--- title: Forming a cluster of Diffusion servers description: What is the purpose of server clusters? Server clusters ensure high availability. --- [Skip to content](https://support.diffusiondata.com/knowledge/forming-a-cluster-of-diffusion-servers#main-content) English Show submenu for translations [Contact Customer Support](https://support.diffusiondata.com/knowledge/kb-tickets/new?hsLang=en) [Customer portal](https://support.diffusiondata.com/tickets-view?hsLang=en) [![DiffusionData Logo](https://support.diffusiondata.com/hs-fs/hubfs/DiffusionData-Logo-Primary-Trimmed.png?width=445&height=64&name=DiffusionData-Logo-Primary-Trimmed.png)](https://www.diffusiondata.com) Open main navigation Close main navigation - English Show submenu for translations - [Contact Customer Support](https://support.diffusiondata.com/knowledge/kb-tickets/new) - [Customer portal](https://support.diffusiondata.com/tickets-view) How can we help you? - There are no suggestions because the search field is empty. 1. [Help Center](https://support.diffusiondata.com/knowledge?hsLang=en) 2. [Getting Started](https://support.diffusiondata.com/knowledge/getting-started?hsLang=en) # Forming a cluster of Diffusion servers **What is the purpose of server clusters?** Server clusters ensure high availability. Diffusion uses a datagrid to share session and topic information between Diffusion servers within a cluster, providing high availability for clients connecting to load-balanced servers. ![article15\_image1](https://support.diffusiondata.com/hs-fs/hubfs/article15_image1.jpg?width=218&height=236&name=article15_image1.jpg) If you have two or more Diffussion servers, you can form a Diffusion cluster. Please refer \<\> on how to [setup multiple Diffusion servers on your local machine](https://26595832.hs-sites-eu1.com/knowledge/setting-up-multiple-servers-on-the-same-machine?hsLang=en). Do you have a license which allows replication? Session replication is a paid feature of Diffusion. You cannot form a cluster or replicate topics and/or sessions if you don't have a **Diffusion Distribution** or **Diffusion Enterprise** license. To form a Diffusion cluster, you need to enable replication. This can be done by changing the replication flag from **false** to **true** in the **Replication.xml** file in the **etc** folder. ``` ``` Ensure that you set this flag as **true** for all the servers which you want to be part of the cluster. - Save the changes you've made on the **.xml** files. If you don't have a license and you start Diffusion servers, you'll most likely end up seeing the below: ``` C:\Work\Diffusion 6.7.2\bin>diffusion.bat2021-11-12 15:22:42.493|INFO|main|PUSH-000578|||Diffusion 6.7.2 "Russell" - Build #4fbb8bff, 9724@PTLT72, Oracle Corporation Java HotSpot(TM) Client VM 1.8.0_311.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:22:42.603|INFO|main|PUSH-000166|||Diffusion server 'PTLT72' starting.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:22:42.603|INFO|main|PUSH-000159|||The maximum message size is 32768 bytes.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:22:42.930|WARN|main|PUSH-000676|||This system has 16 available cores which exceeds the license limit of 8 cores. The server thread pools have been constrained to degrade performance.|com.pushtechnology.diffusion.licence.agent.c2021-11-12 15:22:43.207|WARN|main||||******************************************************************************* Thank you for choosing Diffusion. This server is using a restricted default license. It is limited to 1000 topics, 5 user sessions, and 8 CPU cores. Production use is explicitly prohibited. Get a more capable license for this server by joining the Push Technology Developer Community at https://community.pushtechnology.com. Registration is free and allows you to generate a community license that supports single server production deployments and up to 1000 user sessions. You can also generate an evaluation license that grants trial access to premium features available with a commercial license including multi-server clusters for high-availability and scale, topic persistence, and inter-server connections across wide area networks. For a commercial license that supports larger numbers of topics and sessions and production use of the premium features, please contact our sales team. https://www.pushtechnology.com/ sales@pushtechnology.com +44 20 3588 0900*******************************************************************************|com.pushtechnology.diffusion.licence.agent.LicenceLoaderImpl2021-11-12 15:22:43.574|INFO|main|PUSH-000795|||Configuration replication is enabled.