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Posted to users@tomcat.apache.org by "Reich, Matthias" <ma...@siemens-enterprise.com> on 2010/06/08 12:26:19 UTC
Connection is closed when CometEvent.close is called during an
event
Hello,
I am using a CometProcessor servlet in a long-poll scenario, and recently had
a closer look at the life span of connections that are used for poll requests.
I noticed that connections are closed by Tomcat whenever a poll request
was answered (and closed) directly during processing of the BEGIN event.
In our application this happens for one out of three poll requests approximately
and thus should not be neglected.
I had a look into the source code and found the reason in the
CoyoteAdapter.event method - it sets the 'error' flag to true in this situation.
I modified the code (in 6.0.20 and 6.0.26) so that the error flag is not set.
With that change Tomcat kept the connection open:
...
// Calling the container
connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
if (!error && !response.isClosed() && (request.getAttribute(Globals.EXCEPTION_ATTR) != null)) {
// An unexpected exception occurred while processing the event, so
// error should be called
request.getEvent().setEventType(CometEvent.EventType.ERROR);
request.getEvent().setEventSubType(null);
error = true;
connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
}
if (response.isClosed() || !request.isComet()) {
if (status==SocketStatus.OPEN) {
//CometEvent.close was called during an event.
request.getEvent().setEventType(CometEvent.EventType.END);
request.getEvent().setEventSubType(null);
// don't set the error flag here - otherwise the connection will be closed
// whenever a long poll is answered already during event handling:
// error = true;
connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
}
res.action(ActionCode.ACTION_COMET_END, null);
} else if (!error && read && request.getAvailable()) {
// If this was a read and not all bytes have been read, or if no data
// was read from the connector, then it is an error
request.getEvent().setEventType(CometEvent.EventType.ERROR);
request.getEvent().setEventSubType(CometEvent.EventSubType.IOEXCEPTION);
error = true;
connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
}
return (!error);
...
In my first tests I did not observe any undesired side effects of the change.
However, I did not yet do extensive tests - especially not with a steaming client.
Do you agree that this should be considered a bug and fixed in the next Tomcat
release?
Regards,
Matthias
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Re: Connection is closed when CometEvent.close is called during
an event
Posted by Pid <pi...@pidster.com>.
On 08/06/2010 11:26, Reich, Matthias wrote:
> Hello,
>
> I am using a CometProcessor servlet in a long-poll scenario, and recently had
> a closer look at the life span of connections that are used for poll requests.
>
> I noticed that connections are closed by Tomcat whenever a poll request
> was answered (and closed) directly during processing of the BEGIN event.
> In our application this happens for one out of three poll requests approximately
> and thus should not be neglected.
>
> I had a look into the source code and found the reason in the
> CoyoteAdapter.event method - it sets the 'error' flag to true in this situation.
>
> I modified the code (in 6.0.20 and 6.0.26) so that the error flag is not set.
> With that change Tomcat kept the connection open:
>
> ...
> // Calling the container
> connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
>
> if (!error && !response.isClosed() && (request.getAttribute(Globals.EXCEPTION_ATTR) != null)) {
> // An unexpected exception occurred while processing the event, so
> // error should be called
> request.getEvent().setEventType(CometEvent.EventType.ERROR);
> request.getEvent().setEventSubType(null);
> error = true;
> connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
> }
> if (response.isClosed() || !request.isComet()) {
> if (status==SocketStatus.OPEN) {
> //CometEvent.close was called during an event.
> request.getEvent().setEventType(CometEvent.EventType.END);
> request.getEvent().setEventSubType(null);
>
> // don't set the error flag here - otherwise the connection will be closed
> // whenever a long poll is answered already during event handling:
> // error = true;
>
> connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
> }
> res.action(ActionCode.ACTION_COMET_END, null);
> } else if (!error && read && request.getAvailable()) {
> // If this was a read and not all bytes have been read, or if no data
> // was read from the connector, then it is an error
> request.getEvent().setEventType(CometEvent.EventType.ERROR);
> request.getEvent().setEventSubType(CometEvent.EventSubType.IOEXCEPTION);
> error = true;
> connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
> }
> return (!error);
> ...
>
> In my first tests I did not observe any undesired side effects of the change.
> However, I did not yet do extensive tests - especially not with a steaming client.
>
> Do you agree that this should be considered a bug and fixed in the next Tomcat
> release?
You'll need one of the devs with Async knowledge to look at this, I think.
p
> Regards,
> Matthias
>
>
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>
RE: Connection is closed when CometEvent.close is called during an
event
Posted by "Reich, Matthias" <ma...@siemens-enterprise.com>.
