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From Joachim Draeger ...@joachim-draeger.de>
Subject Re: IMAP Draft: Cluster
Date Fri, 07 Jul 2006 07:47:56 GMT
Am Donnerstag, den 06.07.2006, 15:39 +0200 schrieb Ahmed Mohombe:
> > Am Donnerstag, den 06.07.2006, 10:47 +0200 schrieb Ahmed Mohombe:
> >>> In a worst case, when nobody is ill or on holidays, you have at least
> >>> one open connection per user all the time. 
> >> How would one handle in the JVM and code these very long lasting connections?
> > 
> > Why? Are there issues? 
> > I'm not aware of any problems with many sockets treated by many threads
> > running a long time apart from the not avoidable network, cpu and memory
> > consumption. 
> Well, if too many resources are for long time "occupied" and not released,
> the JVM won't even get the change to do GC on those object trees.

Is it like in Windows 95 that had unavoidable crashed after 21 days? ;-)
Or like in Windows 98 that had to be rebooted to get back full ;-)
performance?
Well that would deny using Java on servers at all. You mean if I set a
variable that was bound for 1 week to null it won't ever by gc'ed?
A socket that has been opened for 24h can't be closed?
Well I know many servers, just like James, are fighting with memory
leaks and class loader issues. But those problems are mostly self-made
and maybe seldom related to JVM Bugs...
Could you give a concrete example? 

> The problem is also that those resources are limited (at least for most
> of the people).

Memory and cpu, of course. They will limit the possible number of
threads and sockets. I don't think we will get into limits just because
of number of threads running or number of sockets opened.

> > In this case the IMAP server should run on a separate machine not to
> > influence smtp/spooling on high loads.
> Having a second machine is already a too high system requirement. Many
> small companies that have their server to an ISP would gladly use IMAP
> but only on that one machine.

Using one machine will always mean that you have to limit performance to
play safe and have reserves. This will first be done by an obligatory
connection limit. After having benchmarks results we could see how a
dynamic connection limit could make sense.

But the best performance will be achieved with a dedicated machine. 

> > Is that the direction of your question?
> This would be the purpose: to guarantee somehow that at least those who
> are in (or do not overpass a limit) get decent handing even if more
> users try to access the service.

n users should be able to get at least one idle connection, n/x
expensive operations can be run simultaneously, managed by a scheduler. 

I'm really happy about such proposals. Even if they are not
first-priority features, we can keep them in mind while doing
design/implementation not to make decisions that would block them.

> >>>> Clustering at DB level would mean to not support dbfile, file and other

> >>>>   types of repositories.
> >>> Clustering at DB level would just mean following some rules in the
> >>> implementation. DBMS are designed to be accessed simultaneously from
> >>> multiple hosts.
> >>> dbfile mmh, good point. I just made a few thoughts. There are also
> >>> various solutions to access a network file-system without a single point
> >>> of failure. 
> >> What about JCR? (e.g. the Jackrabbit implementation). Could
> >> it solve some of the problems, or it is too far away from making
> >> it usable as a "file system" for IMAP?
> > 
> > Well, I don't have any knowledge about JCR. I tried to quickly browse
> > some docs/faqs. 
> > But it seems to be able to use it reliable Jackrabbit uses a JDBC
> > back-end, too. And I guess it wasn't made to store lots of emails. The
> > overhead of implementing this might be greater than using JDBC directly.
> > This gives us the ability to tune performance.
> > How do you think JCR might help?
> Well, JCR is a standard and can have many types of back-ends (not just JDBC).

Yes, but the questions is which back-ends are already there for
production and what are the features. E.g. jackrabbit site says,
file-based back-end does not support transactions and might by
inconsistent after an unclean JVM shutdown.

> I asked about JCR since IMAP from a user perspective looks more like a DMS. In fact,
> many small companies use the Exchange server(and some Outlook plug-ins) with IMAP exactly
in this 
> way: to access easily all the documents from everywhere and all the time.

Yeah, another example why imap is so resource intensive. :-)

Joachim


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