Product SiteDocumentation Site

6.5. Ensure Resources Run on the Same Host

To reduce the load on any one machine, Pacemaker will generally try to spread the configured resources across the cluster nodes. However, we can tell the cluster that two resources are related and need to run on the same host (or not at all). Here, we instruct the cluster that WebSite can only run on the host that ClusterIP is active on.
To achieve this, we use a colocation constraint that indicates it is mandatory for WebSite to run on the same node as ClusterIP. The "mandatory" part of the colocation constraint is indicated by using a score of INFINITY. The INFINITY score also means that if ClusterIP is not active anywhere, WebSite will not be permitted to run.


If ClusterIP is not active anywhere, WebSite will not be permitted to run anywhere.


Colocation constraints are "directional", in that they imply certain things about the order in which the two resources will have a location chosen. In this case, we’re saying that WebSite needs to be placed on the same machine as ClusterIP, which implies that the cluster must know the location of ClusterIP before choosing a location for WebSite.
[root@pcmk-1 ~]# pcs constraint colocation add WebSite with ClusterIP INFINITY
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
Ordering Constraints:
Colocation Constraints:
  WebSite with ClusterIP (score:INFINITY)
Ticket Constraints:
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Stack: corosync
Current DC: pcmk-2 (version 1.1.18-11.el7_5.3-2b07d5c5a9) - partition with quorum
Last updated: Mon Sep 10 17:08:54 2018
Last change: Mon Sep 10 17:08:27 2018 by root via cibadmin on pcmk-1

2 nodes configured
2 resources configured

Online: [ pcmk-1 pcmk-2 ]

Full list of resources:

 ClusterIP      (ocf::heartbeat:IPaddr2):       Started pcmk-2
 WebSite        (ocf::heartbeat:apache):        Started pcmk-2

Daemon Status:
  corosync: active/disabled
  pacemaker: active/disabled
  pcsd: active/enabled