High availability on remote site

Mark Felder feld at FreeBSD.org
Thu Aug 15 12:18:45 UTC 2013

On Thu, 15 Aug 2013 18:19:35 +0700
Olivier Nicole <olivier.nicole at cs.ait.ac.th> wrote:

> Hi,
> I have been assigned to offer HA on a 3 tiers architecture.
> Data storage tier will be MySQL, so replication is easy.

Keep in mind that MySQL replication has plenty of its own issues. It
does not replicate every SQL command to the slave. Guaranteeing that
data on both servers is identical is also a very tricky process. You
might want to first browse through the sections here to get an idea:


> HA should be implemented only on the Data storage tier, Active/Active,
> but one of the sites is remote!
> When everything is working, each application accesses the local MySQL
> tier, but when the local MySQL becomes unavailable, it should be able
> to automatically move to the other database server.
> I have no access to the application, so I cannot modify it to test if
> local MySQL is working. So I should have an HA mechanism that enforces
> changing the IP address on the database server.

This is easy. Use HAProxy. It can test to see if your local MySQL
instance is up and running and if it detects it is not it will
automatically pass connections to the remote site's MySQL server.
> If both servers are installed at different places, with different
> addresses, would there be a way beside establishing an IP tunnel/VPN
> between both places to have all machines in a single subnet?

This seems unnecessary. Why do you need them to be on the same subnet?

> An image is here http://www.cs.ait.ac.th/~on/HA.gif
> I am really bothered by the IP tunnel, but that's the only way I see to keep HA.

Hopefully I've answered this question for you and you see that you
shouldn't need these to be on the same subnet. 

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