On 09/28/2011 04:47 AM, Alan Wright wrote:
> I am migrating from resin 2 to 4.0.22 and have a single host configured
> with a web-loadbalance tier (1 server) and an application tier (3 servers).
> We have elected to do this for resilience and also to make adding hosts
> at a later date easier to implement.
> We start 3 servers on the app-tier to create a "triad" but it may be
> that we only really need 2 servers to get basic failover resilience. Is
> there any benefit from having a triad or am i just wasting resources
> with the overhead of loading the same application environment 3 times?
Three might be overkill in your configuration. Statistically, it would
give you a better resilience.
A main scenario for the triad was a customer case where they took down
servers periodically for maintenance and had a crash at the same time
(this was before the triad or the hub-and-spoke model and with an
8-server cluster), and therefore some of their persistent sessions were
The three server triad gives you a backup, even when you take down a
triad server deliberately. I don't think that scenario applies in your case.
Another main reason is failover load. On a fully-loaded system, taking
down (or crashing) one of three servers means the other two work 50%
harder. If there were only two, it would be 100% which could cause a
cascading failure. That scenario doesn't apply because your backup
servers are using the same CPU, so you shouldn't have a cascading failure.
In your case, since you only have the extra servers for reliability, and
they're on the same machine, the third server isn't as important.
> Also each of the app-tier servers is pointing at the same webapp
> directory since they are all on the same host. Is this OK or is it
> Additionally the application is started as root and for the app tier we
> use<user> and<group> to change the user. When we try to do the same
> thing in the web-loadbalancer tier the application fails to start. Is
> this normal/to be expected? Is it safe for the web-tier to be running as
> Thanks to those who have responded to my earlier queries.
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