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:22:43.743|ERROR|main|PUSH-000696|||Feature 'SESSION_REPLICATION' is not licensed.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:22:43.744|ERROR|main|PUSH-000164|||Diffusion server not started.|com.pushtechnology.diffusion.api.server.DiffusionServercom.pushtechnology.diffusion.licence.DiffusionLicenceException: Feature 'SESSION_REPLICATION' is not licensed. at com.pushtechnology.diffusion.DiffusionController.start(DiffusionController.java:558) ~[diffusion.jar:6.7.2-4fbb8bff] at com.pushtechnology.diffusion.api.server.DiffusionServer.start(DiffusionServer.java:167) [diffusion.jar:6.7.2-4fbb8bff] at com.pushtechnology.diffusion.api.server.DiffusionServer.main(DiffusionServer.java:290) [diffusion.jar:6.7.2-4fbb8bff] at com.pushtechnology.diffusion.Diffusion.main(Diffusion.java:22) [diffusion.jar:6.7.2-4fbb8bff]2021-11-12 15:22:43.750|WARN|main|PUSH-000168|||Diffusion stopping, reason='DiffusionServer.stop() called' by administrator='anonymous'.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:22:43.753|INFO|main|PUSH-000167|||Diffusion stopped.|com.pushtechnology.diffusion.DiffusionController ``` Diffusion stops. The log message is straightforward that the **Feature 'SESSION\_REPLICATION' is not licensed.** However, if you have a license, Diffusion will start. Each server in a cluster must have a unique name. If you don't assign unique names for the servers, your Diffusion instance will fail to start. The exception in the below log is **Server names in a cluster must be unique but Member \[192.168.1.156\]:5701 - 161a5352-8481-4001-994f-b39c874d353a has an identical server name \[PTLT72\]** ``` C:\Work\Diffusion 6.7.2_2\bin>diffusion.bat2021-11-12 15:25:02.326|INFO|main|PUSH-000578|||Diffusion 6.7.2 "Russell" - Build #4fbb8bff, 15420@PTLT72, Oracle Corporation Java HotSpot(TM) Client VM 1.8.0_311.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:25:02.503|INFO|main|PUSH-000166|||Diffusion server 'PTLT72' starting.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:25:02.504|INFO|main|PUSH-000159|||The maximum message size is 32768 bytes.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:25:03.195|INFO|main|PUSH-000202|||Product license expires in 341 day(s).|com.pushtechnology.diffusion.licence.agent.c2021-11-12 15:25:03.198|INFO|main|PUSH-000767|||This server is licensed to asekhar. License ID 5dc588f1-5158-47c1-90b4-6395d415434d.|com.pushtechnology.diffusion.licence.agent.LicenceLoaderImpl2021-11-12 15:25:03.597|INFO|main|PUSH-000795|||Configuration replication is enabled.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:25:03.801|INFO|main|PUSH-000791|||Session replication is enabled.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:25:03.806|INFO|main|PUSH-000796|||Topic replication is enabled.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:25:05.480|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Hazelcast 4.2 (20210324 - 405cfd1) starting at [192.168.1.156]:5702|com.hazelcast.system2021-11-12 15:25:06.174|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] No discovery strategy is applicable for auto-detection|com.hazelcast.spi.discovery.integration.DiscoveryService2021-11-12 15:25:06.933|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Using Multicast discovery|com.hazelcast.instance.impl.Node2021-11-12 15:25:06.942|WARN|main||||[192.168.1.156]:5702 [dev] [4.2] CP Subsystem is not enabled. CP data structures will operate in UNSAFE mode! Please note that UNSAFE mode will not provide strong consistency guarantees.|com.hazelcast.cp.CPSubsystem2021-11-12 15:25:07.336|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Diagnostics disabled. To enable add -Dhazelcast.diagnostics.enabled=true to the JVM arguments.