Automatic Clustering

This section details the auto-clustering feature of Payara Micro.

The Multicast Discovery

The integration of Hazelcast into Payara Micro allows instances to automatically, and dynamically, cluster together. When two instances are pointed at the same multicast address and port, they will automatically cluster together.

You can see evidence of the automatic clustering by simply starting two instances (with different HTTP port configurations or use --autobindhttp), and you should see the following in the log output:

Members [2] {
Member []:5900 this
Member []:5901

The --startPort option is used to determine which clustering port the Payara Micro instance will first try to bind to. If the port is already bound to (say, by another Payara Micro instance), then the Payara Micro instance will simply increment the startPort value and try again until it finds a port it can bind to.

For example, if there are already two Payara Micro instances running, which have bound ports 5900 and 5901, then a third instance started with a startPort value of 5900 will first try to bind to 5900, then to 5901, then finally settle on 5902.

If you wish to have multiple clusters, then you can make use of the --mcAddress and `--mcPort options to define a different multicast address and port; assign a different address and port to each set of instances you wish to run in a separate cluster and they will automatically make their own cluster on the new multicast address and port.

You can also use --clusterName and --clusterPassword to segregate clusters.

Preventing Multicast Cluster Cross Talk

By default Payara Micro clusters automatically discover other cluster members via multicast. This can lead to the situation whereby different development environments being used by different teams cluster together as they are using the same multicast address and multicast port.

This can lead to confusing errors. To prevent cluster cross-talk make sure that the multicast-address and multicast-port are set to different values on each unique cluster. In the case where this is not possible Payara Micro provides the ability to set a cluster name and a cluster password both through the command line or through the API.

If all the multicast settings are similar, instances will only cluster together if all the instances have the same cluster name and cluster password.

Other forms of discovery

Since Payara Micro 5.181

With the introduction of the Domain Data Grid Payara Micro now includes a number of alternative discovery modes configurable on the command line using the --clustermode command line option. The default cluster mode is multicast as described above.

Domain Discovery Mode

Domain Discovery Mode enables a Payara Micro instance join a Domain Data Grid managed by a Payara Server DAS containing Payara Server Instances. To join a Domain Data Grid start Payara Micro with the domain cluster mode:

java -jar payara-micro.jar --clustermode domain:

The format of the domain cluster mode is domain:ip:port where ip is the ip address that the Hazelcast instance is bound to in the DAS.

Hazelcast is very particular about the IP address used to join a cluster. The IP Address should be exactly as reported by the details

When the DAS starts it will log a message like:

Data Grid Status
Payara Data Grid State: DG Version: 35 DG Size: 2
Instances: {
Address: / UUID: 7c1bd6ca-1574-4e38-9189-effcd4ccafd2 Lite: false This: true Name: server Group: server-config
Address: / UUID: 0f6887b2-9932-4085-a6f8-e40751269023 Lite: false This: false

This shows the IP address and ports that can be used as input into the domain cluster mode.

If you accidentally choose the DAS admin port (by default 4848) as the cluster port you will get a warning like
[2018-03-08T13:51:46.446+0000] [] [WARNING] [] [fish.payara.nucleus.hazelcast.DomainDiscoveryService] [tid: _ThreadID=1 _ThreadName=main] [timeMillis: 1520517106446] [levelValue: 900] You have specified 4848 as the datagrid domain port however this is the default DAS admin port, the default domain datagrid port is 4900

TCPIP Discovery Mode

The TCPIP Discovery Mode can be used to cluster Payara Micro instances when Multicast is not available on the network. This is especially useful in Cloud environments or in containerized deployments like Docker and Kubernetes. For the TCPIP discovery mode you specify a comma separated list of ip:port adddresses where you believe other Payara Micros will be listening.

For example the following command will cluster two Payara Micros together when multicast is not available.

java -jar /opt/PayaraDev/Payara/appserver/extras/payara-micro/payara-micro-distribution/tart/payara-micro.jar --autobindhttp --clustermode tcpip:,

java -jar /opt/PayaraDev/Payara/appserver/extras/payara-micro/payara-micro-distribution/tart/payara-micro.jar --autobindhttp --clustermode tcpip:,

A range of IP addresses can be specified in the list of members for example: This is especially useful if you have a cloud VPC and you have a nummber of Payara Micro instances running across a number of Cloud VMs. Using a range is also useful in Docker environments where the range of IP Addresses is known but dynamic for example

During discovery Payara Micro will test each IP:Port combination until it finds another Payara Micro instance. As long as one instance exists then it will join the existing cluster. If no Payra Micro instances are contactable the instance will create a new cluster with itself as the single member.

Hazelcast is very particular about the IP address used to join a cluster. The IP Address should be exactly as reported by the details
TCPIP cluster mode can also be used to join a Payara Server Domain Data Grid by specifying the ip:port of the DAS Hazelcast instance.


If Hazelcast chooses the incorrect IP Address to bind to (for example a docker interface) the --interfaces command line option can be used to force Hazelcast to use the specified network interface for Hazelcast:

java -jar /opt/PayaraDev/Payara/appserver/extras/payara-micro/payara-micro-distribution/tart/payara-micro.jar --autobindhttp --clustermode tcpip:, --interfaces

Advanced Discovery Modes

Further Discovery Modes can be configured directly in a hazelcast configuration file passed to Payara Micro using the --hzconfigfile command line option. Review the Hazelcast documentation to configure Hazelcast using this file.