Re: "Timeout" request header to tell server to wait for resource to become available

That sounds like exactly the case Prefer: wait=x was designed for.

Note that with HTTP/2 you can set the header field to the actual time
that you are willing to wait, and use PING frames to test (and
maintain) connectivity.

On 9 April 2015 at 06:39, Benjamin Carlyle
<> wrote:
> On 28 March 2015 at 11:46, Martin Thomson <> wrote:
>> I believe that what you want is accomplished by RFC 7240:
>> Prefer: wait=5
>> The units are perhaps suboptimal for your use case (seconds instead of
>> milliseconds), but we might be able to make a change to support finer
>> grained timing.
> I thought I would write in to describe a use case for combining
> Prefer:wait with GET requests. I'm not sure if my case is completely
> compatible with Brendan's. My main use case for HTTP, SPDY, and soon
> h2 is within highly available safety related (not safety-critical)
> SCADA systems. Within these systems there is often a requirement for
> soft realtime transfer of data, that is delivery of information within
> an order of magnitude or two or three of the effectively latency of
> the network. The current preferred way to do this with HTTP is to have
> a "main" URL for a given collection of data, plus a series of "delta"
> URLs. Issuing GET to the main URL returns immediately, and includes a
> Link header to the next delta URL. A client will issue GET to the
> delta URL which includes an time-like identifier for the recent main
> resource representation. The delta response will include a Link header
> to the delta.
> A crude example:
> -> GET /main
> <- 200 OK
> <- Link: </delta/5>; rel="delta"
> <- (current state)
> -> GET /delta/5
> <- 200 OK
> <- Link </delta/7>; rel="next"
> <- (changes from t=5 to t=7)
> The request to the delta URL is a "long poll" where the client is
> willing to wait until content is available at the delta URL it has
> been given. There are two main alternatives to a success response for
> the delta URL:
> 4xx - the delta is invalid for some reason, say a circular buffer is
> keeping track of recent changes on behalf of all clients and the index
> into that buffer that the client holds is no longer valid
> 204 - the delta is valid but no changes have occurred yet. The server
> can validly return 204 at any time to a delta request so can shed
> clients it no longer wants to serve etc.
> I haven't been using the RFC7240 code but I may start using it when we
> deprecate a h2-like internal protocol dating back many years and
> switch to official h2. Currently I'm using a custom header sent in the
> GET request to indicate how long the client is willing to wait for a
> response. Typically this might be around 4s after which the client
> will expect a response - otherwise it might be that the server or TCP
> connection is dead. In this way the 204 response acts as a heartbeat
> message to the client when the change rate is low. I refer to the
> technique as long poll delta encoding, and for synchronisation of data
> across a network between control system components with
> well-controlled failure modes I think it's actually hard to beat -
> partially because this particular interaction is stateless. A client
> only has to make one request at any time to come back into sync and
> the server can drop clients at will without loss of synchronisation
> state. A header like this can also be a hint to layers that do not
> understand the full request semantics to allocate resources to the
> request differently, for example by shifting workload onto different
> thread pool.
> I wrote about the mechanism back in 2012 in case anyone is interested
> in a slightly more complete though slightly out of date treatment of
> the subject:
> Obviously there is a lot more to the story of ensuring good responses
> to failure for HTTP requests. As a blanket rule for these systems
> there exists a time limit T in the order of a few seconds such that
> should any kind of network failure occur clients detect the failure
> and reestablish comms to a new server. Doing a long poll with a
> timeout is one part of that.

Received on Thursday, 9 April 2015 16:50:57 UTC