|com.hazelcast.internal.diagnostics.Diagnostics2021-11-12 15:25:07.348|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] [192.168.1.156]:5702 is STARTING|com.hazelcast.core.LifecycleService2021-11-12 15:25:07.543|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Trying to join to discovered node: [192.168.1.156]:5701|com.hazelcast.internal.cluster.impl.MulticastJoiner2021-11-12 15:25:07.586|INFO|hz.PTLT72.IO.thread-in-0||||[192.168.1.156]:5702 [dev] [4.2] Initialized new cluster connection between /192.168.1.156:61946 and /192.168.1.156:5701|com.hazelcast.internal.server.tcp.TcpServerConnection2021-11-12 15:25:13.668|INFO|hz.PTLT72.priority-generic-operation.thread-0||||[192.168.1.156]:5702 [dev] [4.2]Members {size:2, ver:2} [ Member [192.168.1.156]:5701 - 161a5352-8481-4001-994f-b39c874d353a Member [192.168.1.156]:5702 - 30a4cbd2-6096-43ed-880f-c148eb5af33a this]|com.hazelcast.internal.cluster.ClusterService...2021-11-12 15:25:14.907|ERROR|main|PUSH-000164|||Diffusion server not started.|com.pushtechnology.diffusion.api.server.DiffusionServercom.pushtechnology.diffusion.api.config.ConfigException: Server names in a cluster must be unique but Member [192.168.1.156]:5701 - 161a5352-8481-4001-994f-b39c874d353a has an identical server name [PTLT72] at com.pushtechnology.diffusion.replication.hazelcast.HazelcastInstanceProvider.provide(HazelcastInstanceProvider.java:99) ~[diffusion.jar:6.7.2-4fbb8bff] . . . at com.pushtechnology.diffusion.DiffusionController.configureContainer(DiffusionController.java:607) ~[diffusion.jar:6.7.2-4fbb8bff] at com.pushtechnology.diffusion.DiffusionController.start(DiffusionController.java:552) ~[diffusion.jar:6.7.2-4fbb8bff] at com.pushtechnology.diffusion.api.server.DiffusionServer.start(DiffusionServer.java:167) [diffusion.jar:6.7.2-4fbb8bff] at com.pushtechnology.diffusion.api.server.DiffusionServer.main(DiffusionServer.java:290) [diffusion.jar:6.7.2-4fbb8bff] at com.pushtechnology.diffusion.Diffusion.main(Diffusion.java:22) [diffusion.jar:6.7.2-4fbb8bff]2021-11-12 15:25:14.920|WARN|main|PUSH-000168|||Diffusion stopping, reason='DiffusionServer.stop() called' by administrator='anonymous'.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:25:14.926|INFO|main|PUSH-000167|||Diffusion stopped.|com.pushtechnology.diffusion.DiffusionController ``` **How to assign names to your Diffusion servers?** You can assign names to your servers in the **Server.xml** file in the **etc** folder. - To name the second server as say, server2, replace \ in the file with the below. ``` server2 ``` - Save the changes and start your Diffusion server. ``` C:\Work\Diffusion 6.7.2_2\bin>diffusion.bat2021-11-12 15:28:44.827|INFO|main|PUSH-000578|||Diffusion 6.7.2 "Russell" - Build #4fbb8bff, 16412@PTLT72, Oracle Corporation Java HotSpot(TM) Client VM 1.8.0_311.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:28:44.932|INFO|main|PUSH-000166|||Diffusion server 'server2' starting.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:28:44.932|INFO|main|PUSH-000159|||The maximum message size is 32768 bytes.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:28:45.509|INFO|main|PUSH-000202|||Product license expires in 341 day(s).|com.pushtechnology.diffusion.licence.agent.c2021-11-12 15:28:45.511|INFO|main|PUSH-000767|||This server is licensed to asekhar. License ID 5dc588f1-5158-47c1-90b4-6395d415434d.|com.pushtechnology.diffusion.licence.agent.LicenceLoaderImpl2021-11-12 15:28:45.886|INFO|main|PUSH-000795|||Configuration replication is enabled.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:28:46.067|INFO|main|PUSH-000791|||Session replication is enabled.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:28:46.069|INFO|main|PUSH-000796|||Topic replication is enabled.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:28:47.481|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Hazelcast 4.2 (20210324 - 405cfd1) starting at [192.168.1.156]:5702|com.hazelcast.system2021-11-12 15:28:48.079|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] No discovery strategy is applicable for auto-detection|com.hazelcast.spi.discovery.integration.DiscoveryService2021-11-12 15:28:48.788|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Using Multicast discovery|com.hazelcast.instance.impl.Node2021-11-12 15:28:48.796|WARN|main||||[192.168.1.156]:5702 [dev] [4.2] CP Subsystem is not enabled. CP data structures will operate in UNSAFE mode! Please note that UNSAFE mode will not provide strong consistency guarantees.