Cluster Troubleshooting

Symptoms

Below is a list of potential problems with a Confluence cluster, and their likely solutions. The solutions are listed below.

Which multicast address?

The multicast address and port used by Confluence can be found on the Cluster Administration page, or in confluence.cfg.xml in the Confluence home directory.

Debugging tools

Listed below are some debugging tools that help determine what the status of the multicast traffic is:

Add multicast route

Multicast networking requirements vary across operating systems. Some operating systems require little configuration, while some require the multicast address to be explicitly added to a network interface before Confluence can use it.

Usually, adding a route for all multicast traffic to use the correct interface will fix multicast traffic. The example below is for a Ubuntu Linux system:

route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0

To support multiple applications using multicast on different interfaces, you may need to specify a route specific to the Confluence multicast address.

Check firewall

Ensure your firewall allows UDP traffic on the multicast address and port used by Confluence.

Change multicast interface

Confluence might have selected the incorrect interface for multicast traffic, which means it cannot connect to other nodes in the cluster. To override the interface used for multicast traffic after initial setup, edit confluence.cfg.xml in the Confluence home directory and add a property (or change the existing one) to select your desired network interface. For example to tell Confluence to use eth1:

<property name="confluence.cluster.interface">eth1</property>

Increase multicast TTL

The multicast time-to-live (TTL) specifies how many hops a multicast packet should be allowed to travel before it is discarded by a router. It should be set to the number of routers in between your clustered nodes: 0 if both are on the same machine, 1 if on two different machines linked by a switch or cable, 2 if on two different machines with one intermediate router, and so on.

Create a file in the Confluence home directory called tangosol-coherence-override.xml. Add the following to it, setting the TTL value appropriately (1 is the default):

<?xml version='1.0'?>
<coherence>
<cluster-config>
  <multicast-listener>
    <time-to-live system-property='tangosol.coherence.ttl'>1</time-to-live>
  </multicast-listener>
</cluster-config>
</coherence>

Alternatively, simply start Confluence with the system property: -Dtangosol.coherence.ttl=1. Again, 1 is the default value, and you should change it to something appropriate to your network topology.

Check intermediate routers

Advanced switches and routers have the ability to understand multicast traffic, and route it appropriately. Unfortunately sometimes this functionality doesn't work correctly with the multicast management information (IGMP) published by the operating system running Confluence.

If multicast traffic is problematic, try disabling advanced multicast features on switches and routers in between the clustered nodes. These features can prevent multicast traffic being transmitted by certain operating systems.

For best results, use the simplest network topology possible for the cluster traffic between the nodes. For two nodes, that means a single network cable. For larger numbers, try using a single high-quality switch.

Advanced Tangosol configuration

If the solution to your problem involves changes to the Tangosol configuration, these changes should not be made to the Confluence configuration in confluence/WEB-INF/classes/. Instead, to ensure your configuration survives upgrades, make your changes via:

• Tangosol system properties
• creating a tangosol-coherence-override.xml file in the Confluence home directory.

Examples of making these changes are shown in the increasing the TTL section.

Didn't find a solution? Contact Atlassian support

We have dedicated staff on hand to support your installation of Confluence. Please follow the instructions for raising a support request and mention that you're having trouble setting up your Confluence cluster.