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From Jim Schueler <jschue...@eloquency.com>
Subject Re: mod_perl and Transfer-Encoding: chunked
Date Wed, 03 Jul 2013 20:26:19 GMT
Thanks for the prompt response, but this is your question, not mine.  I 
hardly need an RTFM for my trouble.

I drew my conclusions using a packet sniffer.  And as far-fetched as my 
answer may seem, it's more plausible than your theory that Apache or 
modperl is decoding a raw socket stream.

The crux of your question seems to be how the request content gets
magically re-assembled.  I don't think it was ever disassembled in the 
first place.  But if you don't like my answer, and you don't want to 
ignore it either, then please restate the question.  I can't find any 
definition for unchunked, and Wiktionary's definition of de-chunk says to 
"break apart a chunk", that is (counter-intuitively) chunk a chunk.


>             Second, if there's no Content-Length header then how
>             does one know how much
>             data to read using $r->read?   
>
>             One answer is until $r->read returns zero bytes, of
>             course.  But, is
>             that guaranteed to always be the case, even for,
>             say, pipelined requests?  
>             My guess is yes because whatever is de-chunking the

read() is blocking.  So it never returns 0, even in a pipeline request (if 
no data is available, it simply waits).  I don't wish to discuss the 
merits here, but there is no technical imperative for a content-length 
request in the request header.

  -Jim






On Wed, 3 Jul 2013, Bill Moseley wrote:

> Hi Jim,
> This is the Transfer-Encoding: chunked I was writing about:
> 
> http://tools.ietf.org/html/rfc2616#section-3.6.1
> 
> 
> 
> On Wed, Jul 3, 2013 at 11:34 AM, Jim Schueler <jschueler@eloquency.com>
> wrote:
>       I played around with chunking recently in the context of media
>       streaming: The client is only requesting a "chunk" of data.
>        "Chunking" is how media players perform a "seek".  It was
>       originally implemented for FTP transfers:  E.g, to transfer a
>       large file in (say 10K) chunks.  In the case that you describe
>       below, if no Content-Length is specified, that indicates "send
>       the remainder".
>
>       >From what I know, a "chunk" request header is used this way to
>       specify the server response.  It does not reflect anything about
>       the data included in the body of the request.  So first, I would
>       ask if you're confused about this request information.
>
>       Hypothetically, some browsers might try to upload large files in
>       small chunks and the "chunk" header might reflect a push
>       transfer.  I don't know if "chunk" is ever used for this
>       purpose.  But it would require the following characteristics:
>
>         1.  The browser would need to originally inquire if the server
>       is
>             capable of this type of request.
>         2.  Each chunk of data will arrive in a separate and
>       independent HTTP
>             request.  Not necessarily in the order they were sent.
>         3.  Two or more requests may be handled by separate processes
>             simultaneously that can't be written into a single
>       destination.
>         4.  Somehow the server needs to request a resend if a chunk is
>       missing.
>             Solving this problem requires an imaginitive use of HTTP.
>
>       Sounds messy.  But might be appropriate for 100M+ sized uploads.
>        This *may* reflect your situation.  Can you please confirm?
>
>       For a single process, the incoming content-length is
>       unnecessary. Buffered I/O automatically knows when transmission
>       is complete.  The read() argument is the buffer size, not the
>       content length.  Whether you spool the buffer to disk or simply
>       enlarge the buffer should be determined by your hardware
>       capabilities.  This is standard IO behavior that has nothing to
>       do with HTTP chunk.  Without a "Content-Length" header, after
>       looping your read() operation, determine the length of the
>       aggregate data and pass that to Catalyst.
>
>       But if you're confident that the complete request spans several
>       smaller (chunked) HTTP requests, you'll need to address all the
>       problems I've described above, plus the problem of re-assembling
>       the whole thing for Catalyst.  I don't know anything about
>       Plack, maybe it can perform all this required magic.
>
>       Otherwise, if the whole purpose of the Plack temporary file is
>       to pass a file handle, you can pass a buffer as a file handle.
>        Used to be IO::String, but now that functionality is built into
>       the core.
>
>       By your last paragraph, I'm really lost.  Since you're already
>       passing the request as a file handle, I'm guessing that Catalyst
>       creates the tempororary file for the *response* body.  Can you
>       please clarify?  Also, what do you mean by "de-chunking"?  Is
 	>       that the same think as re-assembling?
>
>       Wish I could give a better answer.  Let me know if this helps.
>
>       -Jim
> 
>
>       On Tue, 2 Jul 2013, Bill Moseley wrote:
>
>             For requests that are chunked (Transfer-Encoding:
>             chunked and no
>             Content-Length header) calling $r->read returns
>             unchunked data from the
>             socket.
>             That's indeed handy.  Is that mod_perl doing that
>             un-chunking or is it
>             Apache?
>
>             But, it leads to some questions.   
>
>             First, if $r->read reads unchunked data then why is
>             there a
>             Transfer-Encoding header saying that the content is
>             chunked?   Shouldn't
>             that header be removed?   How does one know if the
>             content is chunked or
>             not, otherwise?
>
>             Second, if there's no Content-Length header then how
>             does one know how much
>             data to read using $r->read?   
>
>             One answer is until $r->read returns zero bytes, of
>             course.  But, is
>             that guaranteed to always be the case, even for,
>             say, pipelined requests?  
>             My guess is yes because whatever is de-chunking the
>             request knows to stop
>             after reading the last chunk, trailer and empty
>             line.   Can anyone elaborate
>             on how Apache/mod_perl is doing this? 
> 
>
>             Perhaps I'm approaching this incorrectly, but this
>             is all a bit untidy.
>
>             I'm using Catalyst and Catalyst needs a
>             Content-Length.  So, I have a Plack
>             Middleware component that creates a temporary file
>             writing the buffer from
>             $r->read( my $buffer, 64 * 1024 ) until that returns
>             zero bytes.  I pass
>             this file handle onto Catalyst.
>
>             Then, for some content-types, Catalyst (via
>             HTTP::Body) writes the body to
>             another temp file.    I don't know how
>             Apache/mod_perl does its de-chunking,
>             but I can call $r->read with a huge buffer length
>             and Apache returns that.
>              So, maybe Apache is buffering to disk, too.
>
>             In other words, for each tiny chunked JSON POST or
>             PUT I'm creating two (or
>             three?) temp files which doesn't seem ideal.
> 
>
>             --
>             Bill Moseley
>             moseley@hank.org
> 
> 
> 
> 
> --
> Bill Moseley
> moseley@hank.org
> 
>
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