> -----Original Message-----
> From: Konstantin Kolinko
> Sent: Friday, June 11, 2010 4:46 AM
> To: Tomcat Users List
> Subject: Re: Connection is closed when CometEvent.close is
> called during an event
>
> Regarding Comet + Keep-Alive, if it does not work, it is worth filing
> an enhancement request against Tomcat 7. It would be easier if there
> were some sample code or better a test case. This new use case has to
> be tested.
>
I have had a look at Tomcat 7 and recognized that with Tomcat 7 I would
probably refactor my application to use Servlet 3 conforming
asynchronous processing instead of the CometProcessor interface.
I assume that keep alive will be supported in conjunction with async
processing?
Then, it would no longer make sense using the CometProcessor interface
to implement long poll.
However, I am not sure if it will be possible for us to change
to Tomcat 7 soon. From the release notes I read that it requires Java 6
and I suspect that we will be bound to Java 5 for quite a while.
Hence, is there also any interest in respective improvements for Tomcat 6?
Then I could provide an example servlet as a patch.
Regards,
Matthias
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RE: Connection is closed when CometEvent.close is called during an
event
Posted by "Reich, Matthias" <ma...@siemens-enterprise.com>.
Konstantin Kolinko wrote:
> 2010/6/11 Reich, Matthias <ma...@siemens-enterprise.com>:
> >
> > The concept of long poll is e.g. described in
> >
> http://www.javaworld.com/javaworld/jw-03-2008/jw-03-asynchhttp
.html?page=6
> >
> > The sequence of events in my situation is as follows:
> > - a poll request is received by the server
> > - the CoyoteAdapter.service method is called and in turn
> > invokes the servlet's event method with a BEGIN event
> > - request.isComet() is still true when the control returns
> > to the CoyoteAdapter.service method
> > - some other thread writes a response and closes the Writer
> > of the response
> > - the CoyoteAdapter.event method is called and in turn
> > invokes the servlet's event method with an END event
> > - the servlet calls event.close()
> > - when the control returns to the CoyoteAdapter.event method
> > we have exactly this situation:
> > response.isClosed() && !request.isComet() &&
> status==SocketStatus.OPEN
> > - thus, if the error flag is set in this situation,
> > the connection will be closed, and a new connection must be opened
> > by the browser for the subsequent poll request
> >
> > According to the above sequence I would expect that the connection
> > is always closed if request.isComet() is still true when control
> > returns to the CoyoteAdapter.service method after processing
> > the BEGIN event - no matter how long it takes from then
> > until the response is written.
> > Surprisingly, I did not always observe this.
> >
> > Anyway, if the error flag is not set in this situation,
> > the connection is kept open.
> >
> >
> >
> > ---
> C:\DOCUME~1\rm041693\LOCALS~1\Temp\CoyoteAdapter.java-revBASE.
svn001.tmp.java Do Jun 10 22:22:20 2010
> > +++
> D:\tomcat\TOMCAT_6_0_26\java\org\apache\catalina\connector\Coy
oteAdapter.java Mo Jun 7 17:30:23 2010
> > @@ -215,7 +215,9 @@
> > //CometEvent.close was called
> during an event.
> >
> request.getEvent().setEventType(CometEvent.EventType.END);
> > request.getEvent().setEventSubType(null);
> > - error = true;
> > + // don't set the error flag -
> otherwise the socket will be closed
> > + // whenever CometEvent.close is
> called during the event
> > + // error = true;
> >
> connector.getContainer().getPipeline().getFirst().event(reques
> t, response, request.getEvent());
> > }
> > res.action(ActionCode.ACTION_COMET_END, null);
> >
>
> Now I understand. Thank you.
>
> I would say that you are trying to combine Comet and Keep-Alive.
Well, it actually was combined (maybe unintentionally) in 6.0.18.
(I retested an older version of our application which uses 6.0.18
and didn't observe that connections were closed in similar situations.)
Since 6.0.20, the connections are closed upon event.close(),
but it took me some time to get aware of this because when the
subsequent request fails due to the close, the browsers
silently retry that request.
>
> In comet to send a portion of data (a response), you do
> writer.flush(). It sends the data over the wire. Doing event.close()
> terminates comet request processing.
>
> If it were possible not to close the connection, it were possible to
> alternate comet and non-comet processing of subsequent requests over
> the same connection, and to process different requests by different
> servlets.
>
> > - some other thread writes a response and closes the Writer
> > of the response
>
> Response object is not thread safe. You must write your response in
> the thread that received your EventType.READ event (or any other
> event) while you are processing that event.
>
> Otherwise, any random result might happen.
