Web application farm is something that most of the organization keeps for the applications that are mission critical and require high availability in their organization. When you have web server farm for an application, you are able to configure all of them in TAC. TAC will distribute traffic equally among all operational servers in the farm. This will ensure high scalability and high availability of the application.
When you enable multiple application server farms in TAC, TAC acts as a load balancer. TAC will handle customer requests and distribute the load among the configured servers in TAC ap-plication properties.
To configure application server farm in TAC, click on “Configure farm…” option under Web Server step in application publishing wizard (next to address field).
Note: You will get this option when you select Farm of application servers in Server type.
Under farm configuration, the administrator is required to set the following options:
Server affinity type: Select server affinity type, Session-based or IP-based.
Response timeout (ms): Configure response timeout for http request. This timeout will determine whether the request is processed in a given time period, or it will be marked as down.
Success threshold attempts: Configure threshold value to verify backend web server status. The success threshold attempts represent to mark server is online after receiving successful response from backend server for the defined verification method.
Fail threshold attempts: Configure threshold value to mark backend web server offline after reaching the threshold value configured in Fail threshold attempts. TAC will mark backend server offline after getting consecutive fail response from the server for the configured threshold attempt.
TAC requires a way to check the backend web server status in the farm to process http request and make them participate in http load balancing. For that, TAC will do server verification by the configured method by the administrator.
In the verification method section, the administrator must configure the verification method that TAC should perform on each web server in the farm to verify web servers in the farm are operational and able to handle web requests. TAC provides two methods: One is send HTTP request; the second method is performing TCP/IP connection to the web servers. Sending HTTP request method is a more reliable way of identifying whether the application is working as expected. The TCP/IP method is faster.
If you select the “send an HTTP request” method, you need to enter the HTTP request method and which http response code to expect in return from the URL path that you defined in the “using URL path” field. It is important to keep in mind that all web servers in the farm have this URL path available and are accessible from TAC server.
The administrator may select the “Follow HTTP” redirects (evaluate final HTTP status) checkbox. If redirects (301 and 302) are enabled to follow, TAC will evaluate the last HTTP status code when redirects are complete. Also, TAC can check whether final HTTP response contains specific text configured in the “Response contains” field.
If, for example, after you enable Send an HTTP request using “GET” method and expect an HTTP 200 status code in response for the URL path defined in the “Using URL path filed”; TAC will attempt to get the response based on the verification method. If TAC does not receive the expected response code in return, TAC will mark the attempt as failed. If this cycle continues to fail consecutively, reaching the fail threshold attempt value, then TAC will mark the server as down or offline. TAC will not use that particular server in the farm for load balancing until TAC receives a successful response for the attempts configured in the Success threshold attempts field consecutively.
If you select “Establish a TCP connection” as the verification method, TAC will try to make TCP connection to the web servers in the farm using the IP and Port configured in TAC web server application properties page. If the TCP connection failed consecutively and the fail threshold attempt limit is reached, TAC will mark the server as down or offline and will not use for load balancing until TAC receives successful TCP connection response consecutively and reaches the success threshold attempt value
Click Verify button to verify the settings and farm configuration. Click OK to exit from the farm configuration.