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From Hermes Flying <>
Subject Re: Fw: Can not understand how maxThreads of Connectors works
Date Thu, 24 Jan 2013 18:03:49 GMT

I am sure that this server.xml is the one used, since there is no other present.
Also as mentioned my plan is to cut network access after a threshold. I used such small values
e.g. 0,1,2 to see what happens. 
Also note that I am not using SUN JVM but IBM. Not sure if this makes a difference

 From: André Warnier <>
To: Tomcat Users List <> 
Sent: Thursday, January 24, 2013 7:14 PM
Subject: Re: Fw: Can not understand how maxThreads of Connectors works
Hermes Flying wrote:
> Hi,
> I don't see how this answers my issue.
> 1) You say 0 threads means 0 requests being processed. This does not happen. Requests
are being processed. No error noticed

It is not only me saying it. The on-line documentation at
says this :


The maximum number of request processing threads to be created by this Connector, which therefore
determines the maximum number of simultaneous requests that can be handled. If not specified,
this attribute is set to 200. If an executor is associated with this connector, this attribute
is ignored as the connector will execute tasks using the executor rather than an internal
thread pool.


Now, considering this, there are a number of possibilities :
- the documentation is totally wrong
- there is a bug in Tomcat
- your Tomcat server is not using this server.xml
- or, it being rather unlikely that processing 0 requests is what a normal user would want,
the Tomcat developers have coded this so that an obviously nonsensical value of 0 would result
in the default (of 200) being applied.

Pick any one of the above.

> 2)You say: "you are setting your parameters in the wrong place". This is not the case
here.I already send an example server.xml. Will copy/paste it again bellow

Yes, but we cannot check from here if this is really the server.xml that your Tomcat is reading.
Are you absolutely sure it is ? How ?

> 3)"It does not seem to make a lot of sense to set up a whole machinery like a host, a
JVM and a Tomcat, just to process one single request at a time." I am not planning to do that,
but I must see how the system behaves in various configuration. Tomcat does not seem to behave
as expected in the trivial case.

The "trivial case" is 0 Threads ? What happens when you set it to 1 ?

> 4) "The default Tomcat settings are chosen by people who know what they are doing, to
obtain a reasonable Tomcat behaviour over a reasonable range of conditions" What does this
actually mean? That we are not supposed to configure Tomcat according to our needs?

Of course you can. But if you are using nonsensical values, do you expect a sensible behaviour
Come on, man. This is open-source software, that you get to use for free.
This is not to say that it is not good software, nor that the developers do not try to make
it as efficient and reliable as possible, nor that the people writing the documentation (also
for free) do not make every effort to write it well and accurately.
On the other hand, it is kind of expected that people using Tomcat and configuring it, would
use a bit of judgment and give the developers a bit of slack.

I am not a developer of Tomcat, but what I tried to provide in my explanation is a guideline
as to how these parameters are supposed to work. That was to help you maybe find the reason
for what appears to you as "not working", but which apparently other people cannot reproduce.

I just tried with Tomcat 6.0.24 under Windows, and when I set
maxThreads="0" in the HTTP Connector, Tomcat starts up without error in the log. But if I
try to access it with a browser, the browser loops saying "connecting.." and never goes past
that point.  If I set maxThreads="1", then Tomcat is answering with the homepage.

Same thing with Tomcat 7.0.21.

So I would say : check that the server.xml below is really the one that Tomcat is using.

(Additionally, I would say that it seems that when Tomcat is configured to not have any Threads
to process requests, well it just does not process any.  Which seems to me like sensible
behaviour under adverse circumstances.)