>
Doesn't the sample code on the documentation page include async
writes as well? When it gets a READ event from one connection
it writes to all open connetions.
With respect to all other connections, this is an async write.
In a long poll scenario, closing the Writer seems to be
the only way to trigger a further event (i.e. an END event)
after the BEGIN event.
>
> BTW, for reference:
> There exists the following documentation page,
> http://tomcat.apache.org/tomcat-6.0-doc/aio.html#CometEvent
> and sample code in
> webapps/examples/WEB-INF/classes/chat/ChatServlet.java
> plus some helper JSPs.
>
> The sample is callable as
> http://localost:8080/examples/jsp/chat/chat.jsp in Tomcat
> 6.0.26 and earlier
> http://localost:8080/examples/jsp/chat/index.jsp in Tomcat 7 and in
> Tomcat 6.0.27 and later
>
>
> Regarding Comet + Keep-Alive, if it does not work, it is worth filing
> an enhancement request against Tomcat 7. It would be easier if there
> were some sample code or better a test case. This new use case has to
> be tested.
I'll try to find the time to prepare something. In the repository
(6.0.26) I have not detected any Comet related test cases. Are there
any test cases you could recommend as an example of how I should
organize it?
What is the best way to deliver the code to you?
>
> The patch you proposing looks promising, but as of now I am
> not sure that
>
> a) both cases when that branch is called are equivalent,
> I mean (response.isClosed()) vs. (!request.isComet()) whether in both
> cases we can go without closing the socket
> Calling CometEventImpl.close() will set both conditions to true.
> I think that response.isClosed() when isComet() == true will mean an
> error and needs closing the socket. Need to think more about it.
>
I think you are right.
If the response is already closed before the container is invoked,
the servlet receives an END event and must call event.close().
If it doesn't, it is an error.
However, the following CAN happen:
- the servlet processes a READ event and therefore
does not call event.close()
- then the response is closed by another thread
- then control returns to CoyoteAdpater.event
- now we have response.isClosed() when isComet() == true
This can only happen in the streaming case, as it requires a
READ event, and even if it is not really an error, I think
it is acceptable to close the socket in this situation.
The servlet programmer knows if the servlet implements
long poll or streaming.
So, maybe the servlet should be enabled to declare
that the connection shall be kept open, e.g. via an
additional method CometEvent.close(boolean keepalive).
Existing code that relies on the socket close would not be
broken if CometEvent.close() would behave like
CometEvent.close(false).
Such an interface extension would also have the charm
of making explicit that closing an event does not only affect
the associated request but also the connection.
> b) how the socket will be processed further and be returned to the
> poller. There are non-comet vs. comet pollers. There are some
> keepAlive settings (like "maxKeepAliveRequests") and those have to be
> respected.
>
For long poll I want to keep the polling connection open
as long as possible. Therefore I am using maxKeepAliveRequests = -1.
Best regards,
Matthias Reich
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Re: Connection is closed when CometEvent.close is called during an
event
Posted by Konstantin Kolinko <kn...@gmail.com>.
2010/6/11 Reich, Matthias <ma...@siemens-enterprise.com>:
>
> The concept of long poll is e.g. described in
> http://www.javaworld.com/javaworld/jw-03-2008/jw-03-asynchhttp.html?page=6
>
> The sequence of events in my situation is as follows:
> - a poll request is received by the server
> - the CoyoteAdapter.service method is called and in turn
> invokes the servlet's event method with a BEGIN event
> - request.isComet() is still true when the control returns
> to the CoyoteAdapter.service method
> - some other thread writes a response and closes the Writer
> of the response
> - the CoyoteAdapter.event method is called and in turn
> invokes the servlet's event method with an END event
> - the servlet calls event.close()
> - when the control returns to the CoyoteAdapter.event method
> we have exactly this situation:
> response.isClosed() && !request.isComet() && status==SocketStatus.OPEN
> - thus, if the error flag is set in this situation,
> the connection will be closed, and a new connection must be opened
> by the browser for the subsequent poll request
>
> According to the above sequence I would expect that the connection
> is always closed if request.isComet() is still true when control
> returns to the CoyoteAdapter.service method after processing
> the BEGIN event - no matter how long it takes from then
> until the response is written.
> Surprisingly, I did not always observe this.
>
> Anyway, if the error flag is not set in this situation,
> the connection is kept open.
>
>
>>
>> 2. The above fragment when using Comet should be equivalent to
>>
>> > if (response.isClosed()) {
>> > if (status==SocketStatus.OPEN) {
>> > //CometEvent.close was called during an event.
