[ClusterLabs] Fwd: Multi cluster

[Ken Gaillot]

On Fri, 2017-08-04 at 18:35 +0200, Jan Pokorný wrote:
> On 03/08/17 20:37 +0530, sharafraz khan wrote:
> > I am new to clustering so please ignore if my Question sounds silly, i have
> > a requirement were in i need to create cluster for ERP application with
> > apache, VIP component,below is the scenario
> > 
> > We have 5 Sites,
> > 1. DC
> > 2. Site A
> > 3. Site B
> > 4. Site C
> > 5. Site D
> > 
> > Over here we need to configure HA as such that DC would be the primary Node
> > hosting application & be accessed from by all the users in each sites, in
> > case of Failure of DC Node, Site users should automatically be switched to
> > there local ERP server, and not to the Nodes at other sites, so
> > communication would be as below
> > 
> > DC < -- > Site A
> > DC < -- > Site B
> > DC < -- > Site C
> > DC < -- > Site D
> > 
> > Now the challenge is
> > 
> > 1. If i create a cluster between say DC < -- > Site A it won't allow me to
> > create another cluster on DC with other sites

Right, your choices (when using corosync+pacemaker) are one big cluster
with all sites (including the data center), or an independent cluster at
each site connected by booth.

It sounds like your secondary sites don't have any communication between
each other, only to the DC, so that suggests that the "one big cluster"
approach won't work.

For more details on pacemaker+booth, see:

http://clusterlabs.org/doc/en-US/Pacemaker/1.1-pcs/html-single/Pacemaker_Explained/index.html#idm139900093104976


> > 2. if i setup all the nodes in single cluster how can i ensure that in case
> > of Node Failure or loss of connectivity to DC node from any site, users
> > from that sites should be switched to Local ERP node and not to nodes on
> > other site.

The details depend on the particular service. Unfortunately I don't have
any experience with ERP, maybe someone else can jump in with tips.

How do users contact the ERP node? Via an IP address, or a list of IP
addresses that will be tried in order, or some other way?

Is the ERP service itself managed by the cluster? If so, what resource
agent are you using? Does the agent support cloning or master/slave
operation?

> > 
> > a urgent response and help would be quite helpful
> 
> From your description, I suppose you are limited to just a single
> machine per site/DC (making the overall picture prone to double
> fault, first DC goes down, then any of the sites goes down, then
> at least the clients of that very site encounter the downtime).
> Otherwise I'd suggest looking at booth project that facilitates
> inter-cluster (back to your "multi cluster") decisions, extending
> upon pacemaker performing the intra-cluster ones.
> 
> Using a single cluster approach, you should certainly be able to
> model your fallback scenario, something like:
> 
> - define a group A (VIP, apache, app), infinity-located with DC
> - define a different group B with the same content, set up as clone
>   B_clone being (-infinity)-located with DC
> - set up ordering "B_clone starts when A stops", of "Mandatory" kind
> 
> Further tweaks may be needed.

-- 
Ken Gaillot <kgaillot at redhat.com>