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Re: [json] Re: jsonrequest and HTTP/1.1 message pipelining

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  • Mark Nottingham
    ... There are several aspects, but if you have an outstanding request on a connection, and another request is queued, deciding whether it s more efficient to
    Message 1 of 7 , Dec 18, 2007
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      On 2007/12/18, at 4:53 PM, Tyler Close wrote:
      > Hi Mark,
      >
      > Thanks for the response. I've got a few questions about your comments
      > and am also wondering if it's feasible to work around the issues you
      > raise.
      >
      > On Dec 17, 2007 4:19 PM, Mark Nottingham <mnot@...> wrote:
      > > Pipelining is often regarded as problematic, especially from the
      > client
      > > side, because of
      > > uneven support in proxies and servers, as well as some uncomfortable
      > > decisions you need
      > > to make about optimisation.
      >
      > Could you elaborate on the optimization issues?
      >
      There are several aspects, but if you have an outstanding request on a
      connection, and another request is queued, deciding whether it's more
      efficient to pipeline or to open a new connection (or, to wait for
      the other connection to clear) isn't always a simple thing to do. If
      the outstanding request takes a long time to process (either because
      the response is very large, or because it takes a lot of server-side
      processing time), it may be better to use your other connection.

      In cases where the resources on the server have low processing
      overhead and are relatively homogenous in size, pipelining works well.
      Subversion is a good example of this, and indeed it benefits from the
      use of pipelining. I'm personally not convinced it's a great solution
      when that isn't the case. YMMV.

      > > Also, non-idempotent methods (e.g., POST, PUT) shouldn't be
      > pipelined, so
      > > this effectively limits it to GET.
      >
      > I remember reading something along these lines in RFC 2616, but the
      > argument never made any sense to me. Perhaps you could clarify the
      > issue. RFC 2616 contains some language about the client not knowing
      > what state the server is in if the connection died with multiple
      > outstanding POST requests, but the same is true if there is even one
      > outstanding POST request. Also, the situation seems to be the same if
      > the client is using multiple non-pipelined connections, since there
      > may be multiple outstanding POST requests.
      >
      Well, it's a SHOULD NOT, not a MUST NOT, but consider a sequence of
      PUT and DELETE requests; if they're pipelined and the connection drops
      in the middle, the client has no idea what state the world is in; if
      it doesn't pipeline, it still has to figure out whether the last
      request was applied, but not the previous ones.

      Also, keep in mind that connections in an intermediary aren't
      necessarily "sticky" to one client; a proxy may be using one single
      persistent connection to send requests from several clients to a
      single server. If pipelining of nonidempotent requests were allowed
      here, the failure cases get really ugly.

      > I expect all the POST
      > requests queued by the client also get sent out regardless of the
      > status of the previous requests, so it seems like the client is in
      > much the same predicament regardless of the use of pipelining.
      >
      Hopefully not...

      > > Even with pipelining on a single connection, you can't make
      > assumptions
      > > about messaging
      > > ordering. Intermediaries are allowed to (and do) split requests up
      > and put
      > > them on
      > > different connections, which may have different routes back to the
      > origin.
      > > Somewhat
      > > pathological, but entirely possible (I've seen configurations
      > which would
      > > allow -- or even
      > > encourage -- this to happen).
      >
      > How about this: If there is no HTTP proxy, pipeline requests;
      > otherwise, send the requests one at a time. So if the client asked
      > that requests be ordered, this is guaranteed and performance is best
      > effort. If the client doesn't care about ordering, but wants best
      > performance, then it uses separately instantiated JSONRequest objects.
      > Sound good?
      >
      You don't always know whether there's an intermediary there;
      interception proxies (aka "transparent proxies") and HTTP accelerators
      aren't apparent to the client.

      --
      Mark Nottingham mnot@...
    • Tyler Close
      Hi Mark, I think message ordering and pipelining are really useful features, so I ld like to kick this around some more to see if there s something that could
      Message 2 of 7 , Dec 31, 2007
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        Hi Mark,

        I think message ordering and pipelining are really useful features, so
        I'ld like to kick this around some more to see if there's something
        that could work. For example, I've seen some web applications that
        implement their own request boxcarring to compensate for the lack of
        pipelining. At this point, they are basically tunneling their own
        protocol through HTTP and so not getting many of the benefits of HTTP,
        such as caching.

        On Dec 18, 2007 9:06 PM, Mark Nottingham <mnot@...> wrote:
        > There are several aspects, but if you have an outstanding request on a
        > connection, and another request is queued, deciding whether it's more
        > efficient to pipeline or to open a new connection (or, to wait for
        > the other connection to clear) isn't always a simple thing to do. If
        > the outstanding request takes a long time to process (either because
        > the response is very large, or because it takes a lot of server-side
        > processing time), it may be better to use your other connection.

        Seems like this logic is something that should be expressed by the end
        client. So if the client issued requests like:

        JSONRequest.post(...);
        JSONRequest.post(...);

        it's saying there is no expected ordering and the browser should use
        separate connections if possible. Whereas if the client issued
        requests like:

        var c = JSONRequest.ordered();
        c.post(...);
        c.post(...);

        it's saying these requests must be ordered and the browser should
        pipeline over a single connection if possible.