>> > request.getEvent().setEventType(CometEvent.EventType.END);
>> > request.getEvent().setEventSubType(null);
>> >
>> > // don't set the error flag here - otherwise the connection
>> will be closed
>> > // whenever a long poll is answered already during event handling:
>> > // error = true;
>> >
>>
>> Response#isClosed():
>> public boolean isClosed() {
>> return outputBuffer.isClosed();
>> }
>
> No, it is not equivalent: response.isClosed() is true after closing
> the Writer or OutputStream, whereas request.isComet() is true
> until event.close() is called.
>
>>
>> If you will not be able to send your answer, why not to close the
>> socket right away?
>
> I was able to send the answer and would like to use the connection
> also for the next poll request. (or for some other request
> the browser decides to send through this connection)
>
>>
>> 3. It would be much more readable, if you provided your changes in the
>> unified diff format. (even better if it were generated with "svn diff"
>> command against sources retrieved from svn).
>>
>
> --- C:\DOCUME~1\rm041693\LOCALS~1\Temp\CoyoteAdapter.java-revBASE.svn001.tmp.java Do Jun 10 22:22:20 2010
> +++ D:\tomcat\TOMCAT_6_0_26\java\org\apache\catalina\connector\CoyoteAdapter.java Mo Jun 7 17:30:23 2010
> @@ -215,7 +215,9 @@
> //CometEvent.close was called during an event.
> request.getEvent().setEventType(CometEvent.EventType.END);
> request.getEvent().setEventSubType(null);
> - error = true;
> + // don't set the error flag - otherwise the socket will be closed
> + // whenever CometEvent.close is called during the event
> + // error = true;
> connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
> }
> res.action(ActionCode.ACTION_COMET_END, null);
>
Now I understand. Thank you.
I would say that you are trying to combine Comet and Keep-Alive.
In comet to send a portion of data (a response), you do
writer.flush(). It sends the data over the wire. Doing event.close()
terminates comet request processing.
If it were possible not to close the connection, it were possible to
alternate comet and non-comet processing of subsequent requests over
the same connection, and to process different requests by different
servlets.
> - some other thread writes a response and closes the Writer
> of the response
Response object is not thread safe. You must write your response in
the thread that received your EventType.READ event (or any other
event) while you are processing that event.
Otherwise, any random result might happen.
BTW, for reference:
There exists the following documentation page,
http://tomcat.apache.org/tomcat-6.0-doc/aio.html#CometEvent
and sample code in
webapps/examples/WEB-INF/classes/chat/ChatServlet.java
plus some helper JSPs.
The sample is callable as
http://localost:8080/examples/jsp/chat/chat.jsp in Tomcat 6.0.26 and earlier
http://localost:8080/examples/jsp/chat/index.jsp in Tomcat 7 and in
Tomcat 6.0.27 and later
Regarding Comet + Keep-Alive, if it does not work, it is worth filing
an enhancement request against Tomcat 7. It would be easier if there
were some sample code or better a test case. This new use case has to
be tested.
The patch you proposing looks promising, but as of now I am not sure that
a) both cases when that branch is called are equivalent,
I mean (response.isClosed()) vs. (!request.isComet()) whether in both
cases we can go without closing the socket
Calling CometEventImpl.close() will set both conditions to true.
I think that response.isClosed() when isComet() == true will mean an
error and needs closing the socket. Need to think more about it.
b) how the socket will be processed further and be returned to the
poller. There are non-comet vs. comet pollers. There are some
keepAlive settings (like "maxKeepAliveRequests") and those have to be
respected.
Best regards,
Konstantin Kolinko
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RE: Connection is closed when CometEvent.close is called during an
event
Posted by "Reich, Matthias" <ma...@siemens-enterprise.com>.
On 10/06/2010 9:49 PM, Konstantin Kolinko wrote:
> 2010/6/8 Reich, Matthias:
> > I modified the code (in 6.0.20 and 6.0.26) so that the
> error flag is not set.
> > With that change Tomcat kept the connection open:
> >
> > ...
> > if (response.isClosed() || !request.isComet()) {
> > if (status==SocketStatus.OPEN) {
> > //CometEvent.close was called during an event.
> > request.getEvent().setEventType(CometEvent.EventType.END);
> > request.getEvent().setEventSubType(null);
> >
> > // don't set the error flag here - otherwise the connection
> will be closed
> > // whenever a long poll is answered already during event handling:
> > // error = true;
> >
>
> 1. I think that I do not understand you. What is your meaning of "long
> poll"? Can you describe your situation as a sequence of events how
> they occur step-by-step on a timeline?
