Display Viewer Applet using Data Server(s) via HTTP

This deployment is chosen when a browser deployment is desired, data updates must be displayed faster than once a second, and Java can be installed on the clients.

In this high availability configuration, as shown below, a single URL is configured for the applet's connection to the rtvdata servlet. This is because the applet security requires that it can only make connections to the host specified in the URL by which it is loaded. The URL specifies a web server that is also a load balancer. A load balancer allows a set of redundant servlets to be accessed by a single URL, directing requests to the servlet that is the least busy. If a servlet fails, the load balancer switches any existing client connections to a different servlet and does not direct any new requests to the failed servlet.

Note in the diagram below, that each rtvdata servlet has a primary and backup data server connection. This is useful when the servlet is available, but its primary data server has failed. Note also that both servlets have the same primary data server. This ensures that, during normal operation, a client will see the same data server state regardless of which servlet the load balancer chooses.

Primary and backup Data Servers can be configured in the Display Builder, by selecting the Data Server tab in the Application Options dialog, or with the Configuration Utility. See the High Availability Data Servers section for details.

High Availability Configuration with Apache and Tomcat

The diagram and example that follow detail how to configure this high-availability deployment with Tomcat 6.0.18 and Apache 2.2.9:

1. On BackendHost1, in Tomcat's server.xml file, modify the Engine entry by adding a jvmRoute:

<Engine name="Catalina" defaultHost="localhost" jvmRoute="tomcat1">

2. On BackendHost2:

<Engine name="Catalina" defaultHost="localhost" jvmRoute="tomcat2">

3. Deploy the rtvdata servlet to Tomcat on BackendHost1 and BackendHost2.

4. On the host running the Apache webserver (load-balancer), make the following changes to the httpd.conf file to enable load balancing:

• Un-comment the LoadModule line for all mod_proxy modules: proxy_module, proxy_ajp_module, proxy_balancer_module, proxy_connect_module, proxy_ftp_module, proxy_http_module.
• Define a proxy for the balancer group of rtvdata servlets. The name of this proxy (rtvdata) is the name by which clients will access the servlets.

# proxy for rtvdata load balancer group
ProxyPass /rtvdata balancer://rtvdata_group stickysession=JSESSIONID

• Define the workers (servlets) for the rtvdata balancer group. The default tomcat ajp connector on port 8009 is used. Note that the value of "route" for each BalancerMember must match the entry in the corresponding server.xml entry for tomcat, as described above.

 # list of actual workers for rtvdata_group
# (route must match jvmRoute in Engine entry in server.xml
# for the correpsonding tomcat instance!)
<Proxy balancer://rtvdata_group>
        BalancerMember ajp://BackendHost1:8009/rtvdata route=tomcat1
        BalancerMember ajp://BackendHost2:8009/rtvdata route=tomcat2
</Proxy>

• For monitoring, enable the apache balancer-manager webapp. This webapp can be viewed in a browser via http://localhost/balance-manager.

# enable balance-manager
<Location /balancer-manager>
        SetHandler balancer-manager
</Location>