        > > How about this: If there is no HTTP proxy, pipeline requests;
        > > otherwise, send the requests one at a time. So if the client asked
        > > that requests be ordered, this is guaranteed and performance is best
        > > effort. If the client doesn't care about ordering, but wants best
        > > performance, then it uses separately instantiated JSONRequest objects.
        > > Sound good?
        > >
        > You don't always know whether there's an intermediary there;
        > interception proxies (aka "transparent proxies") and HTTP accelerators
        > aren't apparent to the client.

        OK then, SSL to the rescue. For the case where the client asks that
        requests be sent in order:

        1. If its an HTTPS connection, open a single connection and pipeline
        the requests.
        2. Otherwise, open a single connection and send requests synchronously.

        For requests that don't use the ordered request API, they can be sent
        as they currently are, over multiple connections.

        This seems like a small amount of complexity to add, while gaining the
        benefits of message ordering and pipelining when making secure
        web-applications.

        I suppose there could be some server-side infrastructure that again
        "helps" the developer by re-ordering requests, but the web-application
        developer is presumably in a better position to do something about
        this. Are there any other gremlins?

        Thanks,
        --Tyler

        --
        Use web-keys for RESTful access-control:
        http://waterken.sourceforge.net/

        Name your trusted sites to distinguish them from phishing sites.
        https://addons.mozilla.org/firefox/957/
      • Mark Nottingham
        Hm. Personally, I wouldn t go this way; you re making a bet that the overhead of setting up SSL/TLS is less than that of working synchronously. If you re just
        Message 3 of 7 , Jan 1, 2008
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          Hm.

          Personally, I wouldn't go this way; you're making a bet that the
          overhead of setting up SSL/TLS is less than that of working
          synchronously. If you're just POSTing stuff to the server, combining
          several things into one request format may be the way to go.

          That having been said -- if you are going to use https, you'll need to
          have an API available that guarantees to give you a single connection
          back. XHR doesn't do that.

          I totally agree that pipelining is useful, and reducing latency is a
          good goal. It's just that the proper place to fix this sort of thing
          is lower in the stack, not higher.

          Cheers,


          On 01/01/2008, at 7:24 AM, Tyler Close wrote:

          > Hi Mark,
          >
          > I think message ordering and pipelining are really useful features, so
          > I'ld like to kick this around some more to see if there's something
          > that could work. For example, I've seen some web applications that
          > implement their own request boxcarring to compensate for the lack of
          > pipelining. At this point, they are basically tunneling their own
          > protocol through HTTP and so not getting many of the benefits of HTTP,
          > such as caching.
          >
          > On Dec 18, 2007 9:06 PM, Mark Nottingham <mnot@...> wrote:
          > > There are several aspects, but if you have an outstanding request
          > on a
          > > connection, and another request is queued, deciding whether it's
          > more
          > > efficient to pipeline or to open a new connection (or, to wait for
          > > the other connection to clear) isn't always a simple thing to do. If
          > > the outstanding request takes a long time to process (either because
          > > the response is very large, or because it takes a lot of server-side
          > > processing time), it may be better to use your other connection.
          >
          > Seems like this logic is something that should be expressed by the end
          > client. So if the client issued requests like:
          >
          > JSONRequest.post(...);
          > JSONRequest.post(...);
          >
          > it's saying there is no expected ordering and the browser should use
          > separate connections if possible. Whereas if the client issued
          > requests like:
          >
          > var c = JSONRequest.ordered();
          > c.post(...);
          > c.post(...);
          >
          > it's saying these requests must be ordered and the browser should
          > pipeline over a single connection if possible.
          >
          > > > How about this: If there is no HTTP proxy, pipeline requests;
          > > > otherwise, send the requests one at a time. So if the client asked
          > > > that requests be ordered, this is guaranteed and performance is
          > best
          > > > effort. If the client doesn't care about ordering, but wants best
          > > > performance, then it uses separately instantiated JSONRequest
          > objects.
          > > > Sound good?
          > > >
          > > You don't always know whether there's an intermediary there;
          > > interception proxies (aka "transparent proxies") and HTTP
          > accelerators
          > > aren't apparent to the client.
          >
          > OK then, SSL to the rescue. For the case where the client asks that
          > requests be sent in order:
          >
          > 1. If its an HTTPS connection, open a single connection and pipeline
          > the requests.
          > 2. Otherwise, open a single connection and send requests
          > synchronously.
          >
          > For requests that don't use the ordered request API, they can be sent
          > as they currently are, over multiple connections.
          >
          > This seems like a small amount of complexity to add, while gaining the
          > benefits of message ordering and pipelining when making secure
          > web-applications.
          >
          > I suppose there could be some server-side infrastructure that again
          > "helps" the developer by re-ordering requests, but the web-application
          > developer is presumably in a better position to do something about
          > this. Are there any other gremlins?
          >
          > Thanks,
          > --Tyler
          >
          > --
          > Use web-keys for RESTful access-control:
          > http://waterken.sourceforge.net/
          >
          > Name your trusted sites to distinguish them from phishing sites.
          > https://addons.mozilla.org/firefox/957/
          >
          >

