+++ /dev/null
-<?xml version="1.0" encoding="ISO-8859-1"?>
-<!DOCTYPE document [
- <!ENTITY project SYSTEM "project.xml">
-]>
-<document url="timeouts.html">
-
- &project;
-<copyright>
- Licensed to the Apache Software Foundation (ASF) under one or more
- contributor license agreements. See the NOTICE file distributed with
- this work for additional information regarding copyright ownership.
- The ASF licenses this file to You under the Apache License, Version 2.0
- (the "License"); you may not use this file except in compliance with
- the License. You may obtain a copy of the License at
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- http://www.apache.org/licenses/LICENSE-2.0
-
- Unless required by applicable law or agreed to in writing, software
- distributed under the License is distributed on an "AS IS" BASIS,
- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- See the License for the specific language governing permissions and
- limitations under the License.
-</copyright>
-<properties>
-<title>Timeouts HowTo</title>
-<author email="rjung@apache.org">Rainer Jung</author>
-<date>$Date: 2009-03-22 00:11:39 +0100 (Sun, 22 Mar 2009) $</date>
-</properties>
-<body>
-<section name="Introduction">
-<br/>
-<p>Setting communication timeouts is very important to improve the
-communication process. They help to detect problems and stabilise
-a distributed system. JK can use several different timeout types, which
-can be individually configured. For historical reasons, all of them are
-disabled by default. This HowTo explains their use and gives
-hints how to find appropriate values.
-</p>
-<p>All timeouts can be configured in the workers.properties file.
-For a complete reference of all worker configuration
-items, please consult the worker <a href="../reference/workers.html">reference</a>.
-This page assumes, that you are using at least version 1.2.16 of JK.
-Dependencies on newer versions will be mentioned where necessary.
-</p>
-<warn>
-Do not set timeouts to extreme values. Very small timeouts will likely
-be counterproductive.
-</warn>
-<warn>
-Long Garbage Collection pauses on the backend do not make a good
-fit with some timeouts. Try to optimise your Java memory and GC settings.
-</warn>
-</section>
-
-<section name="JK Timeout Attributes">
-<br/>
-<subsection name="CPing/CPong">
-<p>
-CPing/CPong is our notion for using small test packets to check the
-status of backend connections. JK can use such test packets directly after establishing
-a new backend connection (connect mode) and also directly before each request gets
-send to a backend (prepost mode).
-Starting with version 1.2.27 it can also be used when a connection was idle
-for a long time (interval mode).
-The maximum waiting time (timeout) for a CPong answer to a CPing and the idle
-time in interval mode can be configured.
-</p>
-<p>
-The test packets will be answered by the backend very fast with a minimal amount of
-needed processing resources. A positive answer tells us, that the backend can be reached
-and is actively processing requests. It does not detect, if some context is deployed
-and working. The benefit of CPing/CPong is a fast detection of a communication
-problem with the backend. The downside is a slightly increased latency.
-</p>
-<p>
-The worker attribute <b>ping_mode</b> can be set to a combination of characters
-to determine, in which situations test packets are used:
-<ul>
-<li><b>C</b>: connect mode, timeout <b>ping_timeout</b> overwritten by <b>connect_timeout</b></li>
-<li><b>P</b>: prepost mode, timeout <b>ping_timeout</b> overwritten by <b>prepost_timeout</b></li>
-<li><b>I</b>: interval mode, timeout <b>ping_timeout</b>, idle time <b>connection_ping_interval</b></li>
-<li><b>A</b>: all modes</li>
-</ul>
-</p>
-<p>
-Multiple values must be concatenated without any separator characters.
-We recommend using all CPing tests. If your application is very latency sensitive, then
-you should only use the combination of connect and interval mode.
-</p>
-<p>
-Activating the CPing probing via <b>ping_mode</b> has been added in version 1.2.27.
-For older versions only the connect and prepost modes exist and must be activated by
-explicitely setting <b>connect_timeout</b> and <b>prepost_timeout</b>.
-</p>
-<p>
-The worker attribute <b>ping_timeout</b> sets the default wait timeout
-in milliseconds for CPong for all modes. By default the value is "10000"
-milliseconds. The value only gets used, if you activate CPing/Cpong probes
-via <b>ping_mode</b>. The default value should be fine, except if you experience
-very long Java garbage collection pauses.
-Depending on your network latency and stability, good custom values
-often are between 5000 and 15000 milliseconds.
