Critical uptime question (Was "Looking for some upgrade advice")

[Boaz Bezborodko]

John Esak wrote:

>Date: Fri, 19 May 2006 07:37:16 -0400
>From: "John Esak" <john at valar.com>
>Subject: OT: RE: Looking for some upgrade advice
>To: "Fplist (E-mail)" <filepro-list at seaslug.org>
>Cc: Rick Walsh <rick at nexusplastics.com>
>Message-ID: <JIECJPPMJGMIINMGGNGAAEHJPBAA.john at valar.com>
>Content-Type: text/plain;	charset="us-ascii"
>
>  
>
>>I suspect the only reason I haven't seen comparable uptimes on my linux
>>systems is because the kernel updates require a reboot.  I talked directly
>>to the 2nd in charge of the kernel, as well as some of the other kernel
>>devs, and the consensus was that if I wanted a hot-swappable kernel, I
>>could go and write the hot-swap code myself.  They didn't consider it a
>>priority, or even desirable.
>>    
>>
>
>As you know, the *last* thing in the world I want to do is start a Linux
>thread here. :-)
>
>BUT... this is something I hadn't considered in our upcoming major move to
>SuSe Linux. We have a situation where the main *nix server (currently SCO
>OpenServer 5.6) can NOT go down at all. Literally, it is used to produce
>various things, mostly bar code lables 365/24/7... with absolutely NO down
>time at all except for two week long vacations during the year and some
>other extremely special circumstances... hardly would I called these
>"planned maintenance"... mor like get in whatever we can because the system
>went down for some unforeseen reason! :-)  Very occasionally, and I mean
>very occasionally, we can stop the constant transactional postings (and
>label printing) for a few minutes... rally, just a few. Otherwise, it
>becomes much like the "I Love Lucy" chocolate factory conveyor belt scene.
>
>What, seriously, are we going to do in this situation. I was kin of hoping
>we could find a *stable* Linux... meaning a kernel that does not need that
>much or *any* patching. Are you talking about real security problems, or
>feature upgrades? We simply can not bring the mahcine down for either
>reason... at least not on *any* kind of ongoing basis.... how in the world
>does *anyone* cope with such a situation.
>
>Yes, yes, I'm constantly considering and devising possible methods to
>de-reference our main databases and CPU's from this immediate hardware
>interface... but to date, I have not come up with anything that would work
>well enough to meet the need. Our systems are currently up-to-the-minute and
>pretty much *have* to stay that way.
>
>Suggestions?
>
>John Esak
>
>  
>
John,

I was thinking about this over the weekend.  It seems to me that you 
could give yourself a whole lot of flexibility if you could somehow 
duplicate the database you're working with.  I think that I could do 
this if the database was not stored on the same machine as that which is 
executing the filePro code.

Here is how I see it working:
Run two different servers each with its own copy of the database files.  
One is the one that is directly accessed by the users while the other 
gets updated with all of its transactions.  Whenever it gets a 
transaction it generates a record of the transaction as a separate file 
for the second database to read.  The second database would have a 
process that will look for these transactions and update the files on 
its database. 

You could set up a controlled switch from the server running the first 
database to the one running the second database.  At the end of each 
transaction executed on the data of the primary database you can have 
code that will check some status flag as to the condition of the server 
of that database.  You can program the process to force the user to exit 
out of the application if it sees a flag that tells it to switch to the 
secondary database files.  Once it exits you change the clients' 
configuration to work from the secondary database upon re-execution.  
All transactions will eventually move on to the second database server 
until all processes have transfered leaving the first free for any 
changes or updates.  In the meantime that secondary database has now 
started acting as the primary database and is building up a list of 
transactions that the original database will have to update to bring it 
to the same condition as the first once it is started up again.  (Or you 
might be able to just copy files.)

I don't know much about Linux, but I could see how an application 
working on the same computer as the data would have a much harder time 
detecting and adjusting for a switch to run off a different server.  But 
if all you're doing is going to a virtual drive similar to how I do it 
now--by running the processes on separate windows machines while they 
look to mapped network drives for the data--it is a much easier process 
to have a script that will exit out of the process, change the mapped 
network drive to point to the new server, and then re execute.

It seems like this will work well enough, but not knowing the actual 
application I don't know if this is a good solution for you.

Boaz
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