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This page was a draft. See TracDev/PluginDevelopment/ExtensionPoints/trac.notification.api.INotificationSubscriber page now.

The Open questions section might still contain unimplemented ideas.

Extension Point : INotificationSubscriber


The INotificationSubscriber subscribes users to notification events.


Trac provides an extendible and flexible notification system. Different people are interested in different kinds of notifications. Notification subscriptions allow administrators and / or users to configure the exact rules used that trigger sending of notifications.


Implementing the interface follows the standard guidelines found in TracDev/ComponentArchitecture and of course TracDev/PluginDevelopment.

The main part of this interface is the match() function. It returns a list of subscriptions, in the form of tuples consisting of:

  • class: The name of the Python class. (This could probably be removed.)
  • distributor: Also known as transport. E.g. the string email. See INotificationDistributor.
  • sid: The session ID of the subscriber. (Can be None if address is provided.)
  • authenticated: 1 for authenticated session IDs, 0 for anonymous session IDs.
  • address: The (email) address to use. (Can be None if sid is provided.)
  • format: The MIME type to be used (e.g. text/plain or text/html.)
  • priority: An integer priority. Smaller numbers have higher priority than bigger numbers. 1 is the highest priority.
  • adverb: Either the string always or never.

Since more than one component can handle the same realms and categories, the priorities and adverbs are used to resolve conflicting subscriptions.

The implementation can use any means to determine if a user is interested in hearing about a given event. Most check that the appropriate conditions apply and then retrieve the required information from the subscription DB table.

The subscriptions in that table are configured in a shared preferences panel that uses two other methods of this interface: The simple description() method returns a description string shown to the user in the preferences panel (or None if the plugin does use the subscriptions DB table.) The requires_authentication() method allows hiding the rule from unauthenticated users. (E.g. because only authenticated users can be ticket owners.)

The default_subscriptions() method describes any default subscriptions that automatically exist without the user configuring subscription DB entries in the preferences. These are also displayed on the preferences panel, but can not be directly modified there. (They usually can be overriden by non-default subscriptions.) The plugin still has to return the respective subscriptions from the matches() method.

Default descriptions should be used when users can be determined by the event itself. For instance, ticket author has a default subscription that is controlled via trac.ini. Default subscriptions should be low priority (i.e. have a priority number much larger than 1, like 100) so that the user can easily override them.


The following example implements a simple subscriber that can trigger notifications when a new ticket is created with a high priority level.

from trac.core import *
from trac.notification.api import INotificationSubscriber
from trac.notification.model import Subscription

class HighPriorityTicketNotificationSubscriber(Component):


    # INotificationSubscriber methods

    def matches(self, event):
        if event.realm != 'ticket':
        if event.category != 'created':

        ticket = event.target
        if ticket['priority'] not in ('blocker', 'critical', 'major'):

        klass = self.__class__.__name__
        for i in Subscription.find_by_class(self.env, klass):
            yield i.subscription_tuple()

    def description(self):
        return "notify me when new high priority tickets are created"

    def requires_authentication(self):
        return False

Available Implementations

Several implementations are part of core Trac:

  • trac.ticket.notification.AllTicketSubscriber
    Allows anyone to subscribe to all ticket change notifications.
  • trac.ticket.notification.TicketOwnerSubscriber
    Allows ticket owners to subscribe to (or unsubscribe from) change notifications for owned tickets.
  • trac.ticket.notification.TicketComponentOwnerSubscriber
    Allows component owners to subscribe to (or unsubscribe from) change notifications for tickets assigned to owned component.
  • trac.ticket.notification.TicketUpdaterSubscriber
    Allows anyone to subscribe to (or unsubscribe from) change notifications for their own ticket changes.
  • trac.ticket.notification.TicketReporterSubscriber
    Allows ticket reporters to subscribe to (or unsubscribe from) change notifications for tickets they created.
  • trac.ticket.notification.CarbonCopySubscriber
    Allows anyone to subscribe to (or unsubscribe from) change notifications for tickets where they are listed in CC.

Additional Information and References

Open Questions

Merge description() and requires_authentication()

These methods are both only used in the preferences panel. Merging them could be easier to understand and more flexible, allowing different requirement checks:

    def get_subscription_preference_description(self, req):
        if req.is_authenticated and 'TICKET_ADMIN' in req.perm:
            return "notify me when an admin is needed"

Remove class name

The class item could be removed from the tuple returned by matches(). It's not really needed and clutters up each plugin with ugly klass = self.__class__.__name__ lines.


  • It helps debugging and can be logged.
    • But if that's needed the NotificationSystem should do so, to avoid mistakes and simplify plugins.
  • The subscription DB table requires that anyway.
    • But it should be replaced there as well, e.g. by a freely chosen rule string.
Last modified 6 months ago Last modified on Mar 22, 2017, 9:04:14 PM
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