IPVS hosting on the Internet

[Dr. Volker Jaenisch]

Hello Thorsten!

Torsten Schlabach schrieb:
> Now if we want to build a reliable service on the Internet (say a
> website, for example) we can have one A record for www.oursite.net point
> to one IP address only. So we would want that IP address to
> transparently fail over between two data centers.
One way to achieve this is to use the DNS.

If you give your domain e.g. yourdomain.com more than one IP e.g.
Datacenter1 : IP = 123.123.123.1
Datacenter2 : IP = 146.234.12.2
the DNS performes a round robin loadbalancing on DNS -> IP resolving.

The first time a webbrowser accesses yourdomain.com it will get the first IP
and your customer lands in DC1. If the next webbrowser accesses 
yourdomain.com
it will land in DC2. Next in DC1 and so on.

The only problem with this approach is to assure that the DNS TTL 
settings are
long enougth so that a typical costomer will not be switched from DC to 
DC within its actual session.

A second and from my point of view the most important is the need for 
synchronisation of
your data e.g. your databases and the filesystem over the WAN between 
the DCs.

The databases you can share across the two locations using a sequoia 
DB-Cluster for e.g. MySQL, PGSQL databases.
The filesystem mirroring can be done using DRBD0.8 and a cluster filesystem.

We have done some testing in that direction. This approach is due to the 
synchronisation not ideal
for high traffic sites with many changes in the shared filesystem or the 
shared DB-Cluster.
If you have a low/medium traffic site with extreme need for high 
availability this scheme may help you.

But please be warned - this approach is definitivly not the cheap one. 
Such a WAN distributed Cluster
is in the order of $100.000 and more.

Best regards,

Volker

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