Well, its not THAT easy. There are two issues-- 1) keeping multiple
copies of data current, and 2) getting the system to recognize a failure
of the primary server and promote a secondary server.
Keeping fairly static data current can be as simple as a nightly backup or
'cp -a' script. Or use rdist or mirror in some cases. More dynamic data
such as named, NIS maps, and such may be kept current by the systems
themselves through master and slave servers. Innd can feed a secondary
site with a complete copy of the news spool. OTOH, Keeping redundant SQL
servers (for example) in sync might be a real neat trick.
The next trick is getting the data out. DNS info is easy-- add the
secondary server to the nameserver list in resolv.conf. If the first
server doesn't respond, the next will be consulted. Switching to a new
ftp, www or other server may involve using a script to monitor the various
servers, and reconfigure a secondary to respond to a given IP address if
the primary goes down. Very doable, but hardly simple. You need to make
sure that the secondary responds appropriately, and shuts down when the
primary comes back online. You must ensure that if data has changed while
the secondary was serving, that the changes are posted to the primary
before it comes online. Finally, you must prevent conflicts for limited
resources like IP addresses. While a simple DNS change *might* solve the
problem (routing www to another server's IP address, for example) such
changes take time to propagate through the Net.
Some tools you may want to consider-- 'amd' allows automounting of
filesystems from multiple redundnat locations. 'rdist' can keep multiple
copies of data in sync. O'reilly's 'CGI Programming" discusses sharing
load ans redundancy in web servers to some slight extent.
Good luck-- truly transparent fault-tolerant systems are not simple.
On Mon, 11 May 1998, Frank Kujawski wrote:
> You have an easy job.
> httpd, point your dns to two different numbers
> named, most computers allow you to select both a primary and secondary
> innd, good luck I have never had any success with this one.
> smtp, set one of your boxes as a secondary mail server
> pop3, do not make redundant
> have fun
>
> Frank
> __________________
> / \
> | [EMAIL PROTECTED] |
> \__________________/
>
> On Mon, 11 May 1998, Jack Hatfield wrote:
>
> > Well, that would be httpd, innd, named. Your basic web, mail, and news
> > servers. I am guessing so do not hold me to this.
> >
> > Is any of this right? I hope I am answering your questions.
> >
> > RocNet Monster
> > RocSoft, Inc.
> > 915 Lyttleton St.
> > Camden SC 29020
> > 803 713-3433
> > -----Original Message-----
> > From: Frank Kujawski <[EMAIL PROTECTED]>
> > To: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
> > Date: Monday, May 11, 1998 9:05 PM
> > Subject: Re: Redundant Servers - Possibility??
> >
> >
> > >The first question is what applications are you running? Many web apps
> > >can be split accross multipul machines.
> > >
> > > Frank
>
>
> --
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