Enabling Cluster Link

Enabling Cluster Link automatically activates distributed caching. The cache is distributed across multiple Liferay DXP nodes running concurrently. Cluster Link does Ehcache replication. The Ehcache global settings are in the portal.properties file.

By default Liferay does not copy cached entities between nodes. If an entity is deleted or changed, for example, Cluster Link sends a remove message to the other nodes to invalidate this entity in their local caches. Requesting that entity on another node results in a cache miss; the entity is then retrieved from the database and put into the local cache. Entities added to one node’s local cache are not copied to local caches of the other nodes. An attempt to retrieve a new entity on a node which doesn’t have that entity cached results in a cache miss. The miss triggers the node to retrieve the entity from the database and store it in its local cache.

Figure 1: Liferay DXPs cache algorithm is extremely efficient.

Figure 1: Liferay DXP's cache algorithm is extremely efficient.

To enable Cluster Link, add this property to portal-ext.properties:


Cluster Link depends on JGroups and provides an API for nodes to communicate. It can

  • Send messages to all nodes in a cluster
  • Send messages to a specific node
  • Invoke methods and retrieve values from all, some, or specific nodes
  • Detect membership and notify when nodes join or leave

When you start @portal@ in a cluster, a log file message shows your cluster’s name (e.g., cluster=liferay-channel-control):

GMS: address=oz-52865, cluster=liferay-channel-control, physical address= 

Cluster Link contains an enhanced algorithm that provides one-to-many type communication between the nodes. This is implemented by default with JGroups’s UDP multicast, but unicast and TCP are also available.

Multicast Over UDP

When you enable Cluster Link, Liferay DXP’s default clustering configuration is enabled. This configuration defines IP multicast over UDP. Liferay DXP uses two groups of channels from JGroups to implement this: a control group and a transport group. If you want to customize the channel properties, you can do so in portal-ext.properties:

cluster.link.channel.name.control=[your control channel name]
cluster.link.channel.properties.control=[your control channel properties]

Please see JGroups’s documentation for channel properties. The default configuration sets many properties whose settings are discussed there.

Multicast broadcasts to all devices on the network. Clustered environments on the same network communicate with each other by default. Messages and information (e.g., scheduled tasks) sent between them can lead to unintended consequences. Isolate such cluster environments by either separating them logically or physically on the network, or by configuring each cluster’s portal-ext.properties to use different sets of multicast group address and port values.

JGroups sets a bind address automatically, using localhost by default. In some configurations, however, localhost is bound to the internal loopback network ( or ::1), rather than the host’s real address. As long as Liferay DXP’s cluster.link.autodetect.address Portal Property points to a server that’s contactable, Liferay DXP uses that server to automatically detect your host’s real address. Here’s the default setting:


Contacting Google may not work if your server is behind a firewall.

An alternative to detecting the host address automatically for the bind address, you can set the bind address manually in your portal-ext.properties file.

  1. Disable address auto-detection by setting the cluster.link.autodetect.address property to an empty value:

  2. Set the following properties to your host’s IP address:

    cluster.link.bind.addr["cluster-link-control"]=[place your IP address or host name here]
    cluster.link.bind.addr["cluster-link-udp"]=[place your IP address or host name here]

Your network configuration may preclude the use of multicast over TCP, so below are some other ways you can get your cluster communicating. Note that these methods are all provided by JGroups.


  1. If you are binding the IP address instead of using localhost, make sure the right IP addresses are declared using these properties:

  2. Test your load and then optimize your settings if necessary.

Unicast over TCP

If your network configuration or the sheer distance between nodes prevents you from using UDP Multicast clustering, you can configure TCP Unicast. You must use this if you have a firewall separating any of your nodes or if your nodes are in different geographical locations.

  1. Add a parameter to your app server’s JVM:

    -Djgroups.bind_addr=[place your IP address or host name here]

    Use the node’s IP address or host name.

  2. Now you must determine the discovery protocol the nodes should use to find each other. You have four choices:

    - TCPPing
    - JDBCPing
    - S3_Ping
    - Rackspace_Ping

    If you aren’t sure which one to choose, use TCPPing. This is used in the rest of these steps; the others are covered below.

