[Fedora-directory-users] How to make application connect to multi-master set-up?

[Les Mikesell]

Eddie C wrote:

> If you custom code your application you can set it up to handle multiple A
> records. But you need to recode your applications each application has 
> to be
> reconfigured each time you add a server to the group.

No it doesn't.  The client just needs to retry on all the IP addresses 
that the DNS request returns.  Add a server, add it's address to DNS, done.

> As to the cost factor. Yes buying a load balancer might cost $2000.00. you
> might be able to ebay one for $1000.(Linux Virtual Server is open source 
> and > GPL but that is another story.)

You can easily pay $30,000 and up for a load balancer.  Remember that it 
needs to be redundant and more reliable than the servers it balances to 
help any.  Then there is maintenance - and repeat for every site.  A 
good client software library routine would fix it for everyone.

 > How much does it really cost to recode  your
> applications, test, and redeploy?

Why do it any other way in the first place?  If you get alternative DNS 
addresses and the one you try first doesn't accept your connection, why 
shouldn't every application do the sensible thing?  If IE can do it...

 > Probably a lot more work then $2000. Our
> LDAP database is the corner stone of our company. We would have to recorde
> 10 applications to achieve our own round robin.

Does this mean you don't have a common library routine that connects to 
the server?

> And would only get some of
> the features of a hardware load balancer.

And you get some the load balancer can't provide.

> Google seems to be taking a hybrid approach. They likely use GEO-DNS,
> mutliple A records. and hardware load balancing. Of course they are
> multi-datacenter.
> 
> Non-authoritative answer:
> Name:    www.l.google.com
> Addresses:  216.239.37.99, 216.239.37.104
> 
> I would be willing to bet that  216.239.37.99 and 216.239.37.104 are
> hardware load balancers.

And you can bet that Google has spent hundreds of thousands on the 
balancing setup with DNS servers that are aware of the state of the 
servers behind a large number of local load balancers.

> For our deployment I have a two node LDAP system (multi master) If I drop
> one of the nodes the IP floats to the other node within a few seconds. We
> did not have to recode any application, just configure them with a floating
> IP address. Some of our developers have built failover into their apps. I
> think its just extra code that there is already a proven solution to. I 
> am a fan of mutli-master and true TCP load balancing, but thats just me.

I use hardware balancers too, but I recognize that most of what they do 
is cover up a problem of dumb clients that don't know enough to try the 
alternate address(es) that they already have.

-- 
   Les Mikesell
    lesmikesell at gmail.com