Re: Duplex communication sample using WSE 2.0 SP1 & ASP.NET 1.1 XML WS

From: Hervey Wilson [MSFT] (herveyw.nospam_at_nospam.microsoft.com)
Date: 10/17/04 

Date: Sun, 17 Oct 2004 10:05:24 -0700

Andreas Mauer wrote:
> Hi,
> I was just wondering if there is somewhere a useful sample that shows how to
> do async. duplex communication within SO-based enterprise solutions using the
> WSE 2.0 SP1 WS-Addressing feauture with ASP.NET 1.1 based XML WS. I know
> there are some samples that show just that using the soap.tcp thing! But I
> need to do this within the ASP.NET 1.1 XML Web Services stack of
> technologies. Maybe there are some good guidance as well of how to realize
> this scenario. The problem I face is that at some initial time a client piece
> of code calls into a web service, this service just consumes another service
> to trigger some long lasting activities. This is a one way communication, but
> what I need is that at some point in thime the long lasting secondary service
> is finished and needs to reply this to the origin consumer service (just like
> a callback), but since this is in the web cloud world, I would like to take
> advantage of the WS-Addressing features. I can't really figure out what would
> be the best way of implementing this duplex way of communicating using the
> WSE 2.0 SP1 with ASP.NET 1.1 based XML Web Sercices.
>
> If someone has done just like this please tell me if there is somewhere some
> good sample available that can be shared within the community!
>

There's a reason that samples for this use soap.tcp instead of HTTP:
HTTP is not a full-duplex protocol; it only allows the client to
initiate requests, not the server. Therefore your long lasting service
cannot call back to the client using the same connection that the client
used; it must create a new connection to the client. Unfortunately, this
is problematic since it requires that the client also be listening on
HTTP (e.g. be an ASP.NET Web Service) and not be behind a firewall or
NAT device that would block the incoming connection.

If your client can be a Web Service itself, it can tell the long-running
server where to send the call back. If not, then you'll likely have to
resort to a polling solution where the client periodically asks the
long-running server whether the work is complete. 

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