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36.2. Server discovery
Server discovery is a mechanism by which servers can propagate their connection details to:
- Messaging clients. A messaging client wants to be able to connect to the servers of the cluster without having specific knowledge of which servers in the cluster are up at any one time.
- Other servers. Servers in a cluster want to be able to create cluster connections to each other without having prior knowledge of all the other servers in the cluster.
Server discovery uses User Datagram Protocol (UDP) multicast to broadcast server connection settings. If UDP is disabled on your network you will not be able to use this, and will have to specify servers explicitly when setting up a cluster or using a messaging client.
36.2.1. Broadcast Groups
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A broadcast group is the means by which a server broadcasts connectors over the network. A connector defines a way in which a client (or other server) can make connections to the server. For more information on what a connector is, refer to Chapter 14, Configuring the Transport.
The broadcast group takes a set of connector pairs, each connector pair contains connection settings for a live and (optional) backup server and broadcasts them on the network. It also defines the UDP address and port settings.
Broadcast groups are defined in
<JBOSS_HOME>/jboss-as/server/<PROFILE>/deploy/hornetq/hornetq-configuration.xml. There can be many broadcast groups per HornetQ server. All broadcast groups must be defined in a broadcast-groups element.
Let us take a look at an example broadcast group from
<JBOSS_HOME>/jboss-as/server/<PROFILE>/deploy/hornetq/hornetq-configuration.xml:
Some of the broadcast group parameters are optional and the defaults will normally be used, but all are specified in the above example for clarity. Each are covered below.
Broadcast Group Parameters
-
name - Each broadcast group in the server must have a unique name in the server.
-
local-bind-address - This is the local bind address that the datagram socket is bound to. If you have multiple network interfaces on your server, you would specify which one you wish to use for broadcasts by setting this property. If this property is not specified then the socket will be bound to the wildcard address, an IP address chosen by the kernel.
-
local-bind-port - If you want to specify a local port to which the datagram socket is bound you can specify it here. Normally you would just use the default value of
-1which signifies that an anonymous port should be used. This parameter is always specified in conjunction withlocal-bind-address.If you are behind a firewall you can utilizelocal-bind-addressandlocal-bind-portto specify a static host and port. However, it is highly unlikely that a cluster would be configured with server instances outside a firewall which would need to communicate with the server which are behind the firewall. -
group-address - This is the multicast address to which the data will be broadcast. It is a class D IP address in the range
224.0.0.0to239.255.255.255, inclusive. The address224.0.0.0is reserved and is not available for use. This parameter is mandatory. -
group-port - This is the UDP port number used for broadcasting. This parameter is mandatory.
-
broadcast-period - This is the period in milliseconds between consecutive broadcasts. This parameter is optional, the default value is
2000milliseconds. -
connector-ref - This specifies the connector and optional backup connector that will be broadcast (see Chapter 14, Configuring the Transport for more information on connectors).
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