I activated the wicket DebugBar in order to trace my session size. When I navigate on the web site, the indicated session size is stable at about 25k.
In the same time, the pagemap serialiazed on the disk continuously grows from about 25k for each page view.
What does that means? From what I understood, the pagemap on disk keeps all the pages. But why the session stays always at about 25k.
What is the impact on a big website. If I have 1000 parallel web sessions, the web server will need 25Mo to hold them and the disk 250Mo (10 pages * 25k * 1000)?
I will make some load test to check.
The debug bar value is telling you the size of your session in memory. As you browse to another page, the old page is serialized to the session store. This provides, among other things, back button support without killing your memory footprint.
So, to answer your first question, the size on disk grows because it is holding historical data while your session stays about the same because it is holding active data.
To answer your second question, its been some time since I have looked at it, but I believe the disk session store is capped at 10MB or so. Furthermore, you can change the behavior of the session store to meet your needs, but that’s a whole different discussion.