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From Jim Schueler <jschue...@eloquency.com>
Subject Re: mod_perl and Transfer-Encoding: chunked
Date Wed, 03 Jul 2013 18:34:55 GMT
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
> 
>
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