Ideally, NodeB ought to "catch up" when it joins the cluster by
syncing the content in the disk page stores at that time.  I think
it's a flaw we should track and fix at some point.

Nice job, Matej!


Matej Knopp-2 wrote:
> 
> Hi all,
> 
> I've committed couple of changes to DiskPageStore to address some
> issues. Mind you the commit is rather fresh and needs to be tested
> properly before 1.3.1 gets out.
> 
> First of all it solves the problem of context reloads (see
> https://issues.apache.org/jira/browse/WICKET-1161). When context is
> reloaded, the DiskPageStore index is no longer lost and all serialized
> page files can be used as if no reload has happened.
> 
> The other improvement is better clustering support
> (https://issues.apache.org/jira/browse/WICKET-1272). The back button
> support is improved in clustered environment and also the memory
> consumption is reduced significantly.
> 
> What's new? Imagine a simplified environment, cluster with two nodes
> (NodeA and NodeB). All sessions from a NodeA are replicated to NodeB
> and vice versa.
> 
> When a page instance is being replicated from NodeA to NodeB, it is
> immediately stored to DiskPageStore on nodeB, rather than kept in
> session. This means that the instance is later accessible on NodeB
> even after another page from the same pagemap has been replicated to
> NodeB, because it's already stored in DiskPageStore. Also it doesn't
> have to be kept in memory, significantly reducing the session state.
> 
> 
> There is one exception though, where the backbutton won't work across
> cluster. Imagine following scenario:
> 
> Session is created to NodeA
> NodeB goes down
> User visits page1, page2, page3 on NodeA  (these pages were not
> replicated to NodeB, because the node is not available)
> NodeB goes up
> 
> only page3 (the last accessed page in it's pagemap) will be available
> on NodeB. page1 and page2 are not available, because when they were
> visited NodeB was not running. page3 is available, because it was
> active in nodeA session, thus got replicated to NodeB when the context
> on NodeB was initialized.
> 
> -Matej
> 
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