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From Andreas Veithen <andreas.veit...@skynet.be>
Subject Using some sort of in-VM transport between Synapse and a web app deployed on the same server?
Date Fri, 04 Jan 2008 23:14:39 GMT
Hi all!

Consider the following use case for Synapse:

* Synapse is deployed as a Web application.
* The Synapse configuration has a service proxy that targets a service  
exposed by another Web application deployed on the same server.
* Both the proxy and the target use HTTP as transport protocol.
* The target Web application neither uses Axis, nor is it reasonable  
to change it in any way.

The obvious solution is to use HTTP to communicate with the target  
service. However using HTTP to communicate between two Web  
applications deployed on the same server, i.e. inside the same VM,  
seems a bit like an overkill. Therefore my question is if there is  
some sort of a local transport implementation that would allow to do  
this in a more efficient way. Obviously the local transport defined in  
Axis2 can't be used here, because it would require changes to the  
target application. Also the fact that each of the two Web  
applications has its own class loader would make this tricky.

It appears that the servlet specification actually defines a way for  
two Web applications hosted in the same container to communicate with  
each other directly, i.e. in VM. Indeed a Web application can obtain  
the ServletContext of another Web application using  
ServletContext#getContext. Usually the server must be configured to  
allow this kind of cross context access, but this is supported by  
Tomcat (by specifying crossContext="true" in the context file). Using  
this mechanism, the Web applications can communicate in two different  
ways:

* By sharing state using get/setAttribute.
* By acquiring a RequestDispatcher for a servlet from the foreign  
context and invoking forward or include on this object.

The first approach can be discarded, again because of class loader  
issues and the fact that it would require changes to the second  
application. The second approach is more promising because it allows  
to invoke the servlet that implements the target service. Note however  
that there is an important restriction imposed by the servlet  
specification on the use of RequestDispatcher#include/forward: "The  
request and response parameters must be either the same objects as  
were passed to the calling servlet’s service method or be subclasses  
of the ServletRequestWrapper or ServletResponseWrapper classes that  
wrap them." This means that these methods can't be called with  
arbitrary HttpServletRequest/Response implementations. Obviously this  
requirement is there to guarantee that the server gets access to the  
original request/response objects. It could be that some servers do  
not enforce this requirement, but for those that do, the practical  
consequence is that the RequestDispatcher approach would only work if  
the original request is received by SynapseAxisServlet, which is  
perfectly compatible with the use case described here.

My question is whether anybody on this list is aware of an existing  
Axis2 transport implementation that uses this approach or if anybody  
here sees any obstacle that would make it impossible to implement this  
kind of transport.

Regards,

Andreas


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