Number of forum topics is cached, is SSI the solution?

We are allowing users to post comments to articles / media items.
Posting comments is made possible by the discussion / forum functionality in GX.
When displaying a mediaItem, we are also showing the number of comments posted to it.

This is currently done by getting the length of the forum topics for the current mediaItem:

$--accolade open--fn:length(mediaItem.current.forum.topics)--accolade close--

This works perfectly.... without caching.

When caching is enabled the following happens:
1. User posts a new comment to an article
2. The new comment is shown in the articles discussion / forum section
3. The number of comments is not updated in the article / media item content itself
4. The number of comments is also not updated in media collection overviews

This behavior is expected: the media item content and media collection are only rebuilt (thrown out of the cache) when a media item changes. Still the number of comments should be updated when a new comment is posted.

How can you let this “forum topic counter” be updated when a new comment is posted?

Should the whole media item be removed from cache when a new comment is posted? This would have some performance impact...

A more elegant solution would be that only the counter is removed from cache. Can this be achieved by making the counter a SSI that listens to the cache updates for the forum?

We have tried the following SSI solution:
1. Create a “topic count” presentation which only displays the number of topics
2. Set a SSI presentation for this “topic count” presentation
3. Render this presentation in the media item content or media collection presentations

After some code wrestling we came with the following...

mediaCollectionElement.jspf

...
<c:set var="currentMediaItem" value="$--accolade open--mediaCollectionElement.mediaItems[index]--accolade close--" ></c>
...
<wm:render presentationName="WPG Forum Topic Count" mediaItem="$--accolade open--currentMediaItem--accolade close--" ></wm>
...

mediaItemContent.jspf

...
<c:set var="mediaItem" value="$--accolade open--presentationcontext.mediaItem--accolade close--" ></c>
...
<wm:render presentationName="Forum Topic Count" mediaItem="$--accolade open--mediaItem--accolade close--" ></wm>
...

forumTopicCount.xml

<presentation>
  <name>Forum Topic Count</name>
  <display-name>Forum Topic Count</display-name>
  <scope>Forum</scope>
  <ssi>
    <presentation>forumTopicCountSsi</presentation>
  </ssi>
</presentation>

forumTopicCountSsi.jspf

<%@ page language="java" session="false" buffer="none" %>
<%@ taglib uri="http://www.gx.nl/taglib/wm" prefix="wm" %>
<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>

<c:set var="passOn" value="cachetimeout,forum" ></c>

<wm:link var="ssiLink" 
	ssiObjectId="$--accolade open--mediaItem.current.forumSettings.id--accolade close--"	ssiObjectClassName="nl.gx.webmanager.cms. forum.ForumSettings"
	cachetimeout="1"    	forum="$--accolade open--mediaItem.current.forum.id--accolade close--"
	passOn="$--accolade open--passOn--accolade close--" ></wm>

$--accolade open--ssiLink.ssiTag--accolade close--

forumTopicCount.jspf

<%@ page language="java" session="false" buffer="none" %>
<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn" %>
<%@ taglib uri="http://www.gx.nl/taglib/wm" prefix="wm" %>

<wm:object var="forum" objectId="$--accolade open--param.forum--accolade close--" objectType="Forum" ></wm>

$--accolade open--fn:length(forum.topics)--accolade close--

The mediaItem variable should be passed down to the SSI presentation which in turn passes the mediaItem ID on to the forumTopicCount presentation.

Without the SSI presentation, the number of topics displays perfectly.
With SSI, it doesn’t display anything.

We have tried SSI debug on the article page / media item content page. The counter is called by the following SSI url:

/web/show?id=83433&langid=43&cachetimeout=1&elementHolder=83435&forum=4&ssiObjectClassName=nl.gx.webmanager.cms.forum.ForumSettings&ssiObjectId=83431

83433 is the id of the “Article Show on” page.
cachetimeout=1 is added for debugging reasons, in the end the cache should be timed out by the ssiObject right?

Getting this SSI URL with the browser returns a totally empty page.
The system log does not print any error stack traces while getting this URL, not even on the Finest Log Level.

We get the same results when caching is turned on and when it’s off.
We are using WebManager 9.8.2

How can we proceed debugging?
Are we building the right SSI link here?

Above all: thanks for reading this and hopefully you can help a fellow developer out.

Sorry for the messy code.
Can't seem to post accolades ({) (}) in my previous post...
I've used --accolade open-- and --accolade close-- instead.