Now that we have a basic but functional active/passive two-node cluster, we’re ready to add some real services. We’re going to start with Apache HTTP Server because it is a feature of many clusters and relatively simple to configure.
Before continuing, we need to make sure Apache is installed on both hosts. We also need the wget tool in order for the cluster to be able to check the status of the Apache server.
# yum install -y httpd wget
# firewall-cmd --permanent --add-service=http
# firewall-cmd --reload
Important
Do not enable the httpd service. Services that are intended to be managed via the cluster software should never be managed by the OS. It is often useful, however, to manually start the service, verify that it works, then stop it again, before adding it to the cluster. This allows you to resolve any non-cluster-related problems before continuing. Since this is a simple example, we’ll skip that step here.
We need to create a page for Apache to serve. On CentOS Stream 8, the default Apache document root is /var/www/html, so we’ll create an index file there. For the moment, we will simplify things by serving a static site and manually synchronizing the data between the two nodes, so run this command on both nodes:
# cat <<-END >/var/www/html/index.html
<html>
<body>My Test Site - $(hostname)</body>
</html>
END
In order to monitor the health of your Apache instance, and recover it if it fails, the resource agent used by Pacemaker assumes the server-status URL is available. On both nodes, enable the URL with:
# cat <<-END >/etc/httpd/conf.d/status.conf
<Location /server-status>
SetHandler server-status
Require local
</Location>
END
Note
If you are using a different operating system, server-status may already be enabled or may be configurable in a different location. If you are using a version of Apache HTTP Server less than 2.4, the syntax will be different.
At this point, Apache is ready to go, and all that needs to be done is to add it to the cluster. Let’s call the resource WebSite. We need to use an OCF resource script called apache in the heartbeat namespace [1]. The script’s only required parameter is the path to the main Apache configuration file, and we’ll tell the cluster to check once a minute that Apache is still running.
[root@pcmk-1 ~]# pcs resource create WebSite ocf:heartbeat:apache \
configfile=/etc/httpd/conf/httpd.conf \
statusurl="http://localhost/server-status" \
op monitor interval=1min
By default, the operation timeout for all resources’ start, stop, and monitor operations is 20 seconds. In many cases, this timeout period is less than a particular resource’s advised timeout period. For the purposes of this tutorial, we will adjust the global operation timeout default to 240 seconds.
[root@pcmk-1 ~]# pcs resource op defaults update timeout=240s
Warning: Defaults do not apply to resources which override them with their own defined values
[root@pcmk-1 ~]# pcs resource op defaults
timeout: 240s
Note
In a production cluster, it is usually better to adjust each resource’s start, stop, and monitor timeouts to values that are appropriate to the behavior observed in your environment, rather than adjust the global default.
After a short delay, we should see the cluster start Apache.
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-2 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Tue Jan 26 19:38:22 2021
* Last change: Tue Jan 26 19:38:19 2021 by root via cibadmin on pcmk-1
* 2 nodes configured
* 2 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-2
* WebSite (ocf::heartbeat:apache): Started pcmk-1
Daemon Status:
corosync: active/disabled
pacemaker: active/disabled
pcsd: active/enabled
Wait a moment, the WebSite resource isn’t running on the same host as our IP address!
Note
If, in the pcs status output, you see the WebSite resource has failed to start, then you’ve likely not enabled the status URL correctly. You can check whether this is the problem by running:
wget -O - http://localhost/server-status
If you see Not Found or Forbidden in the output, then this is likely the problem. Ensure that the <Location /server-status> block is correct.
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.
Note
If ClusterIP is not active anywhere, WebSite will not be permitted to run anywhere.
Important
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
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-2 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Tue Jan 26 19:45:11 2021
* Last change: Tue Jan 26 19:44:30 2021 by root via cibadmin on pcmk-1
* 2 nodes configured
* 2 resource instances configured
Node List:
* 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
Like many services, Apache can be configured to bind to specific IP addresses on a host or to the wildcard IP address. If Apache binds to the wildcard, it doesn’t matter whether an IP address is added before or after Apache starts; Apache will respond on that IP just the same. However, if Apache binds only to certain IP address(es), the order matters: If the address is added after Apache starts, Apache won’t respond on that address.
To be sure our WebSite responds regardless of Apache’s address configuration, we need to make sure ClusterIP not only runs on the same node, but starts before WebSite. A colocation constraint only ensures the resources run together, not the order in which they are started and stopped.
We do this by adding an ordering constraint. By default, all order constraints are mandatory, which means that the recovery of ClusterIP will also trigger the recovery of WebSite.
[root@pcmk-1 ~]# pcs constraint order ClusterIP then WebSite
Adding ClusterIP WebSite (kind: Mandatory) (Options: first-action=start then-action=start)
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
Ordering Constraints:
start ClusterIP then start WebSite (kind:Mandatory)
Colocation Constraints:
WebSite with ClusterIP (score:INFINITY)
Ticket Constraints:
Pacemaker does not rely on any sort of hardware symmetry between nodes, so it may well be that one machine is more powerful than the other.
