[Using Sakai] Who's online feature

[Sam Ottenhoff]

Yes, exactly, the database is capable of a certain level of performance.
 Once queries start piling up (waiting to be processed), the system begins
to melt down as users furiously refresh, the database struggles, and users
can no longer take assessments, grade assignments, or use Sakai.

Say if your database is capable of an arbitrary number of queries (ignoring
that some queries are easy and some are difficult) like 300k queries per
minute. Who's Online in pre-Sakai 2.9 is a simple polling mechanism that
checks with the database on the number of users in a site every 5 seconds
or so.  So if you have 5k users logged into Sakai, that equals 60k database
queries per minute just for Who's Online.  So Who's Online is taking up a
certain amount of your database's performance capacity that could
potentially be really important during times of high demand (e.g., 200
users taking an online assessment at the same time).

Does the 2.9+ Jgroups code introduced for portal chat also replace the old
database polling mechanism?  If yes, I would investigate this approach as
it would be a way of doing presence without the constant volume of queries
to the database (https://jira.sakaiproject.org/browse/SAK-22286).

--Sam


On Mon, Aug 26, 2013 at 8:56 AM, Charles Severance <csev at umich.edu> wrote:

> Elizabeth,
>
> I am looking forward to answers from others - but here is one view.
>
> The "Who is logged in" feature (aka presence) is a great leading indicator
> of performance problems.   It does consume real resources and is easily
> turned off - the feature disappears and the resources are freed up.
>
> What is wonderful about this is that things can feel a little sluggish as
> load increases - you turn off the feature - and poof!  Things speed back
> up.  Kind of like a four hour energy drink.  There are a number of patterns
> that folks follow:
>
> - Turn it off on the first and last weeks of a semester if you see spikes
> in load at those times
> - Turn it off if your base load goes up and you cannot replace hardware
> until the end of the semester
>
> But what you describe below sounds bad if it is not one of the two
> situations above.  If your vendor just is tuning it off "forever", that
> suggests to me that your usage has grown a bit and you need faster or more
> hardware.  Perhaps your vendor can't or does not want to give you more
> hardware - or perhaps you don't want to pay for more hardware - but for
> whatever reason if you are running with presence off all the time - you are
> setting yourself up for a surprise and severe slowdown sometime in the
> future that will be as bad as it gets and last as long as it lasts and you
> will have spent the "quick fix" already.  The problem when a system gets
> slow is that people start clicking even more furiously (like honking horns
> in a traffic jam) - so a little slow can become a big slow pretty quickly.
>
> And once that happens you won't be able to fix by drinking a second
> four-hour energy drink one hour after you drank the first one :)
>
> So for me running with presence turned off all the time is a bad warning
> sign.   I would have a pointed conversation with your vendor.  Presence is
> a nice indication of reasonable headroom in terms of hardware provisioning.
>
> Hope this helps.
>
> /Chuck
>
> On Aug 26, 2013, at 7:49 AM, Elizabeth A. Kiggins <kiggins at uindy.edu>
> wrote:
>
> > Our vendor decided to turn off the "Who's online" feature in Sakai. This
> > was a feature our faculty used regularly. Our vendor tells us this
> > feature slows the system down and is hesitant to turn it back on. I have
> > two questions:
> >
> > How many of you have turned off this feature?
> >
> > If you are using this feature, have you determined that use of the
> > feature slows the system down? If so, is it noticeable?
> >
> > Thank you for your feedback,
> > Beth Kiggins
> >
> > --
> > Elizabeth A Kiggins
> > The Studio Group
> > Faculty Learning/Design Studio
> > University of Indianapolis
> > 1400 E. Hanna Avenue
> > Indianapolis, IN  46227
> >
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