7. Add Apache HTTP Server as a Cluster Service¶
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 is relatively simple to configure.
7.1. Install Apache¶
Before continuing, we need to make sure Apache is installed on both
hosts. We will also allow the cluster to use the wget tool (this is the
default, but curl is also supported) to check the status of the Apache
server. We’ll install httpd (Apache) and wget now.
# dnf 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.
7.2. Create Website Documents¶
We need to create a page for Apache to serve. On AlmaLinux 9, 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
7.3. Enable the Apache Status URL¶
Pacemaker uses the apache resource agent to monitor the health of your
Apache instance via the server-status URL, and to recover the instance if
it fails. On both nodes, configure this URL as follows:
# 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.
7.4. Configure the Cluster¶
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 agent 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, monitor, and other 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
No defaults set
[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
Meta Attrs: op_defaults-meta_attributes
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 for the behavior observed in your environment, rather than adjusting the global default.
Note
If you use a tool like pcs to create a resource, its operations may be
automatically configured with explicit timeout values that override the
Pacemaker built-in default value of 20 seconds. If the resource agent’s
metadata contains suggested values for the operation timeouts in a
particular format, pcs reads those values and adds them to the
configuration at resource creation time.
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-1 (version 2.1.2-4.el9-ada5c3b36e2) - partition with quorum
* Last updated: Wed Jul 27 00:47:44 2022
* Last change: Wed Jul 27 00:47:23 2022 by root via cibadmin on pcmk-1
* 2 nodes configured
* 3 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* fence_dev (stonith:some_fence_agent): Started pcmk-1
* ClusterIP (ocf:heartbeat:IPaddr2): Started pcmk-1
* WebSite (ocf:heartbeat:apache): Started pcmk-2
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.
7.5. 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 else one of them should not run at all, if they cannot run on
the same node). Here, we instruct the cluster that WebSite can only run on
the host where ClusterIP is active.
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.
Note
INFINITY is the default score for a colocation constraint. If you don’t
specify a score, INFINITY will be used automatically.
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-1 (version 2.1.2-4.el9-ada5c3b36e2) - partition with quorum
* Last updated: Wed Jul 27 00:49:33 2022
* Last change: Wed Jul 27 00:49:16 2022 by root via cibadmin on pcmk-1
* 2 nodes configured
* 3 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* fence_dev (stonith:some_fence_agent): Started pcmk-1
* 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
7.6. Ensure Resources Start and Stop in Order¶
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 also starts before WebSite. A colocation constraint ensures
only that the resources run together; it doesn’t affect order in which the
resources are started or stopped.
We do this by adding an ordering constraint. By default, all order constraints
are mandatory. This means, for example, that if ClusterIP needs to stop,
then WebSite must stop first (or already be stopped); and if WebSite needs
to start, then ClusterIP must start first (or already be started). This
also implies that the recovery of ClusterIP will trigger the recovery of
WebSite, causing it to be restarted.
[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:
Note
The default action in an order constraint is start If you don’t
specify an action, as in the example above, pcs automatically uses the
start action.
Note
We could have placed the ClusterIP and WebSite resources into a
resource group instead of configuring constraints. A resource group is
a compact and intuitive way to organize a set of resources into a chain of
colocation and ordering constraints. We will omit that in this guide; see
the Pacemaker Explained
document for more details.
7.7. Prefer One Node Over Another¶
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-2=50
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
Resource: WebSite
Enabled on:
Node: pcmk-2 (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-1 (version 2.1.2-4.el9-ada5c3b36e2) - partition with quorum
* Last updated: Wed Jul 27 00:51:13 2022
* Last change: Wed Jul 27 00:51:07 2022 by root via cibadmin on pcmk-1
* 2 nodes configured
* 3 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* fence_dev (stonith:some_fence_agent): Started pcmk-1
* 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
Wait a minute, the resources are still on pcmk-1!
Even though WebSite now prefers to run on pcmk-2, 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
[ pcmk-1 pcmk-2 ]
fence_dev (stonith:some_fence_agent): Started pcmk-1
ClusterIP (ocf:heartbeat:IPaddr2): Started pcmk-1
WebSite (ocf:heartbeat:apache): Started pcmk-1
pcmk__native_allocate: fence_dev allocation score on pcmk-1: 100
pcmk__native_allocate: fence_dev allocation score on pcmk-2: 0
pcmk__native_allocate: ClusterIP allocation score on pcmk-1: 200
pcmk__native_allocate: ClusterIP allocation score on pcmk-2: 50
pcmk__native_allocate: WebSite allocation score on pcmk-1: 100
pcmk__native_allocate: WebSite allocation score on pcmk-2: -INFINITY
7.8. Move Resources Manually¶
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-2.
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 pcs to get rid of the temporary constraint
automatically after the resource has moved to its destination. Note in the
below that the pcs constraint output after the move command is the same
as before.
[root@pcmk-1 ~]# pcs resource move WebSite pcmk-2
Location constraint to move resource 'WebSite' has been created
Waiting for the cluster to apply configuration changes...
Location constraint created to move resource 'WebSite' has been removed
Waiting for the cluster to apply configuration changes...
resource 'WebSite' is running on node 'pcmk-2'
[root@pcmk-1 ~]# pcs constraint
Location Constraints:
Resource: WebSite
Enabled on:
Node: pcmk-2 (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-1 (version 2.1.2-4.el9-ada5c3b36e2) - partition with quorum
* Last updated: Wed Jul 27 00:54:23 2022
* Last change: Wed Jul 27 00:53:48 2022 by root via cibadmin on pcmk-1
* 2 nodes configured
* 3 resource instances configured
Node List:
* Online: [ pcmk-1 pcmk-2 ]
Full List of Resources:
* fence_dev (stonith:some_fence_agent): Started pcmk-1
* 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
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. |