          --
          Mark Nottingham mnot@...
        • Karthik Kumar
          ... Not all browsers support pipelining. Opera has it enabled, Firefox has it disabled by default; The browser must be able to handle it transparently to the
          Message 4 of 7 , Jan 1, 2008
          • 0 Attachment
            On Jan 2, 2008 11:15 AM, Mark Nottingham <mnot@...> wrote:
            >
            >
            >
            >
            >
            >
            > Hm.
            >
            > Personally, I wouldn't go this way; you're making a bet that the
            > overhead of setting up SSL/TLS is less than that of working
            > synchronously. If you're just POSTing stuff to the server, combining
            > several things into one request format may be the way to go.
            >
            > That having been said -- if you are going to use https, you'll need to
            > have an API available that guarantees to give you a single connection
            > back. XHR doesn't do that.
            >
            > I totally agree that pipelining is useful, and reducing latency is a
            > good goal. It's just that the proper place to fix this sort of thing
            > is lower in the stack, not higher.
            >
            > Cheers,
            >
            > On 01/01/2008, at 7:24 AM, Tyler Close wrote:
            >
            > > Hi Mark,
            > >
            > > I think message ordering and pipelining are really useful features, so
            > > I'ld like to kick this around some more to see if there's something
            > > that could work. For example, I've seen some web applications that
            > > implement their own request boxcarring to compensate for the lack of
            > > pipelining. At this point, they are basically tunneling their own
            > > protocol through HTTP and so not getting many of the benefits of HTTP,
            > > such as caching.
            > >
            > > On Dec 18, 2007 9:06 PM, Mark Nottingham <mnot@...> wrote:
            > > > There are several aspects, but if you have an outstanding request
            > > on a
            > > > connection, and another request is queued, deciding whether it's
            > > more
            > > > efficient to pipeline or to open a new connection (or, to wait for
            > > > the other connection to clear) isn't always a simple thing to do. If
            > > > the outstanding request takes a long time to process (either because
            > > > the response is very large, or because it takes a lot of server-side
            > > > processing time), it may be better to use your other connection.
            > >
            > > Seems like this logic is something that should be expressed by the end
            > > client. So if the client issued requests like:
            > >
            > > JSONRequest.post(...);
            > > JSONRequest.post(...);
            > >
            > > it's saying there is no expected ordering and the browser should use
            > > separate connections if possible. Whereas if the client issued
            > > requests like:
            > >
            > > var c = JSONRequest.ordered();
            > > c.post(...);
            > > c.post(...);
            > >
            > > it's saying these requests must be ordered and the browser should
            > > pipeline over a single connection if possible.
            > >

            Not all browsers support pipelining. Opera has it enabled, Firefox has
            it disabled by default; The browser must be able to handle it
            transparently to the JS app; Synchronous requests with browser-side
            polling may benefit from pipelining; Pipelining effects are good only
            for small-sized requests; So the browser should determine when to
            pipeline or not (depending on immediately receiving the response
            content-length)

            > > > > How about this: If there is no HTTP proxy, pipeline requests;
            > > > > otherwise, send the requests one at a time. So if the client asked
            > > > > that requests be ordered, this is guaranteed and performance is
            > > best
            > > > > effort. If the client doesn't care about ordering, but wants best
            > > > > performance, then it uses separately instantiated JSONRequest
            > > objects.
            > > > > Sound good?
            > > > >
            > > > You don't always know whether there's an intermediary there;
            > > > interception proxies (aka "transparent proxies") and HTTP
            > > accelerators
            > > > aren't apparent to the client.
            > >
            > > OK then, SSL to the rescue. For the case where the client asks that
            > > requests be sent in order:
            > >
            > > 1. If its an HTTPS connection, open a single connection and pipeline
            > > the requests.
            > > 2. Otherwise, open a single connection and send requests
            > > synchronously.
            > >
            > > For requests that don't use the ordered request API, they can be sent
            > > as they currently are, over multiple connections.
            > >
            > > This seems like a small amount of complexity to add, while gaining the
            > > benefits of message ordering and pipelining when making secure
            > > web-applications.
            > >
            > > I suppose there could be some server-side infrastructure that again
            > > "helps" the developer by re-ordering requests, but the web-application
            > > developer is presumably in a better position to do something about
            > > this. Are there any other gremlins?
            > >
            > > Thanks,
            > > --Tyler
            > >
            > > --
            > > Use web-keys for RESTful access-control:
            > > http://waterken.sourceforge.net/
            > >
            > > Name your trusted sites to distinguish them from phishing sites.
            > > https://addons.mozilla.org/firefox/957/
            > >
            > >
            >
            > --
            > Mark Nottingham mnot@...
            >
            >



            --
            Karthik
            http://guilt.bafsoft.net
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