Part 1 - WebLogic Server Clustering

Overview

In this section, we are going to discuss about following information:

• What is a Cluster?
• Benefits of Clusters
• Different Types of Clusters
• Cluster Constraints
• Cluster members communication modes - Multicast & Unicast
• Difference between Unicast & Multicast

What is a Cluster?
It is a group of WebLogic managed server instances running simultaneously and working together to provide High Availability and Scalability.  All managed severs which are part of Cluster is also known as Cluster Members or Cluster Nodes

Benefits of Clustering

• Application Fail-over
• High Availability
• Load Balancing
• Scalability

What are the different Types of Cluster?
Clusters are divided into two types:

1. Vertical Clustering
2. Horizontal Clustering

Vertical Clustering
Grouping of WebLogic managed servers which are hosted on a single machine

Horizontal Clustering
Grouping of WebLogic managed servers which are hosted on more than one machine ( or multiple machines)

Which one is best Vertical or Horizontal Clustering?
In horizontal clustering, if one of machine goes down the client requests will be handled by managed servers of other machine where as in vertical clustering if the machine goes down, the application hosted on these servers will be down and it will impact clients and end users.

Cluster Constraints

• All managed servers in the cluster should be of same WebLogic version
• A Cluster can't span multiple domains. All managed server instance in a cluster must reside in the single domain
• A domain may contain 0 - * clusters

Cluster Members communication modes

All managed WebLogic Server instances in a cluster communicate with one another using two basic network technologies:

IP sockets - provides peer-to-peer communication between cluster members

IP unicast or multicast - cluster members use this to broadcast availability of services and heartbeats that indicate availability

When creating a new cluster, Oracle recommends unicast for messaging within a cluster.The default cluster messaging mode is unicast, when creating a cluster using the Configuration Wizard,. The default cluster messaging mode is multicast, when creating a cluster using WLST, 

Multicast
In Multicast each managed server will communicate with all members of the cluster by using Multicast IP address and port.

Unicast
In Unicast each Managed server will communicate with their group leader by using unicast broadcast channel

Difference between Unicast & Multicast

Multicast	Unicast
It uses multicast IP address and port for message communication	It uses unicast broadcast channel for message communication
All WebLogic versions support Multicast	From WebLogic 10 onwards we have unicast
It is one to many communication	It is one to one communication
Multicast requires additional configurations at OS and Network level	It doesn't require any additional configurations
Multicast uses UDP Protocol	Unicast uses TCP/IP Protocal