Web server plug-ins

Web server plug-ins enable the web server to communicate requests for dynamic content, such as servlets, to the application server. A web server plug-in is associated with each web server definition. The configuration file (plugin-cfg.xml) that is generated for each plug-in is based on the applications that are routed through the associated web server.

A web server plug-in is used to forward HTTP requests from a supported web server to an application server. Using a web server plug-in to provide communication between a web server and an application server has the following advantages:

Each of the supported web server plug-ins runs on a number of operating systems. See Supported Hardware and Software for the product for the most current information about supported web servers.

The default behavior for the web server plug-in is to buffer requests up to 64 kilobytes, and retry the requests if there is no response from the application server. To ensure high availability, and the HTTP requests tend to be large, set the Maximum buffer size used when reading HTTP request content property on the web server plug-in request routing property page in the console to -1. Setting this property to -1 removes the maximum buffer size limit, and enables the web server plug-in to buffer all requests regardless of their size. Requests are retried if the request body fits within the buffer size. To disable all request buffering, and thereby disable retries of requests with request bodies, we can set this property to 0.gotcha


Affinity requests

Affinity requests are requests containing a JSESSIONID. Session affinity means that all requests of the same JSESSIONID are sent to the same application server. For example, if the initial request is sent to clone5, then the next affinity request from that same browser is also sent to clone5 regardless of the weight valued specified for the LoadBalanceWeight property in the plugin-cfg.xml file.

If we select Round robin for the Load balancing option Web server plug-in request routing property, and leave the IgnoreAffinityRequests property in the plugin-cfg.xml file set to its default value of true, the affinity requests do not lower the weight. This behavior might cause an uneven distribution of requests across the servers in environments that make use of session affinity. Setting the IgnoreAffinityRequests property to false causes the weight to be lowered every time an affinity request is received, which results in a more balanced round robin environment.

If we select Random for the Load balancing option property, affinity requests are still sent to the same cloneid, but new requests are routed randomly, and the value specified for the LoadBalanceWeight property is ignored.


Failover

If a request connection exceeds the time limit specified on the ConnectTimeout property in the plugin-cfg.xml file, or a 5xx error is returned from the application server, the web server plug-in marks the server as down, and attempts to connect to the next application server in the list of primary servers specified for the PrimaryServers property in the plugin-cfg.xml file. If the web server plug-in successfully connects to another application server, all requests that were pending for the down application server are sent to this other application server. All other new and affinity requests are sent to other servers, based on whether round robin or random is the setting for the Load balancing option Web server plug-in request routing property.

Failover typically does not occur the first time that the time limit specified on the ServerIOTimeout property in the plugin-cfg.xml file is exceeded for either a request or a response. Instead, the web server plug-in tries to resend the request to the same application server, using a new stream. If the time specified on the ServerIOTimeout property is exceeded a second time, the web server plug-in marks the server as unavailable, and initiates the failover process.

Avoid trouble: Sending a large number of pending requests to the same application server might impact the performance of that application server if a failover situation occurs. We can use the MaxConnections property to limit the number of requests that might be pending for an application server.gotcha


Running multiple web server child processes

We can configure most web servers to start multiple child processes. In this situation, each child process loads its own instance of the web server. When running multiple web server child processes, remember that:


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