Re: rational for rewind()

[Eric Wong <e@80x24.org>]

(resending my original message for the archives, original seems lost,
 Torsten already got this privately)

Torsten Robitzki <Torsten@Robitzki.de> wrote:
> Hello,
> I'm implementing a C++ comet web server, that (tries) to implement rack to 
> adapt ruby applications. Currently I'm reading a body very naively put the 
> body into a ruby String and wrap it with a StringIO to provide the 
> rack.input for the implementation. As I'm going to use the server for 
> uploading images, I would like to implement a real, stream-like object to 
> circumvent the need to buffer the POST body before handing it to the 
> application.

unicorn implements input like tee(1) doing lazy, rewindable buffering:
	http://unicorn.bogomips.org/Unicorn/TeeInput.html

> Now, I've read the rack specs and read about rewind(). To implement 
> rewind(), I would have to store the whole body, even when the upstream 
> application just calculates some kind of checksum on the body, or uploads 
> it to s3. What's the rational behind this part of the specification? Is it 
> possible to not implement rewind() and to tell applications that need 
> rewind(), to keep there own copy of the body, in case it is needed? 

I don't know the rationale, but I suspect it's to make life easier for
application/API authors to know the data will exist for the lifetime
of the request/response cycle.

But I think having rewind as part of the spec sucks.  I seem to recall
there was hope of getting rid of it for Rack 2 (if it ever happens).
However, since I'm not sure if Rack 2 will happen, rewind can already
be disabled and break spec in servers I implement:

unicorn has a "rewindable_input <true|false>" option (default=true):
	http://unicorn.bogomips.org/Unicorn/Configurator.html

yahns is a tri-state: "input_buffering <:lazy|true|false>" option:
	http://yahns.yhbt.net/yahns_config.txt (default=true)

Fwiw, rewindable_input(true) in unicorn is equivalent to
input_buffering(:lazy) in yahns.  This is because unicorn expects nginx
to buffer for slow clients, whereas yahns does not require nginx and
buffers asynchronously even with trickling clients.

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