The concept of long poll is e.g. described in
http://www.javaworld.com/javaworld/jw-03-2008/jw-03-asynchhttp.html?page=6
The sequence of events in my situation is as follows:
- a poll request is received by the server
- the CoyoteAdapter.service method is called and in turn
invokes the servlet's event method with a BEGIN event
- request.isComet() is still true when the control returns
to the CoyoteAdapter.service method
- some other thread writes a response and closes the Writer
of the response
- the CoyoteAdapter.event method is called and in turn
invokes the servlet's event method with an END event
- the servlet calls event.close()
- when the control returns to the CoyoteAdapter.event method
we have exactly this situation:
response.isClosed() && !request.isComet() && status==SocketStatus.OPEN
- thus, if the error flag is set in this situation,
the connection will be closed, and a new connection must be opened
by the browser for the subsequent poll request
According to the above sequence I would expect that the connection
is always closed if request.isComet() is still true when control
returns to the CoyoteAdapter.service method after processing
the BEGIN event - no matter how long it takes from then
until the response is written.
Surprisingly, I did not always observe this.
Anyway, if the error flag is not set in this situation,
the connection is kept open.
>
> 2. The above fragment when using Comet should be equivalent to
>
> > if (response.isClosed()) {
> > if (status==SocketStatus.OPEN) {
> > //CometEvent.close was called during an event.
> > request.getEvent().setEventType(CometEvent.EventType.END);
> > request.getEvent().setEventSubType(null);
> >
> > // don't set the error flag here - otherwise the connection
> will be closed
> > // whenever a long poll is answered already during event handling:
> > // error = true;
> >
>
> Response#isClosed():
> public boolean isClosed() {
> return outputBuffer.isClosed();
> }
No, it is not equivalent: response.isClosed() is true after closing
the Writer or OutputStream, whereas request.isComet() is true
until event.close() is called.
>
> If you will not be able to send your answer, why not to close the
> socket right away?
I was able to send the answer and would like to use the connection
also for the next poll request. (or for some other request
the browser decides to send through this connection)
>
> 3. It would be much more readable, if you provided your changes in the
> unified diff format. (even better if it were generated with "svn diff"
> command against sources retrieved from svn).
>
--- C:\DOCUME~1\rm041693\LOCALS~1\Temp\CoyoteAdapter.java-revBASE.svn001.tmp.java Do Jun 10 22:22:20 2010
+++ D:\tomcat\TOMCAT_6_0_26\java\org\apache\catalina\connector\CoyoteAdapter.java Mo Jun 7 17:30:23 2010
@@ -215,7 +215,9 @@
//CometEvent.close was called during an event.
request.getEvent().setEventType(CometEvent.EventType.END);
request.getEvent().setEventSubType(null);
- error = true;
+ // don't set the error flag - otherwise the socket will be closed
+ // whenever CometEvent.close is called during the event
+ // error = true;
connector.getContainer().getPipeline().getFirst().event(request, response, request.getEvent());
}
res.action(ActionCode.ACTION_COMET_END, null);
>
> Best regards,
> Konstantin Kolinko
>
Regards,
Matthias
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Re: Connection is closed when CometEvent.close is called during an
event
Posted by Konstantin Kolinko <kn...@gmail.com>.
2010/6/8 Reich, Matthias <ma...@siemens-enterprise.com>:
> I modified the code (in 6.0.20 and 6.0.26) so that the error flag is not set.
> With that change Tomcat kept the connection open:
>
> ...
> if (response.isClosed() || !request.isComet()) {
> if (status==SocketStatus.OPEN) {
> //CometEvent.close was called during an event.
> request.getEvent().setEventType(CometEvent.EventType.END);
> request.getEvent().setEventSubType(null);
>
> // don't set the error flag here - otherwise the connection will be closed
> // whenever a long poll is answered already during event handling:
> // error = true;
>
1. I think that I do not understand you. What is your meaning of "long
poll"? Can you describe your situation as a sequence of events how
they occur step-by-step on a timeline?
2. The above fragment when using Comet should be equivalent to
> if (response.isClosed()) {
> if (status==SocketStatus.OPEN) {
> //CometEvent.close was called during an event.
> request.getEvent().setEventType(CometEvent.EventType.END);
> request.getEvent().setEventSubType(null);
>
> // don't set the error flag here - otherwise the connection will be closed
> // whenever a long poll is answered already during event handling:
> // error = true;
>
Response#isClosed():
public boolean isClosed() {
return outputBuffer.isClosed();
}
If you will not be able to send your answer, why not to close the
socket right away?
3. It would be much more readable, if you provided your changes in the
unified diff format. (even better if it were generated with "svn diff"
command against sources retrieved from svn).
Best regards,
Konstantin Kolinko
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