In such cases, you may want to host the resources on the more powerful node when it is available, to have the best performance – or you may want to host the resources on the less powerful node when it’s available, so you don’t have to worry about whether you can handle the load after a failover.
To do this, we create a location constraint.
In the location constraint below, we are saying the WebSite resource prefers the node pcmk-1 with a score of 50. Here, the score indicates how strongly we’d like the resource to run at this location.
[root@pcmk-1 ~]# pcs constraint location WebSite prefers pcmk-1=50
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
Resource: WebSite
Enabled on: pcmk-1 (score:50)
Ordering Constraints:
start ClusterIP then start WebSite (kind:Mandatory)
Colocation Constraints:
WebSite with ClusterIP (score:INFINITY)
Ticket Constraints:
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-2 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Tue Jan 26 19:46:52 2021
* Last change: Tue Jan 26 19:46:40 2021 by root via cibadmin on pcmk-1
* 2 nodes configured
* 2 resource instances configured
Node List:
* 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
Wait a minute, the resources are still on pcmk-2!
Even though WebSite now prefers to run on pcmk-1, that preference is (intentionally) less than the resource stickiness (how much we preferred not to have unnecessary downtime).
To see the current placement scores, you can use a tool called crm_simulate.
[root@pcmk-1 ~]# crm_simulate -sL
Current cluster status:
Online: [ pcmk-1 pcmk-2 ]
ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-2
WebSite (ocf::heartbeat:apache): Started pcmk-2
Allocation scores:
native_color: ClusterIP allocation score on pcmk-1: 50
native_color: ClusterIP allocation score on pcmk-2: 200
native_color: WebSite allocation score on pcmk-1: -INFINITY
native_color: WebSite allocation score on pcmk-2: 100
Transition Summary:
There are always times when an administrator needs to override the cluster and force resources to move to a specific location. In this example, we will force the WebSite to move to pcmk-1.
We will use the pcs resource move command to create a temporary constraint with a score of INFINITY. While we could update our existing constraint, using move allows to easily get rid of the temporary constraint later. If desired, we could even give a lifetime for the constraint, so it would expire automatically – but we don’t do that in this example.
[root@pcmk-1 ~]# pcs resource move WebSite pcmk-1
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
Resource: WebSite
Enabled on: pcmk-1 (score:50)
Enabled on: pcmk-1 (score:INFINITY) (role: Started)
Ordering Constraints:
start ClusterIP then start WebSite (kind:Mandatory)
Colocation Constraints:
WebSite with ClusterIP (score:INFINITY)
Ticket Constraints:
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-2 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Tue Jan 26 19:49:27 2021
* Last change: Tue Jan 26 19:49:10 2021 by root via crm_resource on pcmk-1
* 2 nodes configured
* 2 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-1
* WebSite (ocf::heartbeat:apache): Started pcmk-1
Daemon Status:
corosync: active/disabled
pacemaker: active/disabled
pcsd: active/enabled
Once we’ve finished whatever activity required us to move the resources to pcmk-1 (in our case nothing), we can then allow the cluster to resume normal operation by removing the new constraint. Due to our first location constraint and our default stickiness, the resources will remain on pcmk-1.
We will use the pcs resource clear command, which removes all temporary constraints previously created by pcs resource move or pcs resource ban.
[root@pcmk-1 ~]# pcs resource clear WebSite
Removing constraint: cli-prefer-WebSite
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
Resource: WebSite
Enabled on: pcmk-1 (score:50)
Ordering Constraints:
start ClusterIP then start WebSite (kind:Mandatory)
Colocation Constraints:
WebSite with ClusterIP (score:INFINITY)
Ticket Constraints:
Note that the INFINITY location constraint is now gone. If we check the cluster status, we can also see that (as expected) the resources are still active on pcmk-1.
[root@pcmk-1 ~]# pcs status
Cluster name: mycluster
Cluster Summary:
* Stack: corosync
* Current DC: pcmk-2 (version 2.0.5-4.el8-ba59be7122) - partition with quorum
* Last updated: Tue Jan 26 19:50:52 2021
* Last change: Tue Jan 26 19:50:24 2021 by root via crm_resource on pcmk-1
* 2 nodes configured
* 2 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* ClusterIP (ocf::heartbeat:IPaddr2): Started pcmk-1
* WebSite (ocf::heartbeat:apache): Started pcmk-1
Daemon Status:
corosync: active/disabled
pacemaker: active/disabled
pcsd: active/enabled
To remove the constraint with the score of 50, we would first get the constraint’s ID using pcs constraint –full, then remove it with pcs constraint remove and the ID. We won’t show those steps here, but feel free to try it on your own, with the help of the pcs man page if necessary.
| [1] | Compare the key used here, ocf:heartbeat:apache, with the one we used earlier for the IP address, ocf:heartbeat:IPaddr2 |