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Re: Webserver Redundacy

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You may want to have a look at this

http://homepages.tesco.net/J.deBoynePollard/FGA/dns-round-robin-is- useless.html


"Some desktop clients may even try alternate addresses after a connection time out of 30-45 seconds."

It looks like you are correct, and some browsers seem to implement their round robin
dns like this.

This behavior is unfortunately not specified - and could be changed at any time.

On 28/02/2007, at 4:54 PM, Jim Popovitch wrote:

If this is the case, then you may also have a session persistence

:-) I thought you said that we would have a 50% loss problem.  Are you
just grasping for straws?  :-)

The problem is that there is no specification on how to deal with multiple
IP addresses for the one host.
  A) Do you swap IPs every request?
  B) Do you stay with the one IP till end of session?
  C) Do you stay with the one IP for 30 minutes?
  D) Do you change IP when you get a 'no able to connect'?

As it seems, at the moment, IEs implementation seems to solve your problem.

If the browser switches IP addresses between queries, the other web
server may be asked to respond, in which case, you will not be able to store state in the web server, and will need to store it in the backend - and
this between requests - not visits.

Yes.  But you assume that state is necessary to maintain.  It may not
be. My experience has shown me that browsers don't switch IP addresses
between queries, they use the first one that works and they keep using
it as long as it works. When it quits working they try the next one in
round-robin fashion.

Quoting one of the links above

Microsoft's Internet Explorer caches the results of domain name lookups internally, retaining the results of previous lookups for 30 minutes by default (which can be modified by adjusting the DnsCacheTimeout value of the HKEY_CURRENT_USER\Software\Microsoft \Windows\CurrentVersion\Internet Settings key in the Windows registry). This caching occurs separately within each Internet Explorer process, but an individual Internet Explorer process may be responsible for several web browser windows.

If you assume that you don't need to deal with session persistence due to
experience, you may end up getting bitten when someone changes the
behavior of <insert your favourite browser>.


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