How to set up Tomcat active-active with PCS

An active-active Tomcat setup keeps multiple application instances available so incoming traffic can be balanced across nodes and a single host failure does not become an outage.

In a Pacemaker / Corosync cluster managed with pcs, a systemd unit can be registered as a cluster resource and monitored for health. Cloning that resource runs an instance on each eligible node, while the cluster tracks failures and restarts the unit when monitoring detects a problem.

Cluster control changes the normal service lifecycle, so the underlying Tomcat unit should remain disabled in systemd and only started through the cluster. Active-active does not replicate deployments or runtime state, so application files and configuration must be identical on every node and session state or uploads should use a shared backend or load balancer stickiness. Production clusters typically require fencing (STONITH) and a load balancer with health checks, since pcs does not distribute traffic.

Steps to set up Tomcat active-active with PCS:

Confirm the cluster is online with quorum.

$ sudo pcs status
Cluster name: clustername
Cluster Summary:
  * Stack: corosync (Pacemaker is running)
  * Current DC: node-01 (version 2.1.6-6fdc9deea29) - partition with quorum
  * 3 nodes configured
  * 0 resource instances configured
##### snipped #####

Identify the Tomcat systemd service unit name.
```
$ systemctl list-unit-files --type=service | grep -E '^tomcat.*\.service'
tomcat10.service                             disabled        enabled
```
Strip the trailing .service when using the systemd: resource type.

Related: How to manage Pacemaker cluster services with systemctl in Linux
Disable the Tomcat unit outside the cluster so Pacemaker controls start and stop actions.
```
$ sudo systemctl disable --now tomcat10.service
```
Leaving Tomcat enabled in systemd can start it automatically at boot even when the cluster intends it to be stopped, causing conflicting state and confusing resource status.

Related: How to manage Pacemaker cluster services with systemctl in Linux
Create the Tomcat service resource.
```
$ sudo pcs resource create tomcat_service systemd:tomcat10 op monitor interval=30s
```
Use systemd:tomcat10 when that unit is present.

Related: How to create a Pacemaker resource
Clone the Tomcat service resource across nodes.
```
$ sudo pcs resource clone tomcat_service clone-max=2 clone-node-max=1
```
Omit clone-max to start an instance on every cluster node by default.

Related: How to create a cloned resource in Pacemaker
Related: How to set Pacemaker clone resource options

Verify the cloned resource status.

$ sudo pcs status resources
  * Clone Set: tomcat_service-clone [tomcat_service]:
    * Started: [ node-01 node-02 ]

Confirm Tomcat is listening on the expected connector port on each node.

$ sudo ss --listening --numeric --tcp --processes 'sport = :8080'
State  Recv-Q Send-Q Local Address:Port Peer Address:PortProcess
LISTEN 0      100                *:8080            *:*    users:(("java",pid=220191,fd=38))

Replace 8080 with the port configured by the Connector in server.xml.

Configure the load balancer or DNS to distribute traffic across active nodes.

Session-dependent applications require sticky sessions or an external session store to avoid user logouts when requests move between nodes.

Related: How to plan traffic distribution for active-active PCS services
Run a failover test after distribution is updated.

Related: How to run a Pacemaker failover test with PCS