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Re: Last Call: <draft-snell-http-prefer-14.txt> (Prefer Header for HTTP) to Proposed Standard

From: James M Snell <jasnell@gmail.com>
Date: Fri, 5 Oct 2012 11:28:21 -0700
Message-ID: <CABP7RbeTd=43Pq56raCXOVO6NZhOX3yKb1_LL--EqroUEXiCzw@mail.gmail.com>
To: Martin Thomson <martin.thomson@gmail.com>
Cc: Julian Reschke <julian.reschke@gmx.de>, ietf@ietf.org, SM <sm@resistor.net>, HTTP Working Group <ietf-http-wg@w3.org>
How about the following change:

<snip>

  The "wait" preference can be used to establish an upper bound on the
  length of time, in seconds, the client expects it will take the server
  to process the request once it has been received. In the case that
  generating a response will take longer than the time specified,
  the server, or proxy, can choose to utilize an asynchronous processing
  model by returning -- for example -- a "202 Accepted" response.

  ABNF:
    wait = "wait" BWS "=" BWS delta-seconds

  It is important to consider that there are many -- largely
  unpredictable -- factors that can influence the amount of time it
  takes a server to process a request. The period of time specified
  is not intended to be treated as a strictly defined "hard limit"
  but rather as a hint about the client's expectation.

  For example, a server receiving the following request might choose
  to respond asynchronously if processing the request will take longer
  than 10 seconds:

    POST /collection HTTP/1.1
    Host: example.org
    Content-Type: text/plain
    Prefer: return-asynch, wait=10

    {Data}

</snip>

On Fri, Oct 5, 2012 at 11:12 AM, Martin Thomson <martin.thomson@gmail.com>wrote:

> On 5 October 2012 10:42, James M Snell <jasnell@gmail.com> wrote:
> > I could drop the Date header recommendation altogether and stress in the
> > text that good clock synchronization and predictable latency is required
> for
> > the wait preference to be used effectively.
>
> The feature is useful, I agree.  The problem is that - as defined -
> the server needs to guess something about the times on the client in
> order to implement this reliably.
>
> Relying on clock synchronization is not realistic.  Even in controlled
> environments, errors are commonplace.
>
> Even the simple case shows a problem:
>   a: client sends request
>   b: server receives request
>   c: time passes
>   d: server responds to request
>   e: client receives response
>
> You require that the time be a measure of a->e.  The server has no way
> to determine what that time is.
>
> An alternative would be to make the requirement apply to b->d.  That
> is something that the server has direct control over.  The client then
> gains a little extra work, but at least they are in a position to
> measure a->b + d->e.  In any case, with low or predictable latency, I
> doubt that the addition of a->b + d->e will have any significant
> impact on whether the information is useful to the client.  Especially
> given that times are expressed in seconds, not microseconds.
>
Received on Friday, 5 October 2012 18:29:09 GMT

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