a 3rd party app will not suffice:

• The built-in implementation is broken for a large set of use cases (above); the fact that Django actively encourages this method of messaging is a bad thing
• Reusable apps don't know what 3rd party system to use and hence cannot rely on providing session feedback

Existing 3rd Party Apps

There are a number of good, pre-existing applications out there that support more robust functionality, so there is no need to re-implement a solution from scratch for inclusion in Django. The existing solutions can be combined or modified to meet Django's needs. This section is meant to evaluate some of the different session/cookie message/notification engines out there for potential inclusion in Django as a contrib app. It is a work in progress so please contribute (your notification engine here) or other changes as you see fit.

Criteria

Technical criteria necessary for inclusion in the core:

• Support message passing for anonymous users
• Uses the session only as a fallback: Avoid database/cache queries if possible, but support larger messages that don't fit in a cookie (> 4kb) when needed
• Don't lose messages if they're not displayed to the user (lazy message loading)
• Signs cookie-based messages
• Avoids ​pickling because of the obvious security concerns
• Provide a standard, intuitive interface so that reusable apps can provide feedback related to the current session
• Supports different, configurable "classes" (e.g., info, warning, error, etc.) of messages

Community criteria necessary for inclusion in the core:

• Needs community approval/support
• Needs to be the "de facto" standard implementation

Available Options

Name and Link	Anonymous user support	Session Fallback	Lazy loads messages	Signed cookies	Avoids pickling	Standard interface	Classed Messages
​django-notify	yes	yes	yes	yes	no	yes	yes (via "tags")
​django-flash	yes	no	no	yes	no	no	yes (but not in a standard way)
​django-flash-status	yes	no	no	yes	no	yes	yes, "errors" or "statuses"
​django-cnotes	yes	no	no	yes	no	yes	no

• yes means yes, always
• sometimes means under certain conditions (e.g., an engine might support large messages if the session is used, and avoid DB access if a cookie is used, but not both)
• no means no, not in the current implementation

Please update this table with new options or corrected information as necessary.

Potential API

django-notify is the current front-runner in terms of technical features so a sampling of the potential API is included below. Feel free to include the API(s) of other solutions as well for comparison. Discussion of the API should take place on the ​django-developers list.

django-notify

Add the middleware to your MIDDLEWARE_CLASSES setting (after SessionMiddleware):

    'django_notify.middleware.NotificationsMiddleware',

Add the context processor into your TEMPLATE_CONTEXT_PROCESSORS setting:

    'django_notify.context_processors.notifications',

Add a temporary notification message like this:

    request.notifications.add('Hello world.')
    # You can optionally provide a string containing tags (which is usually represented as HTML classes for the message).
    request.notifications.add('Your rating is over 9000!', 'error')

Use notifications in your template:

{% if notifications %}
<ul class="notifications">
        {% for message in notifications %}
        <li{% if message.tags %} class="{{ message.tags }}"{% endif %}>{{ message }}</li>
        {% endfor %}
</ul>
{% endif %}

Other Considerations

• This app will ideally take advantage of the cookie signing API that is also proposed for 1.2.