In this Fusion Middleware Training article, you will learn the following

Basic cluster architecture

Benefits of Oracle WebLogic cluster

Multi-tier cluster architecture

Communication among clustered server instances

Key criteria for selecting suitable cluster architecture

So what exactly is a weblogic cluster in Oracle Fusion Middleware?

It is a logical group of managed servers within a domain. It Supports features to provide high availability for the following

Whole Servers

Web Applications and services

EJB Applications

JMS

JDBC resources

What exactly does clustering provide ?

The following types of objects can be clustered:

Servlets

JSP

EJB

Remote Method Invocation (RMI) objects

Java Messaging Service (JMS) destinations

Java Database Connectivity (JDBC) connections

What are Proxy Servers for HTTP Clusters ?

Proxy servers are used to provide load balancing and failover for a cluster. They:

Are the client’s first level of interaction with the cluster

Give the cluster its single server appearance

A proxy server can be either software based or hardware based

A software-based proxy server may be a WebLogic servlet, Web server plug-in, or a third-party application

A hardware-based proxy server is typically a physical load balancer

Load-Balancing Clustered EJB Objects

• WebLogic Server supports the following load-balancing algorithms for clustered EJB objects:

• Round-robin

• Weight-based

• Random

• Parameter-based routing (programmatic)

• Server affinity configuration enables calls to objects to remain with the same server and minimizes client-side load balancing.

Road Map

• Oracle WebLogic cluster introduction

• Cluster architecture

• Considerations for selecting an appropriate cluster architecture

• Basic cluster architecture

• Multitier cluster architecture

• Proxy servers

• Cluster communicatio

Selecting a Cluster Architecture

Consider the following factors when selecting a suitable architecture:

Performance

Efficient state persistence

Optimal load balancing

Effective failover

Reliable communication

There are two primary cluster architectures to choose from:

Basic cluster architecture

Multitier architecture

So what are the advantages of clusters in Fusion Middleware

Easy administration

Flexible load balancing ó Robust security

And what are the disadvantages of clusters in Oracle Fusion Middleware

It cannot load-balance EJB method calls.

Load-balancing across the tiers may become unbalanced.

The Web tier and the business logic with services can be separated into two clusters.

Multi-tier Cluster Architecture

What is a basic Cluster Proxy Architecture

This is similar to the basic cluster architecture, except that static content is hosted on HTTP servers

Proxy Web Server Plug-In Versus Load Balancer

There are many advantages to using a physical load balancer instead of the proxy plug-in:

There is no need to configure the client plug-ins.

It eliminates the proxy layer, thereby reducing the number of connections.

There are more sophisticated load-balancing algorithms

There are a number of disadvantages as well for Clusters in Oracle Fusion Middleware

Additional administration

Explicit configuration of “sticky” sessions for stateful Web applications

Proxy Plug-Ins

Proxy plug-ins:

Delegate dynamic content requests to WLS servers and balance load

across a cluster in a round-robin fashion

Route HTTP requests to back-end WLS instances based on session cookie or URL rewriting

Avoid routing to failed servers in the cluster

Oracle HTTP Server containsmod_wl_ohs, which is a plug-in for WLS by default.

WLS provides plug-ins to other major Web servers as well.

OHS as a Proxy Web Server

Oracle HTTP Server (OHS) is a Web server that:

Is based on Apache

Serves static and dynamic content

Supports content generation in many languages, such as Java, C, C++, PHP, PERL, or PL/SQL

Contains a WebLogic Server plug-in (mod_wl_ohs) by default

Can be easily integrated with other Oracle Fusion Middleware components

Can be managed using the Fusion Middleware Control along with other components

Request Flow When Using OHS

The client sends an HTTP request to OHS for access to a Java EE application.

The mod_wl_ohs plug-in at OHS receives the request and determines from the cookie (in request) which WLS server should serve the request.

If no cookie exists, the request is assigned to the next available WLS server in the cluster (round-robin algorithm).

The WLS server that responds places the appropriate cookie in the response.

OHS routes the response to the client (with the cookie).

WLS HttpClusterServlet

HttpClusterServlet:

Is deployed in the default Web application of the proxy server

Delivers client requests in round-robin style to servers in the cluster

Server Communication in a Cluster

• WebLogic Server instances in Fusion Middleware cluster communicate with one another using:

• IP sockets, which are the conduits for peer-to-peer communication between clustered server instances

• IP unicast or multicast, which server instances use to broadcast availability of services and heartbeats that indicate continued availability

• Multicast broadcasts one-to-many communications among clustered instances.

• Unicast is an alternative to multicast to handle cluster messaging and communications. The unicast configuration is much easier because it does not require cross-network configuration that multicast requires

One-to-Many Communications

Oracle WebLogic Server uses one-to-many communication for:

• Clusterwide JNDI updates

• Cluster “heartbeats”

Because all one-to-many communications occur over IP multicast, when you design a cluster, consider the following factors:

• If your cluster spans multiple subnets, your network must be configured to reliably transmit messages.

• A firewall can break IP multicast transmissions.

• The multicast address should not be shared with other applications.

• Multicast storms may occur.

Considerations When Using Unicast

Unicast messaging type:

• Is much easier to configure because it does not require cross-network configuration that multicast requires

• Reduces potential network errors that can occur from multicast address conflicts

You cannot mix and match cluster messaging types within a cluster

Peer-to-Peer Communications

Oracle WebLogic Server uses peer-to-peer communications for:

• Accessing nonclustered or pinned objects that reside on a remote server instance in the cluster

• Replicating HTTP session states and stateful session EJB states between a primary and a secondary server

• Accessing the clustered objects that reside on a remote server instance (typically, in a multitier cluster architecture)

Clusterwide JNDI Naming Service

Each WebLogic Server in a cluster builds and maintains its own local copy of the clusterwide JNDI tree, which lists the services offered by all members of the cluster.

Name Conflicts and Resolution

Cluster-level JNDI conflicts may occur when new services are added to the cluster.

In case of name conflicts, local binding may succeed, but the binding of other object names from other servers will fail.

To avoid cluster-level JNDI conflicts, you must deploy all replica-aware objects to all WebLogic Server instances in a cluster

Ajay Sharma

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