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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