-You can overwrite the timeout used for connect and prepost mode with
-<b>connect_timeout</b> and <b>prepost_timeout</b>.
-Remember: don't use extremely small values.
-</p>
-<p>
-The worker attribute <b>connect_timeout</b> sets the wait timeout
-in milliseconds for CPong during connection establishment. You can use it
-if you want to overwrite the general timeout set with <b>ping_timeout</b>.
-To use connect mode CPing, you need to enable it via <b>ping_mode</b>.
-Since JK usually uses persistent connections, opening new connections is a
-rare event. We therefore recommend activating connect mode.
-Depending on your network latency and stability, good values often
-are between 5000 and 15000 milliseconds.
-Remember: don't use extremely small values.
-</p>
-<p>
-The worker attribute <b>prepost_timeout</b> sets the wait timeout
-in milliseconds for CPong before request forwarding. You can use it
-if you want to overwrite the general timeout set with <b>ping_timeout</b>.
-To use prepost mode CPing, you need to enable it via <b>ping_mode</b>.
-Activating this type of CPing/CPong adds a small latency to each
-request. Usually this is small enough and the benefit of CPing/CPong is more important.
-So in general we also recommend using <b>prepost_timeout</b>.
-Depending on your network latency and stability, good values often
-are between 5000 and 10000 milliseconds.
-Remember: don't use extremely small values.
-</p>
-<p>
-Until version 1.2.27 <b>ping_mode</b> and <b>ping_timeout</b> did not
-exist and to enable connect or prepost mode CPing you had to set <b>connect_timeout</b>
-respectively <b>prepost_timeout</b> to some reasonable positive value.
-</p>
-</subsection>
-
-<subsection name="Low-Level TCP Timeouts">
-<p>
-Some platforms allow to set timeouts for all operations on TCP sockets.
-This is available for Linux and Windows, other platforms do not support this,
-e.g. Solaris. If your platform supports TCP send and receive timeouts,
-you can set them using the worker attribute <b>socket_timeout</b>.
-You can not set the two timeouts to different values.
-</p>
-<p>
-JK will accept this attribute even if your platform does not support
-socket timeouts. In this case setting the attribute will have no effect.
-By default the value is "0" and the timeout is disabled.
-You can set the attribute to some seconds value (not: milliseconds).
-JK will then set the send and the receive timeouts of the backend
-connections to this value. The timeout is low-level, it is
-used for each read and write operation on the socket individually.
-</p>
-<p>
-Using this attribute will make JK react faster to some types of network problems.
-Unfortunately socket timeouts have negative side effects, because for most
-platforms, there is no good way to recover from such a timeout, once it fired.
-For JK there is no way to decide, if this timeout fired because of real network
-problems, or only because it didn't receive an answer packet from a backend in time.
-So remember: don't use extremely small values.
-</p>
-<p>
-For the general case of connection establishment you can use
-<b>socket_connect_timeout</b>. It takes a millisecond value and works
-on most platforms, even if <b>socket_timeout</b> is not supported.
-We recommend using <b>socket_connect_timeout</b> because in some network
-failure situations failure detection during connection establishment
-can take several minutes due to TCP retransmits. Depending on the quality
-of your network a timeout somewhere between 1000 and 5000 milliseconds
-should be fine. Note that <code>socket_timeout</code> is in seconds, and
-<code>socket_connect_timeout</code> in milliseconds.
-</p>
-</subsection>
-
-<subsection name="Connection Pools and Idle Timeouts">
-<p>
-JK handles backend connections in a connection pool per web server process.
-The connections are used in a persistent mode. After a request completed
-successfully we keep the connection open and wait for the next
-request to forward. The connection pool is able to grow according
-to the number of threads that want to forward requests in parallel.
-</p>
-<p>
-Most applications have a varying load depending on the hour of the day
-or the day of the month. Other reasons for a growing connection pool
-would be temporary slowness of backends, leading to an increasing
-congestion of the frontends like web servers. Many backends use a dedicated
-thread for each incoming connection they handle. So usually one wants the
-connection pool to shrink, if the load diminishes.
-</p>
-<p>
-JK allows connections in the pool to get closed after some idle time.
-This maximum idle time can be configured with the attribute
-<b>connection_pool_timeout</b> which is given in units of seconds.
-The default value is "0", which disables closing idle connections.
