[rdo-list] tripleo ha topology

[Dan Sneddon]

Actually, we have been trying to move away from Pacemaker where
possible, but certain services are not perfectly active/active, so we
need an additional layer on top to ensure that connections go to the
right server(s). Galera is one of the services that still benefits from
pcs. Andrew Beekhof has a great overview of the changes in progress
here [1]. From that doc:

  "Generally we use round-robin to distriute load amongst
  instances of active/active services, however Galera and
  Qpid use the stick-table options to ensure that incoming
  connections to the virtual IP (VIP) should be directed to
  only one of the available backends.

  In Galera's case, although it can run active/active, this
  helps avoid lock contention and prevent deadlocks. It is
  used in combination with the httpchk option that ensures
  only nodes that are in sync with its peers are allowed to
  handle requests.

  [snip]

  Although galera supports active/active configurations, we
  recommend active/passive (enforced by the load balancer)
  in order to avoid lock contention."

Another service which is currently active/passive is Cinder. In theory,
this can be made active/active, but there are lingering concerns about
edge cases that make active/passive safer for now.

We are likely moving away from Pacemaker entirely in the long run, but
there are some technical hurdles to overcome before we can do that.

The good news is that most services do not require pcs, and rely only
on systemd. You can now distribute systemd services between multiple
sets of servers, perhaps moving Neutron services to their own set of
servers, or perhaps moving the active/active services to a horizontally
scalable set of API servers. The remaining services which require pcs
will have to be colocated on a 3-node Pacemaker cluster for now.

[1] - https://github.com/beekhof/osp-ha-deploy/blob/master/ha-openstack.md
-- 
Dan Sneddon         |  Senior Principal OpenStack Engineer
dsneddon at redhat.com |  redhat.com/openstack
dsneddon:irc        |  @dxs:twitter

On 12/14/2016 01:59 PM, Dotan, Alon wrote:
> Hey,
> 
> Some can provide link or explanation about the use of haproxy and
> pacemaker together in tripleo?
> 
> Why need pcs if there is galera and haproxy?
> 
> Thanks,
> 
> 
> 
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