The web server plug-ins are used to establish and maintain
persistent HTTP and HTTPS connections to application servers.
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 an application
server, it will not be closed unless the application server closes
it for one of the following reasons:
- If the Use Keep-Alive property is selected and the time
limit specified on the Read timeout or Write timeout property
for the HTTP inbound channel has expired.
- The maximum number of persistent requests which can be processed
on an HTTP inbound channel has been exceeded. This number is set using
the Maximum persistent requests property that is specified
for the HTTP inbound channel.
- The Application Server is shutting down.
Even if the application server closes a connection, the plug-in
will not know that it has been closed until it tries to use it again.
The connection will be closed if one of the following events occur:
- 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 IBM® HTTP
Server, the number of httpd processes that are kept alive 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.
Avoid trouble: Sometimes, if a heavy request
load is stopped or decreased abruptly on a particular application
server, a lot of the plug-in's connections to that application server
will be in CLOSE_WAIT state. Because these connections will be closed
the first time the plug-in tries to reuse them, having a large number
of connections in CLOSE-WAIT state should not affect performance
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