Trouble understanding the handler pipeline

[Utkarsh Srivastava]

Yes, thanks, its clear now. I think this deserves a mention in the docs,
otherwise its too mysterious.

Utkarsh

On Tue, Mar 10, 2009 at 3:11 PM, Trustin Lee <tlee at redhat.com> wrote:

> On Wed, Mar 11, 2009 at 6:30 AM, Utkarsh Srivastava <utkarsh at gmail.com>
> wrote:
> > Hi,
> >
> > I am a day old to netty and still finding my way around. Looks like a
> really
> > slick framework!
> >
> > I am having a little trouble understanding the handler pipeline (yes, I
> have
> > read the api docs for ChannelPipeline, ChannelHandler and ChannelEvent).
> To
> > take a concrete example, the server pipeline in the example time server
> has
> > the following pipeline:
> >
> >         ChannelPipeline p = pipeline();
> >         1. p.addLast("frameDecoder", new
> > LengthFieldBasedFrameDecoder(1048576, 0, 4, 0, 4));
> >         2. p.addLast("protobufDecoder", new
> > ProtobufDecoder(LocalTimeProtocol.Locations.getDefaultInstance()));
> >
> >         3. p.addLast("frameEncoder", new LengthFieldPrepender(4));
> >         4. p.addLast("protobufEncoder", new ProtobufEncoder());
> >
> >         5. p.addLast("handler", new LocalTimeServerHandler());
> >         return p;
> >
> > For a given request, handlers are executed in the order 1,2,5,4,3, right?
> > Thats the only order that makes sense.
> >
> > But thats totally different from the order they are added to the pipeline
> in
> > the code.
>
> That's a good question.  When an event goes upstream, the order is 1,
> 2, 3, 4, 5.  When an event goes downstream, the order is 5, 4, 3, 2,
> 1.  However, ChannelPipline does some trick to shorten the stack
> depth.
>
> 3 and 4 don't implement ChannelUpstreamHandler, and therefore the
> actual evaluation order of an upstream event will be: 1, 2, and 5.
>
> 1, 2, and 5 don't implement ChannelDownstreamHandler, and therefore
> the actual evaluation order of a downstream event will be: 4 and 3
>
> If 5 extends SimpleChannelHandler, it means 5 implements both
> ChannelUpstreamHandler and ChannelDownstreamHandler.  Therefore the
> evaluation order for upstream and downstream events will be 125 and
> 543.
>
> Sounds clear? :)
>
> HTH,
> Trustin
>
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