handling HTTP redirects

[Douglas Tody]

Hi All -

Redirects are an important part of the HTTP protocol and I think
need to be supported in the production (2ndgen) protocols which are
HTTP based - hence SSA for example addresses redirects.  That said,
as a pragmatic matter much client software may not support redirects
(does not now certainly) hence I suggest we be conservative about
using this feature in actual service implementations to allow client
software time to catch up.

	- Doug



On Wed, 4 Jun 2008, Derriere Sebastien wrote:

> 
>   Hello registry, (CC'ed to DAL, FU2 registry)
> 
>   I don't remember this topic being discussed in registry, so I'm
> opening the can of worms. The point is how to handle HTTP redirects
> in using some of the resources capabilities.
>   In the vizier resource descriptions, I can point to some URLs that
> serve for example a cone search capability. Example of such query:
> http://vizier.u-strasbg.fr/viz-bin/votable/-dtd/-A?-source=J/ApJS/115/1/table2&RA=187.209000&DEC=-1.938000&SR=50.00000
>   This accessURL uses the HTTP protocol, and in this case can
> return a 302 redirect code indicating the document has moved,
> and gives the new location (used in this case for load balancing
> to some other machine producing the VOTable).
>   Most clients supporting the http protocol will just work fine
> (I tested firefox, lynx, wget), and just transparently follow
> the redirection and return the final VOTable.
>   However, in some cases, what you get is the 302 message
> (curl does this, unless you invoke curl -L "http://...").
> 
>   In the SSA document, section 8.4 (General HTTP response rules),
> this behaviour is explicitly addressed:
> 
> > A server may send an HTTP Redirect message (using HTTP response codes as
> > defined in IETF RFC 2616) to an absolute URL that is different from the
> > valid request URL that was sent by the client. HTTP Redirect causes the
> > client to issue a new HTTP request for the new URL. Several redirects
> > could in theory occur. Practically speaking, the redirect sequence ends
> > when the server responds with a SSA response. The final response shall
> > be a SSA response that corresponds exactly to the original request (or
> > a service exception).
> 
>   And I think this is the right thing to do. Because if you try to access
> a service via the HTTP protocol, you should use a client that follows the
> rules of the HTTP protocol. If not, don't blame the service!
> 
>   I've had a few remarks from people thinking the links were broken,
> just because they used a library that basically ignored the HTTP status
> codes (not receiving "200 OK" does not mean it's an error!).
>   Is it reasonable to assume the following for registry resource descriptions?
> :
> "An accessURL pointing to some HTTP address should be handled using
> the HTTP protocol". Services should make sure they follow http rules,
> and clients too.
> 
> Sebastien.
> -- 
>     _______
>    /  ~   /, Sebastien Derriere   mailto:derriere at astro.u-strasbg.fr
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>