|com.hazelcast.cp.CPSubsystem2021-11-12 15:28:49.161|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Diagnostics disabled. To enable add -Dhazelcast.diagnostics.enabled=true to the JVM arguments.|com.hazelcast.internal.diagnostics.Diagnostics2021-11-12 15:28:49.171|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] [192.168.1.156]:5702 is STARTING|com.hazelcast.core.LifecycleService2021-11-12 15:28:49.333|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] Trying to join to discovered node: [192.168.1.156]:5701|com.hazelcast.internal.cluster.impl.MulticastJoiner2021-11-12 15:28:49.371|INFO|hz.server2.IO.thread-in-0||||[192.168.1.156]:5702 [dev] [4.2] Initialized new cluster connection between /192.168.1.156:61990 and /192.168.1.156:5701|com.hazelcast.internal.server.tcp.TcpServerConnection2021-11-12 15:28:55.458|INFO|hz.server2.priority-generic-operation.thread-0||||[192.168.1.156]:5702 [dev] [4.2]Members {size:2, ver:4} [ Member [192.168.1.156]:5701 - 161a5352-8481-4001-994f-b39c874d353a Member [192.168.1.156]:5702 - e52a435a-f3bd-45bc-bef9-36f8335f714a this]|com.hazelcast.internal.cluster.ClusterService2021-11-12 15:28:56.421|INFO|main||||[192.168.1.156]:5702 [dev] [4.2] [192.168.1.156]:5702 is STARTED|com.hazelcast.core.LifecycleService2021-11-12 15:28:57.624|INFO|main|PUSH-000808|||JMX has loaded a TLS keystore from 'C:\Work\Diffusion 6.7.2_2\etc\sample.keystore'.|com.pushtechnology.diffusion.management.RemoteJMXService2021-11-12 15:28:57.641|WARN|main|PUSH-000656|||Unable to start connector 'Management' on port 1099, address already in use.|com.pushtechnology.diffusion.management.ManagementServiceImpl2021-11-12 15:28:57.647|INFO|main|PUSH-000651|||Cluster member HazelcastClusterMember[PTLT72, 192.168.1.156:8090, 8b7e511a-15a9-4d7b-b4a4-b46e42c3e3bb] joined the cluster.|com.pushtechnology.diffusion.replication.hazelcast.HzClusterService2021-11-12 15:28:57.647|INFO|main|PUSH-000651|||Cluster member HazelcastClusterMember[server2, 192.168.1.156:8092, c5d3c113-7d34-4a61-848a-611fd7c39aa4] joined the cluster.|com.pushtechnology.diffusion.replication.hazelcast.HzClusterService2021-11-12 15:28:57.688|INFO|main|PUSH-000193|||Connector 'Client Connector' has loaded a TLS keystore from 'C:\Work\Diffusion 6.7.2_2\etc\sample.keystore'.|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:57.759|INFO|main|PUSH-000362|||Started HTTP service for 'default-prometheus-service'.|com.pushtechnology.diffusion.webserver.services.HTTPService2021-11-12 15:28:57.780|INFO|main|PUSH-000337|||Added file service virtual host 'default' to serve files from 'html' to requests where the host header matches '.*'.|com.pushtechnology.diffusion.webserver.services.fileservice.FileService2021-11-12 15:28:57.787|INFO|main|PUSH-000072|||Connector 'Client Connector' started, listening on 0.0.0.0/0.0.0.0:8081.|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:57.791|INFO|main|PUSH-000712|||Connector 'Client Connector' has scheduled pings every 90000ms.|com.pushtechnology.diffusion.comms.connector.PingScheduler2021-11-12 15:28:58.092|INFO|main|PUSH-000806|||The file service is serving files at https://192.168.1.156:8081 .|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:58.092|INFO|main|PUSH-000806|||The file service is serving files at https://192.168.56.214:8081 .|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:58.094|INFO|main|PUSH-000193|||Connector 'High Volume Connector' has loaded a TLS keystore from 'C:\Work\Diffusion 6.7.2_2\etc\sample.keystore'.|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:58.094|INFO|main|PUSH-000072|||Connector 'High Volume Connector' started, listening on 0.0.0.0/0.0.0.0:8092.|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:58.095|INFO|main|PUSH-000712|||Connector 'High Volume Connector' has scheduled pings every 90000ms.|com.pushtechnology.diffusion.comms.connector.PingScheduler2021-11-12 15:28:58.390|INFO|main|PUSH-000806|||The file service is serving files at https://192.168.1.156:8092 .|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:58.390|INFO|main|PUSH-000806|||The file service is serving files at https://192.168.56.214:8092 .