On Mon, 2006-10-09 at 13:26 -0400, Sam Berlin wrote:
> I agree with Robert that it is much easier to go from an event driven
> model to a blocking model. If the first layer that HttpNIO exposes is
> blocking, there'd need to be additional hacking below that in order to
> remove the blocking / thread-based layer. On the other hand, if the
> first layer it exposes is non-blocking, it's relatively trivial to add
> a thread ontop of that and expose an additional blocking layer.
> It is difficult to think of many scenarios that require (or a better
> with) non-blocking I/O, but I would caution against excluding them
> from HttpClient's scope. If HttpClient 4.0 had been ready a year or
> so ago (with an exposed non-blocking layer), we would definitely have
> used it in LimeWire as the basis of file-transfers. As-is, we
> invented our own minimalistic non-blocking state-based http transport
> for downloads.
> If the non-blocking layer is there, I guarantee that folks will be
> able to find a use for it. Whereas if only a blocking layer is there,
> those developers looking for the high-performance asyncronous model
> will have to go elsewhere.
Sam, Robert, et al
Simply for practical reasons while there are only two guys hacking (me
and Roland) we ought not spread out efforts too thin. A full-blown
even-driven API will take time to get right. I think it is more
important to get HttpCore ALPHA3 release that covers 95% of use cases
out the door rather sooner than later. Beyond that it is just a matter
of priorities and available time.
> On 10/9/06, Robert Olofsson <robo@khel...> wrote:
> > Since my proxy is almost fully nio/event based I would like to share
> > a few comments.
> > Oleg Kalnichevski wrote:
> > > I am still quite skeptical about usefulness of a fully event-driven HTTP
> > > transport for one simple reason: asynchronous (non-blocking) I/O
> > > transport makes no sense of what so ever if the process of content
> > > generation or content consumption is asynchronous (blocking).
> > There are many things that may block here are a few examples:
> > *) DNS-lookup
> > *) File reading writing
> > *) Database access
> > *) All higher level api:s that only give you a stream.
> > *) Calls to Runtime.exec
> > That DNS lookups are also totally single threaded in native code in
> > some systems does not make things better.
> > > If one
> > > needs a worker thread to generate / process content anyways, what is the
> > > point of having an even driven transport?
> > Agreed.
> > One objection here may be that you do not need one worker thread for all
> > of the content generation, but that usually does not make things better.
> > You will still need the worker thread for the _slow_and_blocking_
> > operation.
> > So if content generation/modification uses any of the above then using
> > workers simplify things a lot.
> > > I see only a few scenarios
> > > where the third choice (event callbacks) may prove advantageous,
> > > primarily in HTTP proxies and gateways.
> > Except that http proxies does lots of dns lookups so they will block
> > a lot. My proxy spawns worker thread only when they need to, but it
> > complicates some part of the code.
> > That my proxy also modifies the content and caches the data will mean
> > lots of other blocking calls in some of the code paths.
> > > I think ultimately we need both options. I suggest we start with the
> > > second option, release ALPHA3 and then consider implementing the third
> > > option before ALPHA4 / BETA1.
> > One thing to keep in mind:
> > It is easier to go from event driven to a blocking model than to do the
> > reverse. This may be an argument to go for number 3 (full nio).
> > If you go for 3 then make it easy to use a few selector-threads
> > otherwise the system will use only 1 cpu (or 1 core).
> > /robo
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