-</p>
-<p>
-We generally recommend values around 10 minutes, so setting
-<b>connection_pool_timeout</b> to 600 (seconds). If you use this attribute,
-please also set the attribute <b>connectionTimeout</b> in the AJP
-Connector element of your Tomcat server.xml configuration file to
-an analogous value. <b>Caution</b>: connectionTimeout is in milliseconds.
-So if you set JK connection_pool_timeout to 600, you should set Tomcat
-connectionTimeout to 600000.
-</p>
-<p>
-JK connections do not get closed immediately after the timeout passed.
-Instead there is an automatic internal maintenance task
-running every 60 seconds, that checks the idle status of all connections.
-The 60 seconds interval
-can be adjusted with the global attribute worker.maintain. We do not
-recommend to change this value, because it has a lot of side effects.
-Until version 1.2.26, the maintenance task only runs, if requests get
-processed. So if your web server has processes that do not receive any
-requests for a long time, there is no way to close the idle connections
-in its pool. Starting with version 1.2.27 you can configure an independent
-watchdog thread when using Apache 2.x with threaded APR or IIS.
-</p>
-<p>
-The maximum connection pool size can be configured with the
-attribute <b>connection_pool_size</b>. We generally do not recommend
-to use this attribute in combination with Apache httpd. For
-Apache httpd we automatically detect the number of threads per
-process and set the maximum pool size to this value. For IIS we use
-a default value of 250 (before version 1.2.20: 10),
-for the Sun Web Server the default is "1".
-We strongly recommend adjusting this value for IIS and the Sun Web Server
-to the number of requests one web server process should
-be able to send to a backend in parallel. You should measure how many connections
-you need during peak hours without performance problems, and then add some
-percentage depending on your growth rate etc. Finally you should check,
-whether your web server processes are able to use at least as many threads,
-as you configured as the pool size.
-</p>
-<p>
-The JK attribute <b>connection_pool_minsize</b> defines,
-how many idle connections remain when the pool gets shrunken.
-By default this is half of the maximum pool size.
-</p>
-</subsection>
-
-<subsection name="Firewall Connection Dropping">
-<p>
-One particular problem with idle connections comes from firewalls, that
-are often deployed between the web server layer and the backend.
-Depending on their configuration, they will silently drop
-connections from their status table if they are idle for to long.
-</p>
-<p>
-From the point of view of JK and of the web server, the other side
-simply doesn't answer any traffic. Since TCP is a reliable protocol
-it detects the missing TCP ACKs and tries to resend the packets for
-a relatively long time, typically several minutes.
-</p>
-<p>
-Many firewalls will allow connection closing, even if they dropped
-the connection for normal traffic. Therefore you should always use
-<a href="#Connection Pools and Idle Timeouts">connection_pool_timeout and
-connection_pool_minsize</a> on the JK side
-and connectionTimeout on the Tomcat side.
-</p>
-<p>
-Furthermore using the boolean attribute <b>socket_keepalive</b> you can
-set a standard socket option, that automatically sends TCP keepalive packets
-after some idle time on each connection. By default this is set to "False".
-If you suspect idle connection drops by firewalls you should set this to
-"True".
-</p>
-<p>
-Unfortunately the default intervals and algorithms for these packets
-are platform specific. You might need to inspect TCP tuning options for
-your platform on how to control TCP keepalive.
-Often the default intervals are much longer than the firewall timeouts
-for idle connections. Nevertheless we recommend talking to your firewall
-administration and your platform administration in order to make them agree
-on good configuration values for the firewall and the platform TCP tuning.
-</p>
-<p>
-In case none of our recommendations help and you are definitively having
-problems with idle connection drops, you can disable the use of persistent
-connections when using JK together with Apache httpd. For this you set
-"JkOptions +DisableReuse" in your Apache httpd configuration.
-This will have a huge negative performance impact!
-</p>
-</subsection>
-
-<subsection name="Reply Timeout">
-<p>
-JK can also use a timeout on request replies. This timeout does not
-measure the full processing time of the response. Instead it controls,
-how much time between consecutive response packets is allowed.
-</p>
-<p>
-In most cases, this is what one actually wants. Consider for example
-long running downloads. You would not be able to set an effective global
-reply timeout, because downloads could last for many minutes.
-Most applications though have limited processing time before starting
-to return the response. For those applications you could set an explicit
-reply timeout. Applications that do not harmonise with reply timeouts
-are batch type applications, data warehouse and reporting applications
-which are expected to observe long processing times.