|com.pushtechnology.diffusion.comms.connector.ConnectorImpl2021-11-12 15:28:58.471|INFO|main|PUSH-000165|||Diffusion server started.|com.pushtechnology.diffusion.DiffusionController2021-11-12 15:28:58.569|INFO|ForkJoinPool.commonPool-worker-8|PUSH-000834|||The server has restored shared configuration from the cluster.|com.pushtechnology.diffusion.comms.connector.ReadinessConditionsImpl2021-11-12 15:28:58.629|INFO|ForkJoinPool.commonPool-worker-8|PUSH-000648|||Cluster member HazelcastClusterMember[PTLT72, 192.168.1.156:8090, 8b7e511a-15a9-4d7b-b4a4-b46e42c3e3bb] is connected to this server.|com.pushtechnology.diffusion.clustering.peers.InboundPeerConnection2021-11-12 15:28:58.746|INFO|InboundThreadPool Thread_5|PUSH-000835|||The server has restored replicated topics from the cluster.|com.pushtechnology.diffusion.comms.connector.ReadinessConditionsImpl ``` You've now got your second server started. If you have more than two servers to be part of the cluster, repeat the above steps. Thus, you will have a cluster of Diffusion servers. **Where can you see the cluster?** Open **Management Console** of all servers (in this example, [http://localhost:8080/](http://localhost:8080/) and [http://localhost:8081/](http://localhost:8081/)) Go to Network from the menu on the left pane and that shows the cluster. **Server 1:** ![article15\_image2](https://support.diffusiondata.com/hs-fs/hubfs/article15_image2.jpg?width=688&height=299&name=article15_image2.jpg) **Server 2:** ![article15\_image3](https://support.diffusiondata.com/hs-fs/hubfs/article15_image3.jpg?width=688&height=294&name=article15_image3.jpg) You can also see the name of the server (as you named (server2) in the Server.xml) on the top right. Are you wondering how the Diffusion servers are auto detected to form a cluster? Diffusion uses a datagrid to share session and topic information between Diffusion servers within a cluster, providing high availability for clients connecting to load-balanced servers. Diffusion uses **Hazelcast™** as its datagrid. Hazelcast is a third-party product that is included in the Diffusion server installation and runs within the Diffusion server process. The datagrid is responsible for the formation of clusters and the exchange of replicated data. These clusters operate on a peer-to-peer basis and by default there is no hierarchy of servers within the cluster. More details on this can be read in the [Diffusion User Manual](https://docs.pushtechnology.com/docs/latest/manual/html/designguide/solution/ha/ha.html) (Design Guide \> Designing your solution \> Server clusters for high availability). The Hazelcast documentation on [Setting Up Clusters](https://docs.hazelcast.com/imdg/4.1.2/clusters/setting-up-clusters) describes Hazelcast clusters, the discovery mechanisms such as Auto detection, TCP, Multicast used by cluster members and native clients to form a Hazelcast cluster. In Diffusion, you can find the **hazelcast.xml** file in the **etc** folder. For instance, if you want to discover your cluster members by TCP, you - set the **tcp-ip enabled** attribute to **true** - add **member** elements **within** the **tcp-ip element** For example: ```                                 127.0.0.1             ```   - [Administer](https://support.diffusiondata.com/knowledge/administer?hsLang=en) - [Build](https://support.diffusiondata.com/knowledge/build?hsLang=en) - [Getting Started](https://support.diffusiondata.com/knowledge/getting-started?hsLang=en) - [Install & Upgrades](https://support.diffusiondata.com/knowledge/install-upgrades?hsLang=en) - [Management & Monitoring](https://support.diffusiondata.com/knowledge/management-monitoring?hsLang=en) - [Security](https://support.diffusiondata.com/knowledge/security?hsLang=en) - [Support](https://support.diffusiondata.com/knowledge/support?hsLang=en) - [Troubleshooting](https://support.diffusiondata.com/knowledge/troubleshooting?hsLang=en) - [Cloud](https://support.diffusiondata.com/knowledge/cloud?hsLang=en) [![Chill listening crop-3](https://support.diffusiondata.com/hs-fs/hubfs/DiffusionData%20Favicon.png?width=24&height=24&name=DiffusionData%20Favicon.png "Chill listening crop-3")](http://diffusiondata.com) diffusiondata.com Help Center Copyright © 2026, DiffusionData