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From GOMEZ Henri <hgo...@slib.fr>
Subject RE: Design Review for ajp13's changes: WAS problem w/ ajp13 - if Tomc at is shutdown
Date Thu, 08 Mar 2001 22:54:03 GMT
>First off, it's GREAT that you are working on this -- it's a 
>very heavily requested improvement.

> 1) This work will end up being committed in the 3.3 branch, 
>rather than the
>3.2 branch.  3.2 is only bug fixes now, not new features.  The 
>mod_jk C code
>is very similar in 3.3 and 3.2, but not identical.  I can adapt your
>patches, but if you wanted to work against 3.3, that would make things
>easier.

mod_jk from 3.2 and 3.3 is now different. Only 3.3 support Apache 2.0

> 2) You seem to have managed to puzzle out some of the 
>workings of mod_jk
>already, but, FYI, I just committed some internal 
>documentation a few days
>ago.  It's in the 3.3 branch, in 
>src/native/mod_jk/common/jk_service.h --
>may be worth taking a look at.
>
> 3) For option (1), I have a few questions
>
>  - Is there a way in which data could be lost?  Specifically, 
>as you state,
>the send() will return without error, and then it will only 
>get the error on
>the following read().  Is all the data always preserved so that simply
>retrying will work correctly?  I think most of that state is in the
>jk_ws_service_t object -- is it possible a read pointer will 
>be advanced and
>data will be lost?  This may be acceptable, but I'd like to 
>understand it...

A send to a closed or (in our case half-closed) socket will 95%
of the time return a positive number (#bytes sent). It's sad
but even if the IP stack known the socket was closed you'll know
about it only at the next read/recv or select on readstate !
Checking send < 0 is not a great help here.

> - You only retry once.  If there are a number of connections 
>open (from a
>single Apache process), isn't it possible that Tomcat has come 
>back up, and
>that the next connection obtained (from the endpoint cache), 
>will also be
>stale?  Would it make sense in this case to trigger a shutdown 
>of all the
>connections currently in the cache (and then retry once)?  
>That would make
>sense if there were no other ways to get a ECONNRESET error.  

ECONNRESET MUST'NT be checked like this, mod_jk code run
on Apache 1.3 AND 2.0 and this one is multithreaded.

> - Or, more generally, just so I (and everyone) can 
>understand, how does
>this new code deal with the following stages:
>
>  1) TC and Apache both up and running
>
>  2) TC is shutdown
>If mod_jk is in the middle of handling a request, what 
>happens?  There was
>an infinite loop in the 3.2.1 code, but that's been fixed in 
>3.2.2 and 3.3.

Apache send datas (no error) and then wait reply with recv.
There Apache got the error. We must restart the request sent
at least one time. Little code to reorganize in ajp13_worker.

>  3) TC is shutdown, Apache is still up.  While TC is down, 
>requests come
>in.  How are they handled?  Are there any loops Apache gets stuck in?

Apache will determine that TC is down and try another socket. But we must
be carefull here with load-balancing configs.

>  4) TC starts back up.  Now requests get handled smoothly again? 

Yes, but only if the socket were closed by Apache before.

> 4) For option (2):
>
> - If the user has Win32, you're just punting, correct?  Why 
>is that?  I
>know nothing about Win32 socket programming, but I'm 
>curious...  You say
>you're testing on Win2k -- does Win2k support select(), but 
>win32 doesn't? 
>Does anyone know about how widely select() is supported?

select is just to much time consuming to be used at EACH request.
We must handle the potential error not loose to many cpu cycles
when everything is fine.

I'm -1 using select and errno.

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