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<document url="timeouts.html">

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<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>
