If you want to use the session failover functionality, a system configuration that uses a load balancer and distributes requests on multiple J2EE servers, is a prerequisite. Moreover, it is necessary to deploy a database or an EADs server for storing HTTP session information that is created on each J2EE server.

The following figure shows the prerequisite configuration for using the database session failover functionality or the EADs session failover functionality.

Figure 5-3 Prerequisite configuration for the session failover functionality

• Load balancer
  For using the session failover functionality, use of a load balancer is a prerequisite.
  If you are using the load balancer of a redirector, you cannot use the session failover functionality. If you concurrently use the session failover functionality and load balancer of a redirector, the following problems occur:
  
  • You cannot balance a load of the J2EE server itself with a load balancer.
  • You cannot send a health check of the load balancer to any J2EE server. Even when a failure occurs on a J2EE server, redirector transfers requests to the normal J2EE server. As a result, you cannot detect the failure on the J2EE server with the load balancer.
    

    Reference note

    Distributing a request with a load balancer

    Requests are distributed with a load balancer. This distributes the load, achieving stable operations of the system and improving processing performance.

    The load balancing performed by using a load balancer has the advantage that the load related to the load balancing processing is not applied to the Web server and J2EE server. Methods for the request distribution vary according to the load balancer.

    The following figure shows an example of distributing requests by using a load balancer.

    Figure 5-4 Example of request distribution by using a load balancer

    
    

• The J2EE server or Web server
  For using the session failover functionality, arrange one or more J2EE servers and Web servers in one system. We recommend that you arrange two or more servers in the preparation for a J2EE server failure.
  
• EADs client
  When using the EADs session failover functionality, an EADs client is required on the J2EE server for operating data on the EADs sever from a J2EE server. The following table describes the software required for an EADs client.
  

  Table 5-4 Software required for an EADs client

  Category	Product name
  EADs Client	Elastic Application Data store Client for Application Server 02-00

• Database
  When using the database session failover functionality, a database is required as a storage location of session information. The following table describes mapping of the databases, which you can use as a storage destination of session information, JDBC drivers, and resource adapters.
  

  Table 5-5 Mapping of databases, JDBC drivers, and resource adapters which you can use

  Database	JDBC driver	Resource adapter#
  HiRDB	HiRDB Type4 JDBC Driver	DBConnector_HiRDB_Type4_CP.rar
  Oracle	Oracle JDBC Thin Driver	DBConnector_Oracle_CP.rar
  		DBConnector_CP_ClusterPool_Root.rar
  		DBConnector_Oracle_CP_ClusterPool_Member.rar

  # DB Connector is the resource adapter used in the database session failover functionality. For details on the settings required for DB Connector used in the database session failover functionality, see 6.9 DB Connector Settings.

  
  For details on the system configuration for using the database session failover functionality, see 3.10.1 Configuration that uses a database (the database session failover functionality) in the uCosminexus Application Server System Design Guide.
  If the configuration that uses the database session failover functionality satisfies the conditions described here, you need not redesign from the system configuration. You can use the database session failover functionality if you implement the functionality settings and perform parameter tuning.
  
• EADs server
  When using the EADs session failover functionality, an EADs server, which you can use to store session information, is required. Use the EADs server used in the EADs session failover functionality only in the EADs session failover functionality. You can also improve availability by increasing the number of EADs servers. A set of multiple EADs servers is called an EADs cluster.
  The following table describes the software required for EADs server.
  

  Table 5-6 Software required for an EADs server

  Category	Product name
  EADs server	Elastic Application Data store for Application Server 02-00

  For details on the system configuration for using the EADs session failover functionality, see 3.10.2 Configuration in which EADs server is placed on a different machine from the J2EE server (the EADs session failover functionality) or 3.10.3 Configuration in which the EADs server is placed on the same machine as the J2EE server (the EADs session failover functionality) in the uCosminexus Application Server System Design Guide.
  Note that you cannot use an EADs application in the EADs session failover environment.