-</p>
-<warn>
-If JK aborts waiting for a response, because a reply timeout fired,
-there is no way to stop processing on the backend. Although you free
-processing resources in your web server, the request
-will continue to run on the backend - without any way to send back a
-result once the reply timeout fired.
-</warn>
-<p>
-JK uses the worker attribute <b>reply_timeout</b> to set reply timeouts.
-The default value is "0" (timeout disabled) and you can set it to any
-millisecond value.
-</p>
-<p>
-In combination with Apache httpd, you can also set a more flexible reply_timeout
-using an httpd environment variable. If you set the variable JK_REPLY_TIMEOUT
-to some integer value, this value will be used instead of the value in
-the worker configuration. This way you can set reply timeouts more flexible
-with mod_setenvif and mod_rewrite depending on URI, query string etc.
-If the environment variable JK_REPLY_TIMEOUT is not set, or is set to a
-negative value, the default reply timeout of the worker will be used. If
-JK_REPLY_TIMEOUT contains the value "0", then the reply timeout will be disabled
-for the request.
-</p>
-<p>
-In combination with a load balancing worker, JK will disable a member
-worker of the load balancer if a reply timeout fires. The worker will then
-no longer be used until it gets recovered during the next automatic
-maintenance task. Starting with JK 1.2.24 you can improve this behaviour using
-<b><a href="../reference/workers.html">max_reply_timeouts</a></b>. This
-attribute will allow occasional long running requests without disabling the
-worker. Only if those requests happen to often, the worker gets disabled by the
-load balancer.
-</p>
-</subsection>
-</section>
-
-<section name="Load Balancer Error Detection">
-<br/>
-<subsection name="Local and Global Error States">
-<p>
-A load balancer worker does not only have the ability to balance load.
-It also handles stickyness and failover of requests in case of errors.
-When a load balancer detects an error on one of its members, it needs to
-decide, whether the error is serious, or only a temporary error or maybe
-only related to the actual request that was processed. Temporary errors
-are called local errors, serious errors will be called global errors.
-</p>
-<p>
-If the load balancer decides that a backend should be put into the global error
-state, then the web server will not send any more requests there. If no session
-replication is used, this means that all user sessions located on the respective
-backend are no longer available. The users will be send to another backend
-and will have to login again. So the global error state is not transparent to the
-users. The application is still available, but users might loose some work.
-</p>
-<p>
-In some cases the decision between local error and global error is easy.
-For instance if there is an error sending back the response to the client (browser),
-then it is very unlikely that the backend is broken.
-So this situation is a typical example of a local error.
-</p>
-<p>
-Some situations are harder to decide though. If the load balancer can't establish
-a new connection to a backend, it could be because of a temporary overload situation
-(so no more free threads in the backend), or because the backend isn't alive any more.
-Depending on the details, the right state could either be local error or global error.
-</p>
-</subsection>
-<subsection name="Error Escalation Time">
-<p>
-Until version 1.2.26 most errors were interpreted as global errors.
-Starting with version 1.2.27 many errors which were previously interpreted as global
-were switched to being local whenever the backend is still busy. Busy means, that
-other concurrent requests are send to the same backend (successful or not).
-</p>
-<p>
-In many cases there is no perfect way of making the decision
-between local and global error. The load balancer simply doesn't have enough information.
-In version 1.2.28 you can now tune, how fast the load balancer switches from local error to
-global error. If a member of a load balancer stays in local error state for too long,
-the load balancer will escalate it into global error state.
-</p>
-<p>
-The time tolerated in local error state is controlled by the load balancer attribute
-<b>error_escalation_time</b> (in seconds). The default value is half of <b>recover_time</b>,
-so unless you changed <b>recover_time</b> the default is 30 seconds.
-</p>
-<p>
-Using a smaller value for <b>error_escalation_time</b> will make the load balancer react
-faster to serious errors, but also carries the risk of more often loosing sessions
-in not so serious situations. You can lower <b>error_escalation_time</b> down to 0 seconds,
-which means all local errors which are potentially serious are escalated to global errors
-immediately.
-</p>
-<p>
-Note that without good basic error detection the whole escalation procedure is useless.
-So you should definitely use <b>socket_connect_timeout</b> and activate CPing/CPong
-with <b>ping_mode</b> and <b>ping_timeout</b> before thinking about also tuning
-<b>error_escalation_time</b>.
-</p>
-</subsection>
-</section>
-
-</body>
-</document>
Code Review / bottlenecks.git / blobdiff