High Availability FreeBSD www cluster

Matthew Seaman m.seaman at infracaninophile.co.uk
Tue Aug 5 10:30:40 UTC 2008

Rudi Kramer - MWEB wrote:
> Michael Christie:
>> I want to cluster some freeBSD servers, The purpose of this is to
>> learn.  I would like to  run some basic services like www and mail on
> a
>> test network. I would like to set up the servers so if one server
> falls
>> over the other will take over the services automatically, load
>> balanceing would be good as well. I have googled, I could be looking
> in
>> the wrong place , there seems not to be much in regard to seting up
>> freebsd in a cluster, lots on linux. I have looked at the High
>> Availability Linux project , I see on the front page that it will run
> on
>> freebsd.
>> So I am a bit lost and i am wanting to learn how to cluster freebsd
> web
>> and mail servers, I have looked at  Beowulf clusters, which seem to
> give
>> computers more grunt, Can some on on the list please advise me on what
>> clustering softwhere i need to get started and if the High
> Availability
>> Linux project softwhere will do the job.
> I also did some research a while ago and found Wackamole. It looks
> pretty interesting as you don't need a central "director" server but all
> servers in the cluster check each other. It's also in the ports tree :-)
> Site: http://www.backhand.org/wackamole/ 
> Port: /usr/ports/net/wackamole

There's clustering and clustering.  Neither of the two applications
the OP mentioned needs anything like as tight a coupling as what many commercial 'cluster' solutions provide, or that compute-cluster solutions
like Beowulf or Grid Engine[!] provide.

WWW clustering requires two things:

    * A means to detect failed / out of service machines and 
      redirect traffic to alternative servers

    * A means to delocalize user sessions between servers

The first requirement can be handled with programs already mentioned
such as wackamole/spread or hacluster -- or another alternative is 
hoststated(8)[*] on OpenBSD.  You can use mod_proxy_balancer[+] on recent 
Apache 2.2.x  to good effect.  Certain web technologies provide this
sort of capability directly: eg. mod_jk or the newer mod_proxy_ajp13
modules for apache can balance traffic across a number of back-end tomcat 
workers: of course this only applies to sites written in Java.

If you're dealing with high traffic levels and have plenty of money to 
spend, then a hardware load balancer (Cisco Arrowpoint, Alteon Acedirector, 
Foundry ServerIron etc.) is a pretty standard choice.

The second requirement is more subtle.  Any reasonably complicated
web application nowadays is unlikely to completely stateless.  Either
you have to recognise each session and direct the traffic back to the
same server each time, or you have to store the session state in a way
that is accessible to all servers -- typically in a back-end database. 
Implementing 'sticky sessions' is generally slightly easier in terms of 
application programming, but less resilient to machine failure.  There
are other alternatives: Java Servlet based applications running under
Apache Tomcat can cluster about 4 machines together so that session
state is replicated to all of them.  This solution is however not at
all scalable beyond 4 machines, as they'll quickly spend more time passing
state information between themselves than they do actually serving incoming 
web queries.

Mail clustering is an entirely different beast.  In fact, it's two
different beasts with entirely different characteristics.

The easy part with mail is the MTA -- SMTP has built in intrinsic concepts 
of fail-over and retrying with alternate servers.  Just set up appropriate 
MX records in the DNS pointing at a selection of servers and it all should 
work pretty much straight away.  You may need to share certain data between 
your SMTP servers (like greylisting status, Bayesian spam filtering, 
authentication databases) but the software is generally written with this 
capability built in.

The hard part with mail clustering is the mail store which provides the
IMAP or POP3 or WebMail interface to allow users to actually read their 
mail.  To my knowledge there is no freely available opensource solution
that provides an entirely resilient IMAP/POP3 solution.  Cyrus Murder
comes close, in that it provides multiple back-end mail stores, easy 
migration of mailboxes between stores and resilient front ends.  The 
typical approach here is to use a high-spec server with RAIDed disk 
systems, multiple PSUs etc. and to keep very good backups.



[!] http://gridengine.sunsource.net/

[*] hoststated(8) integrates with the traffic redirection capabilities of 
pf(4) to provide pretty much the same sort of functionality as a hardware 
loadbalancer via a firewall machine, but a lot cheaper.

[+] http://httpd.apache.org/docs/2.2/mod/mod_proxy_balancer.html

Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                  Kent, CT11 9PW

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