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

From: Jamie Lokier <jamie@shareable.org>
Date: Fri, 12 Mar 2004 11:41:53 +0000
To: ietf-http-wg@w3.org
Message-ID: <20040312114153.GA17780@mail.shareable.org>

Dear all,

I have been implementing an HTTP/1.1 server which consumes the request
entity lazily, as need by the application.

If the application decides that it can generate some or all of the
response before it finishes reading the whole request entity, is that
allowed?

Can the application generate all the response without reading any of
the request entity and without sending 100 (Continue) - is that
allowed?

I've been studying RFC 2616 and don't see an answer.

RFC 2616, 8.2.2:

  8.2.2 Monitoring Connections for Error Status Messages

     An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
     the network connection for an error status while it is transmitting
     the request. If the client sees an error status, it SHOULD
     immediately cease transmitting the body. If the body is being sent
     using a "chunked" encoding (section 3.6), a zero length chunk and
     empty trailer MAY be used to prematurely mark the end of the message.
     If the body was preceded by a Content-Length header, the client MUST
     close the connection.

I'm not entirely sure if this means _network_ errors (such as TCP
shutdown or reset), or HTTP Error Status codes 4xx and 5xx.  I guess
it means HTTP Error Status codes, because of the implication that the
connection might continue.

1. Regarding my first question, whether the response can be partly
transmitted before all of the request entity has been read.

If the above section means HTTP Error Status codes, then the answer is
no if the status is an error code.  In other words, if the server
wanted to generate an error response, but use data from the request
entity to do it, incrementally, it would have to read all of the
request entity that it needs before transmitting any of the response.
Is this right?

What is the answer if the status is not an error code.  Can the server
begin transmitting the response entity before all of the request
entity has been read?  (Assume that the server is intelligent enough
to avoid deadlock by always reading request data when it arrives).

One simple example would be a server which serves the file it is being
transmitted, or a transformation of it.  It would be quite nice to
upload a file, and have the transformed version generated
incrementally and downloaded as the upload proceeds.  (This is
obviously possible using two separate connections, but my question is
is it supported by HTTP/1.1 over a single connection?)

2. Regarding my second question, whether a non-error response can be
safely transmitted without reading the request entity.  Is it allowed?
Does the connection have to be made non-persistent; i.e. closed afterwards?

If 100 (Continue) isn't transmitted, some clients may start
transmitting their request entities anyway.  Therefore there's no way
for the server to determine whether data received from the client is
the request entity or the next request, if it wants to try avoiding
100 (Continue) to produce a non-error response without using the
request entity.

This comes up because the server will transmit 100 (Continue) only
when the application begins to read the request entity via the server,
naturally.  If the application generates a 200 OK response complete
with entity, and has not read the request entity, what must the server
do?  What should it do?

Does it have to read the request entity anyway if it wants to keep the
connection persistent?  (If so, it might be worth doing that if the
request entity is known to be very small).

Or can it avoid reading the request entity completely, by not sending
100 (Continue) and somehow determining when the next request arrives
on that connection.  Is that possible?

Thanks for your answers :)
-- Jamie
Received on Friday, 12 March 2004 06:41:54 GMT

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