[xwiki-devs] [Brainstorm] Notifications filters capabilities and performances.

Hi developers. I am trying to add a new filter to the notifications to be able to follow pages that are marked with a given tag. And it leads me to some questions about the technical implementation of the notifications. To remind the context: notifications are computed on top of the events recorded by the event stream (a.k.a activity stream). We take events from the event stream SQL table, we apply some transformations on them, and we display them to the user. Then we have implemented the ability to filter on these events: for example "don't show events concerning the document A nor the wiki B". Filters are implemented with 2 distinct ways: 1/ SQL injections: each filter can add SQL elements in the query we make to fetch the events from the event stream table. We made this mechanism so we can let the database do a lot of the filtering process. After all, it's its job and it's supposed to perform well. To be precise, Clement has even created an Abstract Syntax Tree (AST) so it's easier to inject some content in the query and it creates an abstraction over the SQL language so we can even consider to change the storage of the event stream someday. The bad thing is that some complex filtering are difficult to write with the SQL language (event with the AST) or even impossible. 2/ Post-filtering: after the events have been fetched from the database, each filter can still decide to keep or filter them. This is useful for complex filtering that cannot be expressed with the SQL language. It is also needed by the real-time notification email sender, because it takes the events immediately when they occurs without fetching them in the database (so SQL filters are bypassed). The bad thing is that some events are loaded in the memory to finally be rejected, and these filters can perform costly operations such as loading documents. Until now, this double mechanism was working quite well, with each mechanism filling the lacks of the other. However, we still have technical limitations in our design: 1/ Users who have a lot of filter preferences can end up with a giant SQL query that is almost impossible to perform by the database. Actually we had a user complaining about an OutOfMemory problem in the HQL to SQL translator ! 2/ I cannot implement the tag filter ! The tag filter is supposed to show events that concern pages that hold a given tag, EVEN IF THE PAGE WAS EXCLUDED BY THE USER. Example of use-case: "I don't want to receive notifications about wiki A except for pages marked with the tag T". And it is not working. First because it is difficult to write a SQL query for that. It requires to make a join clause with the document and the object tables, which our SQL injection mechanism does not support. Even if it were possible, creating a SQL join with the document table will de-facto filter events that do not concern any pages or pages that do not have any objects: so many other filters will be broken. I also don't consider creating a SQL subquery, I think the whole query would became too big. So I decided to just not inject any SQL code for this filter and only implement the post-filtering mechanism. But the other filter "EXCLUDE WIKI A" generates a SQL injection such as "WIKI <> 'WIKI A'" so the events concerning the wiki A are not fetched from the database. Consequence: the tag filter never see the events that it is supposed to keep. It would be actually possible to by-pass the first SQL injections by injecting something like "OR 1=1". But doing something like this is like dropping the all SQL injections mechanism. I see some solutions for this problem: A/ For each tag, create a permanent list of pages that hold it. So I can inject "OR document IN (that_list)". I think this is heavy. B/ Drop the SQL injection mechanism and only rely on the post-filtering mechanism. It would require to load from the database A LOT of events, but maybe we could cache this. C/ Don't drop the SQL injection mechanism completely but use it as little as possible (for example, do not use it for LOCATION filtering). Seems hard to determine when a filter should use this feature or not. D/ Don't implement the "tags" filter, since it is the root of the issue. But it is like sweeping dirt under the carpet! Since we have the OutOfMemory problem with the SQL injections becoming too huge, I am more in favor of solution B or C. But I'm not sure for now, since I do not know how much it would impact the performances and the scalability of the whole notifications feature. This is a complex topic, but I hope this message will inspire you some suggestions or things I have not seen with my own eyes. Thanks for your help, Guillaume