Why use Unicast versus Multicast in Weblogic Clusters

The main benefit of Unicast over Multicast is ease of configuration. Unicast uses TCP communications and this usually requires no additional network configuration. Multicast uses UDP communication and Multicast addresses and this may require some network configuration and an additional effort in selecting the address to be used.

There is a great article from the Oracle A-Team with in-depth explanation: WebLogic Server Cluster Messaging Protocols.

In documentation for WLS 12.1.2 Oracle added Considerations for Choosing Unicast or Multicast in which they suggest to use Multicast in clusters with more than 10 Managed Servers.

In my personal experience I found that Unicast may give some problems in large clusters mainly because it is a new protocol introduced in WLS 10.0 and still suffer from some minor issues.

The main difference between Unicast and Multicast is as follows

Unicast:

Say you have three servers (MS-1,MS-2,MS-3) in a cluster. If they have to communicate with each other, then they have to ping (i.e. heartbeats ) the cluster master for informing it that they are alive.

If MS-1 is the master then MS-2 and MS-3 would send the ping to MS-1

Multicast:

In Multicast, there is no cluster master. Instead each server has to ping each other to inform everyone that they are alive.

So MS-1 would send the ping to MS-2 & MS-3 and at the same time MS-2 would send the ping to MS-1 & MS-3 and MS-3 would ping MS-1 & MS-3.

Thus, in multicast there are more pings sent which makes the congestion in sending the pings much heavier compared to unicast. Because of this, WLS recommends using Unicast instead for less congestion in the network: Oracle Docs: Communications In a Cluster

The principle behind Multicast is that any message is received by all subscribers to the Multicast address. So MS-1 only needs to send 1 network packet to alert all other cluster members of its status. This means a status or JNDI update requires only 1 packet per Cluster for Muticast vs 1 packet per Server (approximately) for Unicast. Multicast also requires no "master" election. Multicast is thus far simpler to code and creates less network traffic.

So, Multicast is great? Not necessarily. It uses UDP datagrams which are inherently unreliable and un-acknowledged, so given the unreliable bearer protocol - Etherent - your message might never turn up (interpret: you fall out of the cluster). The whole concept of Multicast is based on subscription, it is not a 'routable' protocol in the normal sense, so by default routers must discard Multicast packets or risk a network storm. Hence the historical requirement for all cluster members to reside on the same network segment.

These shortcomings of Multicast mean Unicast is the way to go if your cluster spans networks or you're losing too many multicast packets.