Re: application based HA

Andras POTOCZKY wrote:
> Hi List,
>
> I would like to configure an application based HA solution. I mean if 
> my important application stops working (but the active server is still 
> alive), the HA configuration is passing the shared IP to the standby 
> node.
>
> I've found pacemaker is the tool for it but I couldnt find any 
> document or example related to the application level HA.
>
> Can somebody point me how can I figure this out?
>
>

The short answer: Look for documentation on resource agents.  If your 
application has a pre-existing resource agent defined, then you set that 
up and it handles failover.

The longer answer is that application-level failover is difficult, 
because it involves things like:
- IP addresses
- process state
- work queues
- data files
- etc.
All of which are unique to a specific application.

Hence, it becomes a lot easier to:
- address failover at the virtual machine level (IP addresses, 
applications, processes)
- use either a storage area network, or DRBD to make sure that your data 
remains available and consistent

Some applications provide their own application-layer failover - such as 
databases with replication logic built in:
- put them behind a load leveler, and you're done (assuming your 
load-leveler isn't a single point of failure), or,
- if your public-facing DNS name points to multiple IP addresses, then 
you have a level of failover there, or,
- use CARP to support IP address failover

For other applications, it's quite a bit harder:  I've been trying to 
come up with a way to make our email and mailing-list infrastructure HA, 
at the application layer, but there are so many distinct processes 
running, and ques, and such, that just VM-level failover, with DRBD disk 
replication has always been far simpler. (The reason I keep looking is 
that I'd like to migrate to either a BSD or illumos based platform, but 
there's no Xen and no DRBD.)

Miles Fidelman



HAST
I've been wrestling with this one for a while - more in terms of setting 
up HA applications


-- 
In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra

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