TL;DR

This thread is about the way we store notification filter preferences for each user. The constraint is there can be a lot of them (700 is a number a user has recently reported). So how should we store them?

Full text

Definition

So what is a filter preference? It's a generic object that can store many elements, such as a page locations, application names, event types, etc... They describe a configuration about a given filter for a given user. For example, a filter preference can say "for the ScopeNotificationFilter and the user A, include the location Main.WebHome" as it could be "for the UserNotificationFilter and the user A, exclude the user SPAM". It's generic.

The main usage is for page locations (ScopeNotificationFilter). By default, we have the "autowatch" mode enabled. It means every time a user modifies a page, a filter preference for this page and this user is created. So if a user modifies 700 pages, he gets 700 filter preferences.

How are they stored

Currently, we have a simple implementation. There is a generic XClass called "XWiki.Notifications.Code.NotificationFilterPreferenceClass". For each preference, we add an XObject on the user page. It's that simple. But it also means that if a users have 700 filter preferences, she also gets 700 XObjects on her page, and 700 revisions of that page. Which is a pain: it takes a lot of place in the document's cache, and it's heavy to load (lot of SQL queries needed). So we have a big problem here.

Possible solutions

A: Minimize the number of xobjects needed for ScopeNotificationFilter

Currently, one location is represented by 1 filter preference. But most filter preferences are very similar. They almost all say "for the ScopeNotificationFilter, for all event types, for all applications, the filter preference is enabled". The only different part is the actual location. But the "location" field is itself a LIST stored with the "relational storage" option. So we can take advantage of it and store similar preferences into 1 single object.

1 object with 700 locations instead of 700 objects with 1 location.

However, it's a bit harder than this. Event if the NotificationFilterPreferences is generic and can contains many locations, the ScopeNotificationFilter expect it to concern only one location (and then it perform complex operations to sort the filters preferences according to a hierarchy). The UI in the user profile makes the same assumption so it does not handle multiple locations in the same preferences object. Refactoring this is not simple and cannot be done for 10.6.

Variation 1: store only 1 xobject, but make the API return 700 preferences objects anyway

This is the variation I am prototyping. Actually it's ok if the filters and the UI expect only 1 location into the preferences object. All we have to do is to "smash" the xobject into many NotificationFilterPreferences objects that we need internally. It would simply be the responsibility of the Store to detect similarities and to save the minimal amount of XObjects to store a bunch of preferences.

But it means being very smart when loading, creating, updating and deleting a preference. Not having one xobject per filter preference introduces complexity, and complexity can lead to bugs. Again, according to the time frame, it's hard to implement.

Variation 2: use custom mapping

Probably the easiest solution that would help making less SQL queries. The idea is to have a SQL table for notification filter preferences and bind the XObjects to that table. It would still use a lot of place in the document's cache but be more efficient on the database level.

Other Problem 1: it still creates page revisions

As long as we store the filter preferences with xobjects, we create page revisions. We can get rid of those by using some internal API to not create a revision when we save an xobject but I wonder if it's what users want. If a user tries to rollback some changes and don't see all filter preferences it concerns, I think it's not very transparent.

Other Problem 2: Document's cache

Sometime we load the a user document to get the avatar of the user, her name, etc... So we load user documents very frequently, even if the user is not connected! Having 700 filters in the document and cache them with the document even if we don't need them is a big waste of memory.

B: Implement a completely new store with Hibernate

A bit like having a custom mapping. We could create a SQL table and implement an API to handle it. Then, no xobjects would be involved.

Some drawbacks:

• we need to write a custom cache as well.
• the user cannot modify her preferences using the wiki principles (xobjects all the way).

C: Refactor the UI and the ScopeNotificationFilter so they do not assume 1 filter preference = 1 location

This option is still possible. Probably the best because creating 1 filter preferences object per location is an obvious waste of memory. A refactoring of the UI is needed anyway, because we currently have no way to remove a bunch of filter preferences easily (users have to delete the 700 filters preferences manually) so we can kill 2 birds with the same stone.

But again, it requires some work.

Conclusion

That's it. All possible solutions require development effort that is hardly possible to make before 10.6 (and even 10.7, considering I would probably be the one implementing it and I'm not fulltime on the subject and I have holidays soon).