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From Apache Wiki <wikidi...@apache.org>
Subject [Jakarta-httpclient Wiki] Update of "FrequentlyAskedConnectionManagementQuestions" by RolandWeber
Date Sun, 20 Jan 2008 15:32:53 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Jakarta-httpclient Wiki" for change

The following page has been changed by RolandWeber:

- #pragma section-numbers 2
- = Connection Management FAQ =
+ This page has been [http://wiki.apache.org/HttpComponents/FrequentlyAskedConnectionManagementQuestions
+ to the new [http://wiki.apache.org/HttpComponents/ HttpComponents Wiki].
- This document addresses questions about connection management which
- have been raised repeatedly on the HttpClient and HttpComponents mailing lists.
- ----
- [[TableOfContents]]
- -------
- == Connections in TIME_WAIT State  ==
- After running your HTTP application, you use the [http://www.netstat.net/ netstat] command
- and detect a lot of connections in state '''TIME_WAIT'''.
- Now you wonder why these connections are not cleaned up.
- === What is the TIME_WAIT State? ===
- The TIME_WAIT state is a protection mechanism in TCP.
- The side that closes a socket connection orderly will keep the connection
- in state TIME_WAIT for some time, typically between 1 and 4 minutes.
- This happens ''after'' the connection is closed. It does ''not'' indicate a cleanup problem.
- The TIME_WAIT state protects against loss of data and data corruption.
- It is there to help you. For technical details, have a look at the
- [http://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq-2.html#ss2.7
Unix Socket FAQ, section 2.7].
- === Some Connections Go To TIME_WAIT, Others Not ===
- If a connection is orderly closed by your application, it will go to the TIME_WAIT state.
- If a connection is orderly closed by the server, the server keeps it in TIME_WAIT and your
client doesn't.
- If a connection is reset or otherwise dropped by your application in a non-orderly fashion,
- it will not go to TIME_WAIT.
- Unfortunately, it will not always be obvious to you whether a connection is closed orderly
or not.
- This is because connections are pooled and kept open for re-use by default.
- !HttpClient 3.x, !HttpClient 4, and also the standard Java HttpURLConnection do that for
- Most applications will simply execute requests, then read from the response stream,
- and finally close that stream.
- [[BR]]
- Closing the response stream is ''not'' the same thing as closing the connection!
- Closing the response stream returns the connection to the pool, but it will be kept open
if possible.
- This saves a lot of time if you send another request to the same host within a few seconds,
or even minutes.
- Connection pools have a limited number of connections.
- A pool may have 5 connections, or 100, or maybe only 1.
- When you send a request to a host, and there is no open connection to that host in the pool,
- a new connection needs to be opened. But if the pool is already full,
- an open connection has to be closed before a new one can be opened.
- In this case, the old connection will be closed orderly and go to the TIME_WAIT state.
- [[BR]]
- When your application exits and the JVM terminates, the open connections in the pools
- will ''not'' be closed orderly. They are reset or cancelled, without going to TIME_WAIT.
- To avoid this, you should call the {{{shutdown}}} method of the connection pools your
- application is using before exiting.
- The standard Java HttpURLConnection has no public method to shutdown it's connection pool.
- === Running Out Of Ports ===
- Some applications open and orderly close a lot of connections within a short time,
- for example when load-testing a server. A connection in state TIME_WAIT will prevent
- that port number from being re-used for another connection. That is not an error,
- it is the purpose of TIME_WAIT.
- TCP is configured at the operating system level, not through Java.
- Your first action should be to increase the number of ephemeral ports on the machine.
- Windows in particular has a rather low default for the ephemeral ports.
- The [http://www.performancewiki.com/ PerformanceWiki] has tuning tips for the
- common operating systems, have a look at the respective Network section.
- [[BR]]
- Only if increasing the number of ephemeral ports does not solve your problem,
- you should consider decreasing the duration of the TIME_WAIT state.
- You probably have to reduce the maximum lifetime of IP packets, as the duration
- of TIME_WAIT is typically twice that timespan to allow for a round-trip delay.
- Be aware that this will affect ''all'' applications running on the machine.
- Don't ask us how to do it, we're not the experts for network tuning.
- There are some ways to deal with the problem at the application level.
- One way is to send a "Connection: close" header with each request.
- That will tell the server to close the connection, so it goes to TIME_WAIT on the other
- Of course this also disables the keep-alive feature of connection pooling
- and thereby degrades performance. If you are running load tests against a server,
- the untypical behavior of your application may distort the test results.
- [[BR]
- Another way is to not orderly close connections. There is a trick to set
- SO_LINGER to a special value, which will cause the connection to be reset
- instead of orderly closed. Note that the !HttpClient API will not support that
- directly, you'll have to extend or modify some classes to implement this hack.
- [[BR]]
- Yet another way is to re-use ports that are still blocked by a connection in TIME_WAIT.
- You can do that by specifying the SO_REUSEADDR option when opening a socket.
- Java 1.4 introduced the method
- [http://java.sun.com/j2se/1.5.0/docs/api/java/net/Socket.html#setReuseAddress(boolean) Socket.setReuseAddress]
- for this purpose.
- You will have to extend or modify some classes of !HttpClient for this too,
- but at least it's not a hack.
- === Further Reading ===
- [http://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq.html
Unix Socket FAQ]
- [http://java.sun.com/j2se/1.5.0/docs/api/java/net/Socket.html#setReuseAddress(boolean) java.net.Socket.setReuseAddress]
- [http://www.nabble.com/HttpClient-3.1---JavaHttp-TIME_WAIT-differences-tf4951549.html Discussion]
- on the !HttpClient mailing list in December 2007
- [http://www.performancewiki.com/ PerformanceWiki]
- [http://www.netstat.net/ netstat] command line tool

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