[Discuss] Redundant array of inexpensive servers: clustering?

[bill.n1vux at gmail.com (Bill Ricker)]

Hi Rich !

Commercial practice varies. The nicest HA solutions available today do
require apps be "cloud" enabled, which is to say fully virtualized; you can
then in-house them by building your own mini-cloud.

Choice 1 is whether storage is replicated or shared. Shared can be a
cluster FS or a backend storage system (either of which should have
internal redundancy).

Choice 2 is running on OS-on-Iron or in managed VM's/cloud etc.

Choice 3 is whether Apps run hot-hot, hot-cold, or hot-warm on the multiple
nodes.
   For classical web apps, hot-hot load balancing is simple and effective -
either with session state split between encrypted cookie and DB, or with
sticky-sessions in load-balancer.
   Hot-cold failover with restart on 2nd server when failure is detected is
annoying config management but perfectly doable, but must be configured for
each app. We did a lot of this for legacy apps at my last shop. Veritas and
each OS vendor have proprietary managers for this. Since they had multiple
vendors there, Veritas was used. Problems with hot-cold is if App's shared
storage is corrupted, it won't move; and since cold on node 2, the needed
config there isn't always kept updated.

If you make both your servers into VM hosts of any VM brand that allows
load-balancing, and package each of your services into VM images, you can
script auto(re)start of services.

I note the upcoming BLU presentation in OpenStack, which is a one of many
potential toolkits of possible use.
- Bill