> <?xml version='1.0' encoding='utf-8'?>
> <Server port="8005" shutdown="SHUTDOWN">
>   <!--APR library loader. Documentation at /docs/apr.html -->
>   <Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on"
>   <!--Initialize Jasper prior to webapps are loaded. Documentation at /docs/jasper-howto.html
>   <Listener className="org.apache.catalina.core.JasperListener" />
>   <!-- Prevent memory leaks due to use of particular java/javax APIs-->
>   <Listener
>  className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
>   <!-- JMX Support for the Tomcat server.
>  Documentation at /docs/non-existent.html -->
>   <Listener className="org.apache.catalina.mbeans.ServerLifecycleListener" />
>   <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener"
>   <!-- Global JNDI resources
>        Documentation at /docs/jndi-resources-howto.html
>   -->
>   <GlobalNamingResources>
>     <!-- Editable user database that can also be used by
>          UserDatabaseRealm to authenticate users
>     -->
>     <Resource name="UserDatabase" auth="Container"
>               type="org.apache.catalina.UserDatabase"
>               description="User database that can be updated and
>  saved"
>               factory="org.apache.catalina.users.MemoryUserDatabaseFactory"
>               pathname="conf/tomcat-users.xml" />
>   </GlobalNamingResources>
>   <!-- A "Service" is a collection of one or more "Connectors" that share
>        a single "Container" Note:  A "Service" is not itself a "Container",   
    so you may not define subcomponents such as "Valves" at this level.
>        Documentation at /docs/config/service.html
>    -->
>   <Service name="Catalina">
>       <!--The connectors can use a shared executor, you can define one or more
named thread pools-->
>     <!--
>     <Executor name="tomcatThreadPool" namePrefix="catalina-exec-"         maxThreads="150"
>     -->
>             <!-- A "Connector" represents an endpoint by which requests are
>          and responses are returned. Documentation at :
>          Java HTTP Connector: /docs/config/http.html (blocking & non-blocking)
>          Java AJP  Connector: /docs/config/ajp.html
>          APR (HTTP/AJP) Connector: /docs/apr.html
>          Define a non-SSL HTTP/1.1 Connector on port 8080
>     -->
>     <Connector port="8080" maxThreads="0" acceptCount="1"
>  protocol="HTTP/1.1"                connectionTimeout="20000"         
      redirectPort="8443" />
>     <!-- Define an AJP 1.3 Connector on port 8009 -->
>     <Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />
>     <!-- An Engine represents the entry point (within Catalina) that processes
>          every request.  The Engine implementation for Tomcat stand alone
>          analyzes the HTTP headers included with the request, and passes them
>          on to the appropriate Host (virtual host).
>          Documentation at /docs/config/engine.html
>  -->
>     <!-- You should set jvmRoute to support load-balancing via AJP ie :
>     <Engine name="Catalina" defaultHost="localhost" jvmRoute="jvm1">     
       -->     <Engine name="Catalina" defaultHost="localhost">
>       <!-- This Realm uses the UserDatabase configured in the global JNDI
>            resources under the key "UserDatabase".  Any edits
>            that are performed against this UserDatabase are immediately
>            available for use by the Realm.  -->
>       <Realm
>  className="org.apache.catalina.realm.UserDatabaseRealm"
>              resourceName="UserDatabase"/>
>       <!-- Define the default virtual host
>            Note: XML Schema validation will not work with Xerces 2.2.
>        -->
>       <Host name="localhost"  appBase="webapps"
>             unpackWARs="true" autoDeploy="true"
>             xmlValidation="false" xmlNamespaceAware="false">
>       </Host>
>     </Engine>
>   </Service>
> </Server>
> ________________________________
>  From: André Warnier <>
> To: Tomcat Users List <> Sent: Thursday, January 24, 2013
11:53 AM
> Subject: Re: Fw: Can not understand how maxThreads of Connectors works
>  Hermes Flying wrote:
>> Hi,
>> So is there an explanation for this? All I am interested is make sure that after
a limit, clients attempted to connect are stopped based on my configuration on maxThreads
and accept count.
>> But I can not figure out how this works.
> (This all being explained in vernacular language to which experts may object).
> Threads in Tomcat serve to process requests.  Each Thread can process one request.
> 0 Threads = 0 requests being processed.
> n Threads = n requests can be processed simultaneously (kind of).
> Threads "belong to" either a Connector (by default), or an Executor (if you configure
several Connectors to use an Executor, then they use the common pool of Threads of the Executor,
instead of their own individual pool of Threads).
> Having a common pool of Threads between several Connectors is normally more efficient
and allows for a smoother operation.  Otherwise you could have the case that requests arriving
through one Connector (e.g. HTTP) are being starved because this particular Connector has
no more Threads available, while on the other hand another Connector (e.g. AJP) still has
plenty of capacity.
> The "acceptCount" is another matter entirely, working at a deeper level.
> Before a Thread is assigned to process a request,
> - the client requests a TCP connection to the server
> - the server must "accept" this connection. If it doesn't within a certain time, the
client will get an error (connect timeout).
> - when the server accepts the connection, it goes into a queue. The length of that queue
corresponds to the acceptCount of the corresponding Connector.
> (If the accept queue is already full, the connection request will be rejected).
> As long as the server does no further action, an accepted connection stays in the queue
and the client request does not proceed. If that lasts a long time, the client may timeout
(usually saying that the server is not responsive).
> - whenever the server feels like it (for example, when it sees that it has at least one
Thread free to handle a request), it will pop the first connection from the accept queue,
and pass it to a Thread to be processed.
> Now a Thread is assigned to process this request, so one less Thread is available in
the pool of Threads.
> - if another client connection happens now, it goes into the accept queue.
> - whenever the original Thread is done processing the request, the Thread goes back into
the pool of available Threads, and could be assigned to another client request currently sitting
in the accept queue.
> That's roughly how it works.
> If it does not do so in your case, then it must mean that you are setting your parameters
in the wrong place, and Tomcat is either not seeing them at all, or ignoring them because
they are not where they should be.
> The default Tomcat settings are chosen by people who know what they are doing, to obtain
a reasonable Tomcat behaviour over a reasonable range of conditions.
> If you change these settings, you can get a behaviour that is no longer reasonable or
> For example, if you set the accepCount to 1 and maxThreads to 1, then you can have the
following :
> - 1 request accepted and allocated a Thread, thus in process
> - 1 additional request being queued in the accept queue, waiting for a Thread to become
> And any additional client request arriving at that time will be rejected at the TCP level. 
That will hardly result in an error that is understandable by the clients.
> Intuitively, it does not seem to make a lot of sense to set up a whole machinery like
a host, a JVM and a Tomcat, just to process one single request at a time.
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