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Re: Can the response entity be transmitted before all the request entity has been read?

From: Jamie Lokier <jamie@shareable.org>
Date: Mon, 15 Mar 2004 19:07:42 +0000
To: Alex Rousskov <rousskov@measurement-factory.com>
Cc: HTTP WG <ietf-http-wg@w3.org>
Message-ID: <20040315190742.GD9731@mail.shareable.org>

Alex Rousskov wrote:
> 	You seem to be developing an HTTP-to-application proxy rather
> than a pure origin server.

It's not really any different to other well known origin servers.

It is structured like a server+CGI, or Apache+mod_perl.  I used the
term "server application" to indicate that there's a server component
and another component which communicates with the server in a simple
way: reading request entity, and writing response entity

> In that case, the most reliable strategy in the presence of
> questionable traffic is often to switch to a tunnel behavior and
> terminate the connection as soon as the response is sent.

I see that is the most common strategy :).  It seems a bit dubious,
though, because it's easy to imagine application code like this:

     write "Status: 200\n\n";
     write "Thank you for your submission.\n";

     while ($x = read (...)) { store ($x) }

     write "Great, all done.\n";

As you say, there are theoretical and practical requirements for
talking to real HTTP clients and proxies.  One of them may well be
that the response shouldn't be sent before the request is read.

I'd rather put the hard requirements, and every feature that will help
with robustness, in the _server_, rather than document it as a
requirement that applications have to follow.  It's the server's job
to keep the communication good as reliable as possible, insulating the
application.

> It may be a design mistake to give applications control knobs that
> depend on proper HTTP/1.1 support in clients.

Exactly.  You see in the above example, _not_ giving the application a
control knob implies that the server reads the whole request before
sending any of the application's response, or refuses to let the
application read anything after any response has been sent.

That's contrary to most server implementations: they do give the
application control over when to read and write, which is the opposite
of what you're suggesting here.

> Give your applications just the basic knobs that minimize the amount
> of guess work on the proxy. If an application requires fine control
> of 100-Continue or status code sending time, the application will
> not work reliably on top of HTTP. Do not do anything "unusual" or
> border-line, and do not expose HTTP knobs that allow your
> applications to involve your proxy in such "unusual" or border-line
> behavior.

I agree.  I wish the application to do just the basic things people
expect:

    a function to set the status code + response headers
    ability to write the response entity
    ability to read the request entity
    no special requirement on when the request entity is read

> 	More specifically, do not expect clients to receive or support
> 100-Continue by default.

Ok.  I am so far following the advice in RFC 2616, which is to send
100-Continue if Expect: 100-continue is received and it's HTTP/1.1 or
greater, or if it's HTTP/1.1 (exactly that version) and it's POST or PUT.

Is that advice good?

Not sending 100-Continue is always safe, but may introduce delays.

> Do not expect clients to be able to recover from receiving responses
> prior to them sending complete requests.

That's a crucial question.  Should I either enforce that in the server,
by insisting on reading the whole request before allowing any text of
a non-error response to be sent (error can be sent immediately), or
document it as an application requirement: that the application must
do all its reading before it writes anything?

It seems that existing servers, e.g. Apache, thttpd and all the others
don't do either: they allow request to be read by the application when
it likes, the response to be sent before all the request is read if
the application likes, and don't document this as a problem.  It's for
application writers to be aware of it.

Yet, I want to document it if it's a real practical issue, because I
can see good uses for sending response before the request is all read:
a translation or file format server, which writes part of the
translation before all the text is uploaded, and an upload meter,
which writes some text as a large upload progresses, perhaps showing
thumbnails of the uploaded files as each one arrives too.

If these things are not possible, or if they have to be implemented in
other ways such as multiple frames so that uploads are separate
requests from the progressive display, or if they work with most
clients and there are a few known ones where it must be prevented,
then I accept that.

I'd simply like to know whether it's best to program the server to
enforce that, knowing it's a common/rare client weakness, or to not
enforce it but recommend it in the application interface
documentation, or to permit it if it actually works in practice.

> Do not spend time deciding when to close a connection in the
> presence of strange traffic: close as soon as you can, usually right
> after sending the final response (erroneous or otherwise).

My strategy is to copy Apache's well-tested "lingering close":
shutdown(fd,SHUT_WR) followed by reading everything for up to 30
seconds, or until 2 seconds passes with no incoming data, then the
full close().  Immediately fully closing the socket is known to cause
client problems for good reasons.

The other part of my strategy, based on what people have said here, is
to ensure I read the whole request if sending a non-error response,
but not if sending an error response - then I can just follow it with
lingering close.

> This subjective advice is based on testing actual HTTP
> implementations. YMMV.

Your advice is very welcome!  Thank you!  I find it surprising that
response-during-request-upload isn't mentioned in any guidelines that
I've seen as either ok or a problem.  If there is knowledge about this
working or not working with common browser clients, I'd like to know
more.  Maybe I'm one of very few people to consider it :/

-- Jamie
Received on Monday, 15 March 2004 14:07:45 GMT

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