Since the breakage is caused in at least some cases by proxies, it is not
in general safe to let XHR users opt in since they may control the origin
server but generally would not control whatever proxies are between the
server and the user.
Pipelining is a great potential performance improvement and it's sad that
it can't safely be used on the public Internet right now, so I hope we
someday find a way out of the impasse.
There is a world of difference between browsers choosing to do pipelining
where no one gets to opt-in, and XHR opt-in where developers know the origin
server, may even know the full route for all users (as in intranets) and can
make the calculating risk of where they want to try pipelining or not, and
can even code backup solutions when pipelined requests fail. It seems very
presumptious to tell developers that can't take that risk. Why not tell them
they can't use XHR at all, since there are old browsers out there don't
support XHR? Because developers should be given the opportunity to make this
decision. Developers have chosen to use XHR even though there are browsers
that don't support it, and this has led to progress. If they experience too
much pipelining failure they can choose to opt-out. I am very skeptical that
at this point the failure rate is high enough that very many developers
would opt-out.
Ironically, this is probably our best opportunity to get through this
impasse. If web developers can selectively turn on XHR, the few remaining
proxies out there that break under pipelining will start to be singled out,
and more likely to be fixed which in turn will create the pipeline
reliability for browsers to use it more broadly.
Kris
