[Twisted-web] rest webservice and big data.

[L. Daniel Burr]

On Mon, 20 Aug 2007 13:13:09 -0500, Jean-Paul Calderone  
<exarkun at divmod.com> wrote:

[Discusion of streaming file uploads in twisted.web]

>> Someone with deeper knowledge of twisted.web may be able to propose
>> a strategy for implementing streaming file uploads
>
> Don't mind if I do ;)
>

Always good to have a core developer weigh in on these matters ;)

> HTTPChannel already notices the difference between when the headers have
> all been received and when the body has been received entirely. When the
> former occurs, allHeadersReceived is called.  In the base implementation
> this sets up a file-like object into which the body will be written.  It
> would be possible to do something slightly different here in order to
> support streaming uploads: do resource traversal to find the IResource
> the upload is being sent to and then let it deal with bytes received in
> the body of the request.
>

That sounds pretty reasonable.

> The only other things which might not be obvious here.  Changes to
> twisted.web should be backwards compatible so that existing twisted.web
> applications continue to work without being modified.  Implementing
> what I've described above without regard for backwards compatibility  
> would
> probably mean subjecting existing applications to two things:
>

You lost me here.  First you say that changes to twisted.web should be
backwards-compatible, then you go on to describe how to do things in
a non-compatible manner.  I don't have a preference regarding the
issue of compatibility, but I'm not clear as to whether you do.

>   * resource traversal would be performed earlier than usual for the
>     application.  This might have adverse consequences, or it might not.
>     In the absense of any way to know for sure, we shouldn't change this
>     behavior.  So, instead, the code might require a new kind of site, or
>     an attribute to be set on the root resource, or something else of 
>     this sort which would allow new applications to indicate their
>     preference for the new behavior while preserving the existing
>     behavior for existing applications.
>

Sure, this sounds a bit like the way nevow uses Element for doing
things in the new, context-less way, while leaving Fragment in place
to handle the existing, context-laden way.

>   * The body of a request is currently available in the request object
>     itself.  Existing applications won't expect it to be elsewhere, nor
>     will they expect to have to handle the upload as it is happening.  It
>     should be required that resources indicate in some way that they are
>     capable of handling streaming uploads.  This might be done by adding  
>     a new interface which they must implement (since they will need to  
>     provide methods for handling bytes from the upload, this is  
> necessary     anyway).
>
>> but I expect it would be a fair amount of work, and end up looking  
>> similar
>> to what is already in web2.
>
> Well, "fair" is quite subjective, so maybe it is and maybe it isn't ;)

Point taken, although I'll point out that from the perspective of the
original poster, "fair amount of work" might mean "lots of work and
cursing", given the usual newbie experience with twisted and its
learning curve.

> It
> doesn't strike me as a massive undertaking, though.  I think an initial
> patch could probably be done in a day or two.  Allow another couple of  
> days
> (not necessarily elapsed - there might be some latency in finding  
> reviews,
> etc) to get feedback and make whatever improvements are suggested, and  
> that
> would probably be it.
>

Well, that'd be pretty awesome, and would benefit nevow users too.

> FWIW, what I described doesn't resemble the support for this  
> functionality
> in web2 at all, I think.
>

True, and I'm somewhat surprised, given the work currently going on
with web2.  Isn't dialtone's consumer/producer oriented stream stuff
going to be "the way" to do this sort of thing?  Does twisted.web
have to approach it differently, due to design constraints?

Thanks for taking the time to think through this,

L. Daniel Burr