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From Simon Nash <n...@apache.org>
Subject Re: Dynamic service references ?
Date Fri, 18 Nov 2011 11:28:05 GMT
Jon Giddy wrote:
> On 16/11/11 13:29, Simon Nash wrote:
>> Millies, Sebastian wrote:
>>>
>>>
>>> can references in composite files somehow be made dynamic? In 
>>> particular, can I define or override
>>>
>>> the endpoints in a web service reference at run time?
>>>
>>>
>>>
>>> I understand (correctly, I hope, I have not tested this) that I can 
>>> override whatever endpoint is specified in the wsdl by
>>>
>>> specifying the uri attribute on the web service binding.
> I don't think this works, as the Uri has been read during startup before 
> you get a chance to modify it.
>>>
>>>
>>> Is there a way to define Tuscany web service references with dynamic 
>>> endpoints? Or alternatively, could I perhaps use the endpointProperties
>>>
>>> and modify the injected service proxy before making a remote call? Or 
>>> is  there some entirely different route that does not involve steps 
>>> a) to f) above?
>>>
>>>
>>>
>>> How would you deal with this type of situation?
>>>
>>>
>>>
>>
>> It might be possible to use service references for this.  A service
>> reference can be serialized/deserialized to/from XML.  So you could
>> use runtime code to create the XML-equivalent form of the service
>> reference that you need, then deserialize the XML to create the
>> service reference.
> 
> We tried something like this, but failed to get it to work fully.
> 
> We have a client that needs to talk to multiple identical services, and 
> got it working successfully using multiplicity references. However, this 
> is a static solution where the client knows all the services it will use 
> at startup.
> 
> We then tried a solution where, as each service starts up, it creates a 
> ServiceReference to itself, marshals that into Base64, and calls an 
> endpoint on the client, passing in its ServiceReference, which is stored 
> in an internal directory (basically a HashMap). The client is then able 
> to retrieve the endpoints from its internal directory and use them.
> 
> However, a service registering with a client is a little bizarre. We 
> really wanted the service to register its ServiceReference with a 
> directory service, and for the client to retrieve the 
> ServiceReference(s) from the directory service. That is, replace the 
> internal HashMap with an external service. This would be the classic 
> Publish-Find-Bind model of Service Oriented Architecture.
> 
> However, we couldn't get it to work. The client can get the Base64 
> encoded ServiceReference, and perform the same actions as before, but we 
> get a NullPointerException when attempting to use the reference.
> 
> It would be great to know if anyone else succeeds at this, as we feel so 
> close to achieving dynamic binding in Tuscany,...and yet so far.
> 
> Jon.
> 
> 
> 
I think it's unlikely that this could work with Base64 serialization.
This would presumably be a Base64 representation of the Java-serialized
form of the server's ServiceReferenceImpl object.  Deserializing this
on the client machine isn't likely to be successful.

I think the first approach is working because Tuscany recognizes service
references as arguments to service invocations and does special magic
(actually serializes them to XML) when passing them on the wire.
So I'd guess that the Tuscany server is converting Base64 to a
server-side service reference object, and is then converting this object
to XML for passing on the wire to the client.  The Tuscany client
deserializes this XML to a working client-side service reference.

So I think the only approach that hsa a chance of working would be
to store the XML form in the directory and have the client retrieve
this XML form from the directory and deserialize it.

As to the magic formula for doing this....I'll try some experiments,
but I'm very busy with another project right now so it's likely to
be a few days before I have any news.

   Simon


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