14 Load Balancing and Failover

14.1 Overview of Load Balancing and Failover

OracleAS Wireless offers a scalable, reliable server infrastructure through clustering and high availability. The clustering structure includes the following two features.

• Load Balance: mod_oc4j on top of Oracle HTTP Server (OHS) distributes the request workload among multiple OracleAS Wireless server processes.

• Fault Tolerance (Failover): mod_oc4j on top of OHS redirects a client to another working OracleAS Wireless server process if a OracleAS Wireless Server process failure occurs.

14.2 Clustering Architecture

Each OracleAS Wireless Server process which runs on a single Java Virtual Machine (JVM) is referred to as a node. One or more nodes comprise an island. Nodes within an island are capable of serving the same applications, because the session for each client is replicated among all the nodes within an island in preparation of failover. One or more islands together form an OC4J (OracleAS Containers for J2EE) instance for the purpose of load balancing. The entire OC4J instance is linked by mod_oc4j to a simple front-end, Oracle HTTP Server (OHS). Typically, an island has two to four nodes.

By default, the requests from the same client are always redirected to the same OracleAS Wireless Server process. If one process goes down, then the fault tolerance feature is supported for both stateful and stateless requests as follows:

• Stateless Requests – Fault tolerance is achieved by redirecting the client to another working process.

• Stateful Requests – The session state is propagated to the processes within the same island, which enables another process in that same island to pick up the request from a given client if a failover occurs.

14.3 Clustering Configuration

This section describes how to configure the Oracle HTTP Server (OHS), Oracle Process Management and Notification (OPMN), and OracleAS Containers for J2EE (OC4J).

LoadModule oc4j_module libexec/mod_oc4j.so
<IfModule mod_oc4j.c>
Oc4jMount /ptg OC4J_Wireless
Oc4jMount /ptg/* OC4J_Wireless
Oc4jMount /mcs OC4J_Wireless
Oc4jMount /mcs/* OC4J_Wireless
Oc4jMount /mcs/media OC4J_Wireless
Oc4jMount /mcs/media/* OC4J_Wireless
</IfModule>

<ias-component id="wireless" status="enabled">
    <process-type id="OC4J_Wireless" module-id="OC4J">
       <environment>
       ...
       </environment>
       <module-data>
          <category id="start-parameters">
          ...
          </category>
       </module-data>
       ...
       <port id="ajp" range="3301-3400"/>
       <port id="rmi" range="3201-3300"/>
       <port id="jms" range="3701-3800"/>
       <process-set id="OC4J_WirelessIslandA" numprocs="2"/>
       <process-set id="OC4J_WirelessIslandB" numprocs="3"/>
    </process-type>
 </ias-component>

14.4 Configuring OracleAS Wireless for High-Availability Deployment

In Oracle Application Server 10g Release 2 (10.1.2.02), OracleAS Wireless applications cannot be clustered using the Oracle9iAS clustering mechanism. However, you can configure Oracle Application Server 10g Release 2 (10.1.2.02) to achieve a high-availability deployment by completing the following steps.

1. Install the Oracle Application Server 10g Release 2 (10.1.2.02) infrastructure tier on one machine and install multiple middle tiers on separate machines. Ensure that each of these middle-tier installations point to the infrastructure tier.

2. Shut down DCM and all of process by running the command

   [oracle home]/dcm/bin/dcmctl stop

3. Shut down Oracle Enterprise Manager (OEM) using the command

   [oracle home]/bin/emctl stop

4. Verify that the file [oracle home]/opmn/conf/ons.conf exists on each of the mid-tiers. Verify that the infrastructure tier contains IP-address entries for all the other tiers. If not, file and add missing IP-address entries.

5. On each middle tier, increase the number of processes that need to participate in the default island for the OC4J_Wireless OC4J instance to the desired number. This can be done from the Oracle Enterprise Manager Application Server Control or by modifying the file:

   [oracle home]/opmn/conf/opmn.xml.

   For details and concepts of OC4J instance and OC4J islands, refer to Oracle Application Server Containers for J2EE Services Guide

6. In the mod_oc4j configuration file for each middle-tier (that is, [oracle home]/Apache/Apache/conf/mod_oc4j.conf), modify the mount-point entries for the OracleAS Wireless runtime. These entries should be of the form illustrated in Example 14-3 and must be the same for all middle- tier machines.

   Example 14-3 Mount-Point Entries in mod_oc4j.conf

   Oc4jMount /ptg instance://c.host4CPU.mysite.com:OC4J_Wireless, c.host2CPU.mysite.com:OC4J_Wireless 
   
   Oc4jMount /ptg/* instance://c.host4CPU.mysite.com:OC4J_Wireless, c.host2CPU.mysite.com:OC4J_Wireless 
   
   Oc4jMount /mcs instance://c.host4CPU.mysite.com:OC4J_Wireless, c.host2CPU.mysite.com:OC4J_Wireless
   
   Oc4jMount /mcs/* instance://c.host4CPU.mysite.com:OC4J_Wireless,c.host2CPU.mysite.com:OC4J_Wireless 
   
   Oc4jMount /mcs/media instance://c.host4CPU.mysite.com:OC4J_Wireless,c.host2CPU.mysite.com:OC4J_Wireless 
   
   Oc4jMount /mcs/media/* instance://c.host4CPU.mysite.com:OC4J_Wireless,c.host2CPU.mysite.com:OC4J_Wireless 
   
   Oc4jSelectMethod roundrobin:weighted
   Oc4jRoutingWeight host4CPU.mysite.com 2
   Oc4jRoutingWeight host2CPU.mysite.com 1
   
   

   
   

   Note: In Example 14-3, c represents the instance name.

   
   

7. Run [oracle home]/dcm/bin/dcmctl updateConfig to update the DCM repository with the configuration file changes.

   On slow machines, a DCM error (timeout) of the form ADMN-906005 may appear. If this occurs, run the command [oracle home]/dcm/bin/dcmctl getReturnStatus and wait until the command exits. This confirms that the changes have been propagated to the DCM repository.

8. Add the <cluster-config/> tag under the <orion-web-app> tag in [oracle home]/j2ee/OC4J_wireless/application-deployments/ptg/ptg-web/orion-web.xml.

9. Start DCM and all processes by running the command

   [oracle home]/dcm/bin/dcmctl start.

10. Start Enterprise Manager by running the command

    [oracle home]/bin/emctl start

11. Configure a hardware load-balancer to point to the middle-tiers.

Currently, high-availability support is only available for the core server runtime (by default mapped to the URI /ptg/rm).

For more information, refer to the Oracle Application Server Containers for J2EE User's Guide .