Web server plug-in connections

The WebSphere Web server plug-in establishes and maintains persistent connections to an application server's HTTP and HTTPS transports.

When the plug-in is ready to send a request to the application server, it first checks its connection pool for existing connections. If an existing connection is available the plug-in checks its connection status. If the status is still good, the plug-in uses that connection to send the request. If a connection does not exist, the plug-in creates one. If a connection exists but has been closed by the application server, the plug-in closes that connection and opens a new one.

After a connection is established between a plug-in and the application server, it will not be closed unless the application server closes it for one of the following reasons:

• The time limit specified on the ConnectionKeepAliveTimeout HTTP transport custom property has expired.
• The maximum number of requests that can be processed on a single keep alive has been exceeded. (This number is specified by the connectionMaxKeepAliveRequests HTTP transport custom property.)
• The maximum number of concurrent keep alive (persistent) connections across all HTTP transports has been exceeded. (This number is specified by the MaxKeepAliveConnections HTTP transport custom property.)
• The application server is shutting down.

When the application server closes a connection, the plug-in only determines that the connection is closed after attempting to use it to route another request. The connection is closed if one of the following events occurs:

• The plug-in receives a new HTTP request and tries to reuse the existing connection.
• The number of httpd processes drop because the Web server is not receiving any new HTTP requests. (For the IHS Web server, the number of httpd processes that are retained depends on the value specified on the Web server's MinSpareServers directive.)
• The Web server is stopped and all httpd processes are terminated, and their corresponding sockets are closed.

Note: Sometimes, if a heavy request load is stopped or decreased abruptly on a particular application server, several of the plug-in's connections to that application server are in the CLOSE_WAIT state. Because these connections are closed the first time the plug-in tries to reuse them, having a large number of connections in CLOSE-WAIT state does not typically affect performance.