  3. Extract the tcp.xml file from $LIFERAY.HOME/osgi/marketplace/Liferay Foundation - Liferay Portal - Impl.lpkg/com​.​liferay​.​portal​.​cluster​.​multiple​-​[version].​jar/lib​/​jgroups​-​[version].​Final​.​jar/tcp.xml to a location accessible to Liferay DXP. to a location accessible to Liferay DXP. Use this file on all your nodes.

  4. If you’re vertically clustering (i.e., you have multiple servers running on the same physical or virtual system), you must change the port on which discovery communicates for all nodes other than the first one, to avoid TCP port collision. To do this, modify the TCP tag’s bind_port parameter:

    <TCP bind_port="[some unused port]"

    Since the default port is 7800, provide some other unused port.

  5. Add to the same tag the parameter singleton_name="liferay_cluster". This merges the transport and control channels to reduce the number of thread pools. See JGroups documentation for further information.

    Usually, no further JGroups configuration is required. However, in a very specific case, if (and only if) cluster nodes are deployed across multiple networks, then the parameter external_addr must be set on each host to the external (public IP) address of the firewall. This kind of configuration is usually only necessary when nodes are geographically separated. By setting this, clustered nodes deployed to separate networks (e.g. separated by different firewalls) can communicate together. This configuration may be flagged in security audits of your system. See JGroups documentation for more information.

  6. Save the file. Modify that node’s portal-ext.properties file to point to it:


You’re now set up for Unicast over TCP clustering! Repeat this process for each node you want to add to the cluster.


Rather than use TCP Ping to discover cluster members, you can use a central database accessible by all the nodes to help them find each other. Cluster members write their own and read the other members’ addresses from this database. To enable this configuration, replace the TCPPING tag with the corresponding JDBCPING tag:


The above example uses MySQL as the database. For further information about JDBC Ping, please see the JGroups Documentation.

S3 Ping

Amazon S3 Ping can be used for servers running on Amazon’s EC2 cloud service. Each node uploads a small file to an S3 bucket, and all the other nodes read the files from this bucket to discover the other nodes. When a node leaves, its file is deleted.

To configure S3 Ping, replace the TCPPING tag with the corresponding S3_PING tag:


Supply your Amazon keys as values for the parameters above. For further information about S3 Ping, please see the JGroups Documentation.

Other Pings

JGroups supplies other means for cluster members to discover each other, including Rackspace Ping, BPing, File Ping, and others. Please see the JGroups Documentation for information about these discovery methods.

Modifying the Cache Configuration with a Module

It’s recommended to test your system under a load that best simulates the kind of traffic your system must handle. If you serve a lot of message board messages, your script should reflect that. If web content is the core of your site, your script should reflect that too.

As a result of a load test, you may find that the default distributed cache settings aren’t optimized for your site. In this case, tweak the settings using a module. You can install the module on each node and change the settings without taking down the cluster.

We’ve made this as easy as possible by creating the project for you. Download the project and unzip it into a Liferay Workspace, in the workspace’s modules folder. To override your cache settings, you must only modify one Ehcache configuration file, which you’ll find in this folder structure:

  • src
    • main
      • java
        • resources
          • ehcache
            • override-liferay-multi-vm-clustered.xml

In the sample project, this file contains a configuration for the GroupImpl object which handles sites. You may wish to add other objects to the cache; in fact, the default file caches many other objects. For example, if you have a vibrant community, a large portion of your traffic may be directed at the message boards portlet, as mentioned above. To cache the threads on the message boards, configure a block with the MBMessageImpl class:


You can preserve the default settings while customizing them with your own by extracting Liferay’s cluster configuration file and putting it into your module project. You’ll find it in the com.liferay.portal.cache.ehcache.impl.jar file the [Liferay Home]/osgi/portal folder. The file you want is liferay-multi-vm-clustered.xml, in the /ehcache folder inside the com.liferay.portal.cache.ehcache.impl.jar file. Once you have the file, replace the contents of the override-liferay-multi-vm-clustered.xml file above with the contents of this file. Now you’ll be using the default configuration as a starting point.

Once you’ve made your changes to the cache, save the file, build, and deploy the module, and your settings override the default settings. In this way, you can tweak your cache settings so that your cache performs optimally for the type of traffic generated by your site. You don’t have restart your server to change the cache settings. This is a great benefit, but beware: since Ehcache doesn’t allow for changes to cache settings while the cache is alive, reconfiguring a cache while the server is running flushes the cache.

« Clustering SearchAuto Deploy to All Nodes »
Was this article helpful?
0 out of 0 found this helpful