Unicast Vs Multicast In WebLogic Cluster Communications

WebLogic server instances in a cluster communicate with one another using two basic network technologies:
• IP sockets, which are the conduits for peer-to-peer communication between clustered server instances.
• IP unicast or multicast, which is used by server instances to broadcast availability of services and heartbeats that indicate continued availability.

Unicast:

When creating a new cluster, it is recommended that you use unicast for messaging within a cluster. It is much easier to configure because it does not require crossnetwork configuration that multicast requires. It also reduces potential network errors that can occur from multicast address conflicts.

You could consider the following if you are going to use unicast to handle cluster communications:

• All members of a cluster must use the same message type. Mixing between multicast and unicast messaging is not allowed.
• You must use multicast if you need to support a previous version of WebLogic server within your cluster.
• Individual cluster members cannot override the cluster messaging type.
• The entire cluster must be restarted to change the messaging type. Unicast uses TCP socket communication and multicast uses UDP communication.On networks where UDP is not supported, unicast must be used.

Multicast:

For backwards compatibility with previous versions of Oracle WebLogic, you must use multicast for communications between clusters.

Multicast broadcasts one-to-many communications between clustered instances. WebLogic server uses IP multicast for all one-to-many communications among server instances in a cluster. Each WebLogic server instance in a cluster uses multicast to broadcast regular “heartbeat” messages. By monitoring heartbeat messages, server instances in a cluster determine when a server instance has failed (clustered server instances also monitor IP sockets as a more immediate method of determining when a server instance has failed).

IP multicast is a broadcast technology that enables multiple applications to subscribe to an IP address and port number and listen for messages.

A multicast address is an IP address in the range 224.0.0.0 – 239.255.255.255.

If your cluster is divided over multiple subnets, your network must be configured to reliably transmit messages. A firewall can break IP multicast transmissions and the multicast address should not be shared with other applications.

Also, multicast storms may occur, when server instances in a cluster do not process incoming messages on a timely basis, thus generating more network traffic, which causes round trips with no end.

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